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Basics for beginners
Mechanical Ventilation
Definition
It is a method to mechanically assist or replace spontaneous breathing when patients can not do so on their own
General Considerations Mechanical ventilation has resulted in profoundly improved survival from acute and chronic respiratory failure
Mechanical ventilation in the intensive care unit (ICU) is delivered under positive pressure in contrast to normal human breathing in which inspiration induces negative pressure in the thorax This makes the complications of barotrauma and hypotension predictable To achieve ventilation rate tidal volume (VT) fraction of inspired oxygen (FIO2) and positive end-expiratory pressure (PEEP) are selected It is useful to track the product of VT and rate the minute ventilation (E) to assess for complications and readiness to wean A normal E is less then 10 Lmin Pressures in ventilators There are two types of pressures used in ventilators
a Negative pressure ventilators The best example is ldquoiron lungrdquo that creates a negative pressure environment around the patientrsquos chest thus sucking air into the lungs This is a large elongated tank which encases the patient up to the neck The neck is sealed with a rubber gasket so that the petientrsquos face and airway are exposed to the room air This was largely used to treat patients with respiratory paralysis due to poliomyelitis
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b Poistive pressure ventilators This is the main form of ventilators currently used
These ventilators work by increasing the pressure in the patientrsquos airway and thus forcing additional air into the lungs
The fundamental difference between positive pressure and negative pressure is in positive pressure ventilation air is introduced into the lungs forcefully creating positive pressure but in negative pressure ventilation negative pressure is created outside of the lungs that sucks the chest to inflate and creates negative pressure within lungs as a result air flows into the lungs
Ventilation Invasive Vs non-invasive The difference is just use of a tube (ie endotracheal tube tracheostomy tube) If you use endotracheal tube it is invasive ventilation If you do not use it is non-invasive ventilation Non-invasive ventilation can be delivered with the use either of
a Negative pressure ventilators (eg Iron lung) b Positive pressure assistance to the airway by mask (using NIPPV) Non Invasive
Positive Pressure Ventilation (NIPPV) can be delivered by several appliances including face masks nasal masks nasal pillows etc
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Bi-PAP setting
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Nasalface mask
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Nasotracheal intubation
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Positive pressure machines
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Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
triplehelix Mechanical Ventilation
31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Mechanical Ventilation
Definition
It is a method to mechanically assist or replace spontaneous breathing when patients can not do so on their own
General Considerations Mechanical ventilation has resulted in profoundly improved survival from acute and chronic respiratory failure
Mechanical ventilation in the intensive care unit (ICU) is delivered under positive pressure in contrast to normal human breathing in which inspiration induces negative pressure in the thorax This makes the complications of barotrauma and hypotension predictable To achieve ventilation rate tidal volume (VT) fraction of inspired oxygen (FIO2) and positive end-expiratory pressure (PEEP) are selected It is useful to track the product of VT and rate the minute ventilation (E) to assess for complications and readiness to wean A normal E is less then 10 Lmin Pressures in ventilators There are two types of pressures used in ventilators
a Negative pressure ventilators The best example is ldquoiron lungrdquo that creates a negative pressure environment around the patientrsquos chest thus sucking air into the lungs This is a large elongated tank which encases the patient up to the neck The neck is sealed with a rubber gasket so that the petientrsquos face and airway are exposed to the room air This was largely used to treat patients with respiratory paralysis due to poliomyelitis
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3
b Poistive pressure ventilators This is the main form of ventilators currently used
These ventilators work by increasing the pressure in the patientrsquos airway and thus forcing additional air into the lungs
The fundamental difference between positive pressure and negative pressure is in positive pressure ventilation air is introduced into the lungs forcefully creating positive pressure but in negative pressure ventilation negative pressure is created outside of the lungs that sucks the chest to inflate and creates negative pressure within lungs as a result air flows into the lungs
Ventilation Invasive Vs non-invasive The difference is just use of a tube (ie endotracheal tube tracheostomy tube) If you use endotracheal tube it is invasive ventilation If you do not use it is non-invasive ventilation Non-invasive ventilation can be delivered with the use either of
a Negative pressure ventilators (eg Iron lung) b Positive pressure assistance to the airway by mask (using NIPPV) Non Invasive
Positive Pressure Ventilation (NIPPV) can be delivered by several appliances including face masks nasal masks nasal pillows etc
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4
Bi-PAP setting
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5
Nasalface mask
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6
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7
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8
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
b Poistive pressure ventilators This is the main form of ventilators currently used
These ventilators work by increasing the pressure in the patientrsquos airway and thus forcing additional air into the lungs
The fundamental difference between positive pressure and negative pressure is in positive pressure ventilation air is introduced into the lungs forcefully creating positive pressure but in negative pressure ventilation negative pressure is created outside of the lungs that sucks the chest to inflate and creates negative pressure within lungs as a result air flows into the lungs
Ventilation Invasive Vs non-invasive The difference is just use of a tube (ie endotracheal tube tracheostomy tube) If you use endotracheal tube it is invasive ventilation If you do not use it is non-invasive ventilation Non-invasive ventilation can be delivered with the use either of
a Negative pressure ventilators (eg Iron lung) b Positive pressure assistance to the airway by mask (using NIPPV) Non Invasive
Positive Pressure Ventilation (NIPPV) can be delivered by several appliances including face masks nasal masks nasal pillows etc
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4
Bi-PAP setting
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5
Nasalface mask
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6
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7
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8
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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Bi-PAP setting
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Nasalface mask
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8
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
triplehelix Mechanical Ventilation
26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
triplehelix Mechanical Ventilation
28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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Nasalface mask
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Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
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7
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8
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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8
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
triplehelix Mechanical Ventilation
25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
triplehelix Mechanical Ventilation
26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Nasotracheal intubation
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9
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Positive pressure machines
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10
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Negative-pressure ventilation
Uuml Negative-pressure ventilators support ventilation by lowering the pressure surrounding the chest wall during inspiration and reversing the pressure to atmospheric level during expiration These devices augment the tidal volume by generating negative extrathoracic pressure
Uuml Several of these devices such as body ventilators and iron lungs are available and either cover the whole body below the neck or apply negative pressure to the thorax and abdomen
Noninvasive positive-pressure ventilation (NPPV)
Uuml NPPV is delivered by a nasal or face mask therefore eliminating the need for intubation or tracheostomy NPPV can be given by a volume ventilator a pressure-controlled ventilator a bilevel positive airway pressure (BIPAP or bilevel ventilator) device or a continuous positive airway pressure (CPAP) device Volume ventilators are often not tolerated because they generate high inspiratory pressures that result in discomfort and mouth leaks
Uuml NPPV delivers a set pressure for each breath (with a bilevel or standard ventilator in the pressure-support mode) Although positive-pressure support is usually well tolerated by patients mouth leaks or other difficulties are sometimes encountered BIPAP ventilators provide continuous high-flow positive airway pressure that cycles between a high positive pressure and a lower positive pressure
Uuml NPPV may be used as an intermittent mode of assistance depending on patients clinical situations Instantaneous and continuous support is given to the patients in acute respiratory distress As the underlying condition improves ventilator-free periods are increased as tolerated and support is discontinued when the patient is deemed stable In most studies the duration of NPPV use in patients with acute on chronic respiratory failure averages 6-18 hours
Uuml The total duration of ventilator use varies with the underlying disease approximately 6 hours is used for acute pulmonary edema and more than 2 days is used for COPD exacerbation
Mechanisms of action Uuml NPPV decreases the work of breathing and thereby improves alveolar ventilation
while simultaneously resting the respiratory musculature The improvement in gas exchange with BIPAP occurs because of an increase in alveolar ventilation
Uuml Externally applied expiratory pressure (eg positive end-expiratory pressure [PEEP]) decreases the work of breathing by partially overcoming the auto-PEEP which is frequently present in these patients The patients generate a less negative inspiratory force to initiate a breathing cycle
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11
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
triplehelix Mechanical Ventilation
25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
triplehelix Mechanical Ventilation
26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Inhalation and exhalation Uuml In spontaneous mode upon detection of inspiration higher pressure is delivered
until the flow rate falls below the threshold level The expiratory pressure with bilevel pressure support is equivalent to the PEEP and the inspiratory pressure is equivalent to the sum of the PEEP and the level of pressure support
Uuml In timed mode biphasic positive airway pressure ventilation alternates between the inspiratory and expiratory pressures at fixed time intervals which allows unrestricted breathing at both pressures This differs from the spontaneous mode of BIPAP which cycles on the basis of the flow rates of the patients own breathing
Uuml Supplemental oxygen can be connected to the device but a higher flow of oxygen therapy is usually required
Uuml NPPV is more effective in a relaxed patient and is not optimal in an anxious uncooperative patient or a patient fighting the ventilator Patients must be adequately prepared with properly fitting masks and the increase of the inspiratory and the expiratory pressures should occur gradually
Uuml Effectiveness should be determined clinically by improved respiratory distress decreased patient discomfort and improved results from arterial blood gas determinations
BIPAP ventilator versus conventional ventilator
The conventional ventilator offers a number of advantages such as the delivery of precise oxygen concentrations and separate inspiratory and expiratory tubing that minimizes carbon dioxide rebreathing Patient disconnection can be readily detected because monitoring and alarm features are more sophisticated in conventional ventilators than in bilevel systems
Nasal mask versus face mask
No randomized trials have compared nasal masks to full face masks in NPPV Most patients in acute respiratory failure are mouth breathers therefore a facial mask may be preferable in some patients These patients should be carefully observed because of the risk of aspiration
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12
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Neurally Adjusted Ventilatory Assist (NAVA)
Uuml NAVA is a new positive pressure mode of mechanical ventilation where the ventilator is controlled directly by the patients own neural control of breathing
Uuml The neural control signal of respiration originates in the respiratory center and are transmitted through the phrenic nerve to excite the diaphragm These signals are monitored by means of electrodes mounted on a nasogastric feeding tube and positioned in the esophagus at the level of the diaphragm As respiration increases and the respiratory center requires the diaphragm for more effort the degree of ventilatory support needed is immediately provided
Uuml This means that the patients respiratory center is in direct control of the mechanical support required on a breath-by-breath basis and any variation in the neural respiratory demand is responded to by the appropriate corresponding change in ventilatory assistance
Choosing amongst ventilator modes
Uuml Assist-control mode minimizes patient effort by providing full mechanical support with every breath This is often the initial mode chosen for adults because it provides the greatest degree of support
Uuml In patients with less severe respiratory failure other modes such as SIMV may be appropriate
Uuml Assist-control mode should not be used in those patients with a potential for respiratory alkalosis in which the patient has an increased respiratory drive Such hyperventilation and hypocapnia (decreased systemic carbon dioxide due to hyperventilation) usually occurs in patients with end-stage liver disease hyperventilatory sepsis and head trauma Respiratory alkalosis will be evident from the initial arterial blood gas obtained and the mode of ventilation can then be changed if so desired
Uuml Positive End Expiratory Pressure may or may not be employed to prevent atelectasis in adult patients It is almost always used for pediatric and neonatal patients due to their increased tendency for atelectasis
Uuml High frequency oscillation is used most frequently in neonates but is also used as an alternative mode in adults with severe ARDS
Uuml Pressure Regulated Volume Control is another option
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13
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
triplehelix Mechanical Ventilation
22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Guidelines for the use of NPPV in patients with acute respiratory failure
Uuml Blood gas findings o Partial pressure of carbon dioxide in arterial gas (PaCO2) greater than 45
mm Hg o pH less than 735 but more than 710 o PaO2 and fraction of inspired oxygen (FIO2) less than 200
Uuml Clinical inclusion criteria o Signs or symptoms of acute respiratory distress o Moderate-to-severe dyspnea increased over usual o Respiratory rate greater than 24 breaths per minute o Accessory muscle use o Abdominal paradox o Gas exchange o PaCO2 greater than 45 mm Hg and pH less than 735 o PaCO2-to-FIO2 ratio less than 200 mm Hg
Uuml Diagnosis o COPD exacerbation o Acute pulmonary edema o Pneumonia
Uuml Contraindications o Respiratory arrest o Inability to use mask because of trauma or surgery o Excessive secretions o Hemodynamic instability or life-threatening arrhythmia o High risk of aspiration o Impaired mental status o Uncooperative or agitated patient o Life-threatening refractory hypoxemia (alveolar-arterial difference in
partial pressure of oxygen [PaO2] lt60 mm Hg with FIO2 of 1) Uuml Factors predictive of success
o Younger age o Lower acuity of illness (ie acute physiology and chronic health evaluation
[APACHE] score) o Patient able to cooperate o Ability to coordinate breathing with ventilator o Moderate hypercapnia (PaCO2 gt45 mm Hg but lt92 mm Hg) o Moderate acidemia (pH gt710 but lt735) o Improvement in gas exchange and heart and respiratory rates within first 2
hours
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14
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Protocol for Initiation of NPPV in patients with acute respiratory failure
Uuml Position the head of the bed at a 45deg angle Uuml Choose the correct size of mask and initiate ventilator at CPAP (expiratory
positive airway pressure or EPAP) of 0 cm water with a pressure support of 10 cm water
Uuml Hold the mask gently on the patients face until the patient is comfortable and in full synchrony with the ventilator
Uuml Apply wound care dressing on the patients nasal bridge and other pressure points Uuml Secure the mask with head straps but avoid a tight fit Uuml Slowly increase CPAP to more than 5 cm water Uuml Increase pressure support (ie inspired positive airway pressure or IPAP 10-20 cm
water) to achieve maximal exhaled tidal volume (10-15 mLkg) Uuml Evaluate that ventilatory support is adequate which is indicated by an
improvement in dyspnea a decreased respiratory rate achievement of desired tidal volume and good comfort for the patient
Uuml Oxygen supplementation is achieved through NPPV machine-to-machine oxygen saturation of greater than 90
Uuml A backup rate may be provided in the event the patient becomes apneic Uuml In patients with hypoxemia increase CPAP in increments of 2-3 cm water until
FiO2 is less than 06 Uuml Set the ventilator alarms and backup apnea parameters Uuml Ask the patient to call for needs and provide reassurance and encouragement Uuml Monitor with oximetry and adjust ventilator settings after obtaining arterial blood
gas results
Benefits of NPPV
In acute respiratory failure NPPV offers a number of potential advantages over invasive PPV These advantages include the avoidance of intubation-related trauma a decreased incidence of nosocomial pneumonia enhanced patient comfort a shorter duration of ventilator use a reduction in hospital stay and ultimately reduced cost
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15
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
triplehelix Mechanical Ventilation
22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Guidelines for use of NPPV in chronic respiratory failure resulting from restrictive processes
Uuml Uuml Blood gas and clinical criteria
o PaCO2 greater than 45 mm Hg o Nocturnal hypoventilation and symptoms (eg hypersomnolence morning
headache) Uuml Appropriate diagnosis
o Neuromuscular diseases o Thoracic deformity o Obesity hypoventilation syndrome o Obstructive sleep apnea unresponsive to CPAP
Uuml Exclusions o Inability to clear secretions o Moderate-to-severe bulbar involvement o Uncooperative patients o Patients who need continuous ventilatory support
Indication of mechanical ventilation
Uuml Mechanical ventilation is indicated when the patients spontaneous ventilation is inadequate to maintain life
Uuml It is also indicated as prophylaxis for imminent collapse of other physiologic functions or ineffective gas exchange in the lungs
Uuml Because mechanical ventilation only serves to provide assistance for breathing and does not cure a disease the patients underlying condition should be correctable and should resolve over time
Uuml In addition other factors must be taken into consideration because mechanical ventilation is not without its complications
Common medical indications for use include
Uuml Acute lung injury (including ARDS trauma) Uuml Apnea with respiratory arrest including cases from intoxication Uuml Chronic obstructive pulmonary disease (COPD) Uuml Acute respiratory acidosis with partial pressure of carbon dioxide (pCO2) gt 50
mmHg and pH lt 725 which may be due to paralysis of the diaphragm due to Guillain-Barreacute syndrome Myasthenia Gravis spinal cord injury or the effect of anaesthetic and muscle relaxant drugs
Uuml Increased work of breathing as evidenced by significant tachypnea retractions and other physical signs of respiratory distress
Uuml Hypoxemia with arterial partial pressure of oxygen (PaO2) with supplemental fraction of inspired oxygen (FiO2) lt 55 mm Hg
Uuml Hypotension including sepsis shock congestive heart failure
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16
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
triplehelix Mechanical Ventilation
30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
triplehelix Mechanical Ventilation
31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Note Mechanical ventilation does not mandate endotracheal intubation nor does intubation require mechanical ventilation For example endotracheal intubation may be life saving in a case of impending upper airway obstruction or high risk for aspiration without need for a ventilator
Types of ventilators
Ventilation can be delivered via
A Hand-controlled ventilation such as Uuml Bag valve mask Uuml Continuous-flow or Anaesthesia (or T-piece) bag
B A mechanical ventilator
Types of mechanical ventilators include
Uuml Transport ventilators These ventilators are small more rugged and can be powered pneumatically or via AC or DC power sources
Uuml ICU ventilators These ventilators are larger and usually run on AC power (though virtually all contain a battery to facilitate intra-facility transport and as a back-up in the event of a power failure) This style of ventilator often provides greater control of a wide variety of ventilation parameters (such as inspiratory rise time) Many ICU ventilators also incorporate graphics to provide visual feedback of each breath
Uuml PAP ventilators These ventilators are specifically designed for non-invasive ventilation This includes ventilators for use at home in order to treat sleep apnea
Modes of mechanical ventilation
A mode of mechanical ventilation refers to the program by which the ventilator interacts with the patient the relationship between the possible types of breaths allowed by the ventilator and the variables that define inspiration
Inspiration is defined by three variables
Uuml trigger
Uuml limit
Uuml cycle
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17
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
triplehelix Mechanical Ventilation
22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
triplehelix Mechanical Ventilation
25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
triplehelix Mechanical Ventilation
26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
triplehelix Mechanical Ventilation
28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Trigger is the change detected by the ventilator that causes inspiration to begin
Limit is the value that cannot be exceeded during inspiration (volume pressure or flow)
Cycle is the value that when reached terminates inspiration
Using these definitions three breath types are possible
Uuml Full ventilator control (mandatory) the ventilator controls the breathing
Uuml Partial ventilator control (assisted) ventilator assist with patientrsquos breathing
Uuml Full patient control (spontaneous) patient controls the breathing completely
The most commonly used modes in adults are volume limited
In controlled mechanical ventilation (CMV) there is no patient triggering rather all breaths are ventilator triggered limited and cycled CMV is used in patients who make no respiratory effort such as those with neuromuscular paralysis
In assistcontrol ventilation (ACV) by contrast the clinician sets a minimum rate and tidal volume The patient can trigger the ventilator at a more rapid rate and will receive the set volume each time
In intermittent mandatory ventilation (IMV) ventilator-limited (ie volume or pressure) breaths are similarly delivered at a set (minimum) rate but the patient can breathe spontaneously by triggering a demand valve between machine-limited breaths
In current ventilators IMV is modified to synchronized IMV (SIMV) in which the ventilator synchronizes machine breaths with patient effort
In the patient who does not trigger the ventilator CMV ACV and SIMV are qualitatively identical In assist control there is a greater potential for respiratory alkalosis and intrinsic PEEP (PEEPi) or autoPEEP
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18
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
What is auto-PEEP (PEEPi)
This is persistent positive alveolar pressure at end of expiration often with associated hyperinflation due to delivery of full machine breaths for all patient efforts The hyperinflation results from insufficient time for the lungs to empty This causes gas trapping and builds up of positive pressure in the lungs
Is the patient gas trapping ndash Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration)
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19
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
In principle machine breaths in the assistcontrol or IMV modes can be defined by a set volume or pressure
If volume control is selected the tidal volume is fixed and airway pressure varies with the resistance and compliance of the patients lungs and chest wall
If pressure control is selected a fixed inspiratory pressure level is maintained for a set inspiratory time or inspirationexpiration (IE) ratio and tidal volume and flow vary with patient effort and mechanics
Two additional common modes of ventilation are continuous positive airway pressure (CPAP) and pressure support ventilation (PSV)
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20
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
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21
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
triplehelix Mechanical Ventilation
26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
triplehelix Mechanical Ventilation
28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
CPAP is spontaneous breathing with no mandatory or assisted breaths a constant level of pressure is applied to the airway throughout the respiratory cycle The mode CPAP is similar to PEEP which is not a mode but is the addition of baseline positive pressure during mechanical ventilation
In PSV breaths are patient triggered pressure limited and flow cycled That is with no machine backup rate the ventilator assists the patients inspiratory effort with a preset pressure Patients determine their own respiratory rate inspiratory time and tidal volume
PSV can be combined with other modes such as SIMV to assist patient efforts between the set machine breaths
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22
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Although the ventilator performs the full work of breathing in CMV and the patient performs the full work of breathing in CPAP two important points should be emphasized
Uuml First the ideal work of breathing for the mechanically ventilated patient remains unclear (ie the ideal amount that prevents muscle atrophy yet permits rest)
Uuml Second patient work of breathing is not necessarily less in ACV or SIMV than in PSV particularly if patientndashventilator synchrony is optimized in PSV
Several newer alternative ventilatory modes have been developed in recent years in an attempt to combine attractive features of pressure and volume ventilation into a single mode that will deliver the minimum necessary ventilator pressure for a tidal volume goal Modes including mandatory minute ventilation (MMV) automatically titrate the amount of ventilator assistance to changing patient mechanics and breathing drive These modes have not yet been shown to improve clinical end points in prospective trials but are increasingly encountered in general practice
Ventilator Settings
The major variables to set for the volume-controlled modes ACV and SIMV are
not respiratory rate
not tidal volume
not flow rate and pattern
not FIO2 (fraction of inspired oxygen) and
not PEEP level
Respiratory rate
Uuml Although a rate of 10ndash20 breathsmin is generally appropriate for most patients with respiratory failure patients with airflow limitation who are at risk fordeveloping PEEPi may benefit from lower rates and patients with a need for highminute ventilation due to metabolic acidosis need higher rates
Uuml In SIMV it is best to initially deliver at least 80 of minute ventilation withmachine breaths
Uuml In ACV setting the rate about 4 breathsmin below the patient rate ensures that thepatient and not the machine is dictating minute ventilation and yet providesadequate backup if the patient becomes apneic
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23
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Tidal Volume
Uuml Although tidal volumes as high as 10 mLkg (and perhaps higher) may beappropriate for patients without lung disease lower volumes are otherwiseindicated
Uuml In acute respiratory distress syndrome (ARDS) the use of a tidal volume of 6 mLkg (ideal body weight) was associated with improved mortality compared with12 mLkg and should be considered the standard of care
Uuml A tidal volume of 6ndash8 mLkg is often used in obstructive lung disease (asthmaCOPD) to avoid high airway pressures and development of PEEPi
Uuml In fact studies of asthma and ARDS suggest that a lung protective ventilatorystrategy termed permissive hypercapnia may lead to improved outcomes This strategy reduces tidal volumes andor rate and allows a respiratory acidosis to a pH as low as 715ndash720
Uuml Generally when increased ventilation is needed it is more effective to adjustminute ventilation by changes in rate rather than tidal volume becauseincreases in tidal volume occasionally have the paradoxical effect of slowing respiratory rate
Uuml Frequent arterial blood sampling to check CO2 tension in the stable patient can be avoided by noting the minute ventilation needed to achieve a given level ofPaCO2
Flow rate and pattern
Uuml The peak flow rate determines the maximal inspiratory flow delivered by theventilator during inspiration Although 60 Lmin is a common initial peak flowsetting higher flows with subsequent higher peak inspiratory pressure are commonly needed for high ventilatory demand or underlying airway obstruction
Uuml Flow is delivered during the inspiratory period via one of three waveforms
not constant (square wave)
not decelerating (ramp wave) or
not sinusoidal
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24
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Uuml Although important differences in clinical outcome have not been demonstratedfor particular flow patterns decelerating flow is most commonly used because itmay achieve better gas exchange (ie lower alveolarndasharterial oxygen gradient and lower dead space) as well as lower peak inspiratory pressure despite higher meanairway pressure
Uuml To improve oxygenation in ARDS mean airway pressure is increased To do thisin volume-controlled modes the peak inspiratory flow rate is decreased In pressure control ventilation inspiratory time is increased (pressure control) to
What is the difference between constant decelerating and sinusoidal flow waveforms
Flow of gas is calculated in liters per minute Flow commences at the beginning of a breath and stops at the end of the breath Gas flows into the lungs in inspiration and out of the lungs in expiration The pattern of expiratory flow is more or less the same for different modes of ventilation as long as the expiratory phase is long enough to prevent gas trapping
The normal flow pattern of gas moving in and out of the lungs is sinusoidal
In volume control ventilation a variety of different wave patterns can be used
In clinical practice constant and decelerating flow patterns are used the latter is preferred In constant decelerating and sinusoidal flow patterns the inspiratory flow rate is equal to the peak flow rate but the mean flow rate is higher in constant flow patterns rather than the other two This suggests that this pattern will cause more shearing injury to the lung parenchyma Therefore a decelerating flow pattern is probably the most effective flow pattern ndash it ensures peak flow early in inspiration while simultaneously minimizing flow during the phase of the inspiratory cycle in which the patient is least likely to need it
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25
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
reach an IE ratio greater than 1 a practice termed inverse ratio ventilation (IRV)
Uuml IRV is uncomfortable for the patient often requiring deep sedation or evenparalysis and carries the risk of gas trapping it should be used selectively and byexperienced clinicians
Fraction of inspired osygen (FIO2)
Uuml The FIO2 is typically started at 10 (100 oxygen)
Uuml Although the literature does not stipulate a cutoff for a safe level of inspired oxygen in humans concerns over oxygen radical-mediated lung injury have led to the common practice of decreasing the FIO2 below 05ndash06 as soon as feasible
Uuml Pulse oximetry may be used to titrate FIO2 with one study suggesting that a threshold of 92 in light-skinned patients and 95 in darker-skinned patients ensures adequate oxygenation
Uuml Measurement of partial pressure of arterial oxygen by arterial blood gas (PaO2gt55ndash60 mm Hg is typically acceptable) is recommended at the start of ventilation to verify accuracy of pulse oximetry for each patient
Positive End Expiratory Pressure (PEEP)
Uuml PEEP is selected to improve oxygenation
Uuml It can also be used to improve work of breathing and inspiratory triggering inpatients with PEEPi
Uuml Potential adverse effects of PEEP include elevation of intracranial pressure andhemodynamic compromisehypotension
Uuml PEEP improves oxygenation mostly by recruiting lung units
Uuml Paradoxically PEEP may sometimes worsen oxygenation by
not decreasing cardiac output and thereby the oxygen saturation of mixed venous blood returning to the lungs
not directing pulmonary blood flow to more consolidated airspaces bycompressing alveolar capillaries in nondiseased more compliant airspacesor
not promoting right-to-left interatrial shunting Because high levels of PEEP reduce cardiac output and impair oxygen delivery to tissues measurementof the effect of PEEP on oxygen delivery termed a best PEEP trial maybe helpful
Uuml In patients with airflow limitation (eg COPD) PEEPi increases the work of breathing by increasing the inspiratory effort needed to initiate ventilator flow Insuch patients with dynamic airflow limitation and expiratory airway collapse
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26
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
triplehelix Mechanical Ventilation
30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
addition of ventilator PEEP up to 85 of the PEEPi level may improve inspiratory triggering and work of breathing However given the potential forworsening of hyperinflation particularly for patients with asthma PEEP for this purpose should be used cautiously A stable inspiratory plateau pressure (Pplat)after addition of ventilator PEEP suggests absence of worsened hyperinflation
PEEP is an adjuvant to the mode of ventilation used to help maintain functional residual capacity (FRC) At the end of expiration the PEEP exerts pressure to oppose passive emptying of the lung and to keep the airway pressure above the atmospheric pressure The presence of PEEP opens up collapsed or unstable alveoli and increases the FRC and surface area for gas exchange thus reducing the size of the shunt For example if a large shunt is found to exist based on the estimation from 100 FiO2 then PEEP can be considered and the FiO2 can be lowered (lt 60) in order to maintain an adequate PaO2 thus reducing the risk of oxygen toxicity
In addition to treating a shunt PEEP may also be useful to decrease the work of breathing In pulmonary physiology compliance is a measure of the stiffness of the lung and chest wall The mathematical formula for compliance (C) = change in volume change in pressure The higher the compliance the more easily the lungs will inflate in response to positive pressure An underinflated lung will have low compliance and PEEP will improve this initially by increasing the FRC since the partially inflated lung takes less energy to inflate further Excessive PEEP can however produce overinflation which will again decrease compliance Therefore it is important to maintain an adequate but not excessive FRC
Indications
PEEP can cause significant haemodynamic consequences through decreasing venous return to the right heart and decreasing right ventricular function As such it should be judiciously used and is indicated for adults in two circumstances
bull If a PaO2 of 60 mmHg cannot be achieved with a FiO2 of 60 bull If the initial shunt estimation is greater than 25
If used PEEP is usually set with the minimal positive pressure to maintain an adequate PaO2 with a safe FiO2 As PEEP increases intrathoracic pressure there can be a resulting decrease in venous return and decrease in cardiac output A PEEP of less than 10 cmH2O is usually safe in adults if intravascular volume depletion is absent Lower levels are used for pediatric patients Older literature recommended routine placement of a Swan-Ganz catheter if the amount of PEEP used is greater than 10 cmH2 for hemodynamic monitoring More recent literature has failed to find outcome benefits with
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27
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
triplehelix Mechanical Ventilation
30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33triplehelix Mechanical Ventilation
33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
routine PA catheterisation when compared to simple central venous pressure monitoring] If cardiac output measurement is required minimally invasive techniques such as oesophageal doppler monitoring or arterial waveform contour monitoring may be sufficient alternatives PEEP should be withdrawn from a patient until adequate PaO2 can be maintained with a FiO2 lt 40 When withdrawing it is decreased through 1-2 cmH2O decrements while monitoring haemoglobin-oxygen saturations Any unacceptable haemoglobin-oxygen saturation should prompt reinstitution of the last PEEP level that maintained good saturation
Positioning
Prone (face down) positioning has been used in patients with ARDS and severe hypoxemia It improves FRC drainage of secretions and ventilation-perfusion matching (efficiency of gas exchange) It may improve oxygenation in gt 50 of patients but no survival benefit has been documented
Sedation
Most intubated patients receive sedation through a continuous infusion or scheduled dosing to help with anxiety or psychological stress Daily interruption of sedation is commonly helpful to the patient for reorientation and appropriate weaning
Prophylaxis
bull To protect against ventilator-associated pneumonia patients bed is often elevated to about 30deg
bull Deep vein thrombosis prophylaxis with heparin or sequential compression device is important in older children and adults
bull A histamine receptor (H2) blocker or proton-pump inhibitor may be used to prevent gastrointestinal bleeding which has been associated with mechanical ventilation
Modification of settings
Uuml In adults when 100 FiO2 is used initially it is easy to calculate the next FiO2 to be used and easy to estimate the shunt fraction The estimated shunt fraction refers to the amount of oxygen not being absorbed into the circulation In normal physiology gas exchange (oxygencarbon dioxide) occurs at the level of the alveoli in the lungs The existence of a shunt refers to any process that hinders this gas exchange leading to wasted oxygen inspired and the flow of un-oxygenated blood back to the left heart (which ultimately supplies the rest of the body with unoxygenated blood)
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28
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33triplehelix Mechanical Ventilation
33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
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36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Uuml When using 100 FiO2 the degree of shunting is estimated by subtracting the measured PaO2 (from an arterial blood gas) from 700 mmHg For each difference of 100 mmHg the shunt is 5 A shunt of more than 25 should prompt a search for the cause of this hypoxemia such as mainstem intubation or pneumothorax and should be treated accordingly If such complications are not present other causes must be sought after and PEEP should be used to treat this intrapulmonary shunt Other such causes of a shunt include
not Alveolar collapse from major atelectasis not Alveolar collection of material other than gas such as pus from pneumonia water
and protein from acute respiratory distress syndrome water from congestive heart failure or blood from haemorrhage
Monitoring the Ventilated Patient Managing a patient on the mechanical ventilator necessitates monitoring respiratory physiological variables These variables track progression and resolution of disease complications of mechanical ventilation patient comfort work of breathing and likelihood of successful patient liberation from the ventilator
RESPIRATORY MECHANICS
Variables indicated on all mechanical ventilators include exhaled tidal volume and airway pressure For the
patient on volume-cycled ventilation for whom breath-to-breath volume is constant airway pressure at any
moment depends on
Uuml the impedance of the respiratory system to air delivery (ie respiratory system compliance and
airflow resistance)
Uuml patient effort and
Uuml patient synchrony with the ventilator
Respiratory system refers to the lung (parenchyma and airways) and its surrounding chest wall (pleura and
thoracoabdominal cage) Although lung and chest wall mechanics may be distinguished with the use of
invasive tools such as the esophageal balloon this is rarely needed Specifically it is important to
remember that pressures caused by changes in compliance of the chest wall such as pneumothorax or even
abdominal distention are transmitted to the lung
Pressure at the airway opening (Pao) will increase with any increase in PEEP PEEPi flow resistance
(eg bronchospasm) or tidal volume and with any decrease in compliance (eg pneumothorax)
Pao increases progressively during inspiration with volume delivery by the ventilator until it reaches its
peak the peak inspiratory pressure (PIP) at the moment the full tidal volume is delivered
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29
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
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38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
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39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
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40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
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41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
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47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
Pao likewise normally decreases to atmospheric (or to PEEP if PEEP is programmed into the ventilator) at
the end of exhalation as the respiratory system empties The downstream pressure in the alveolar
compartment (which includes PEEP and PEEPi) reflecting respiratory system compliance can be
measured at the airway opening in isolation from the additional pressure generated in the airways by
stopping flow (ie making flow zero) and thereby making pressure generated by flow through the airways
zero
In practice the maximum pressure of the alveolar compartment reached at completion of the tidal volume
or plateau pressure (Pplat) can be monitored by programming a 10-s end-inspiratory pause of zero flow
The PIP ndash Pplat difference equals the flow-resistive pressure (ie the pressure generated by flow along the
airways)
These measurements apply best to volume-controlled ventilation in which flow and tidal volume are
programmed By contrast in pressure-controlled ventilation in which a constant pressure is applied to the
airway opening for a prescribed inspiratory time PIP is often equal to Pplat as can be demonstrated by
observing an end-inspiratory period of zero flow on the ventilator flow-time graph
The monitoring variables of static compliance (Cstat) resistance to airflow (R) and intrinsic PEEP (PEEPi)
can be easily derived at the bedside These variables are used to adjust the ventilator follow disease
progression and monitor response to therapy
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30
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
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32
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33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
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34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
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35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
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43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
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45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
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49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
THE MECHANICALLY VENTILATED PATIENT IN ACUTE RESPIRATORY DISTRESS A useful approach to the ventilated patient who develops acute hypoxemia is depicted below By observing PIP and measuring Pplat the problem can often be localized either to the alveolar or the airways compartment and an immediate differential diagnosis can be generated Specifically elevation of PIP with an unchanged Pplat (ie PIPndashPplat increased) may indicate mucus plugging or bronchospasm Elevation of both PIP and Pplat in parallel (ie PIPndashPplat unchanged) may indicate pneumothorax pulmonary edema or pneumonia Alternatively if neither is elevated the possibility of a vascular event altering gas exchange should be considered (ie pulmonary embolism)
Normally at end exhalation (before the next delivered breath) the tidal volume in the alveolar
compartment fully empties and both expiratory airflow and alveolar pressure fall to zero However if
elevated airflow resistance slows alveolar emptying beyond the available expiratory period
particularly in settings of decreased expiratory time (eg elevated respiratory rate) andor decreased
alveolar driving pressure (eg emphysema) positive alveolar pressure or PEEPi may persist at end
exhalation PEEPi is often initially detected by observing the flow graphic on the ventilator for
persistent flow at end-expiration PEEPi is measured by programming a 10-s end-expiratory pause of
zero flow into the ventilator PEEPi may cause hypoventilation hypotension or a false elevation in
pulmonary capillary wedge pressure Strategies to minimize PEEPi include decreasing the respiratory
rate use of bronchodilators or addition of PEEP
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31
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
triplehelix Mechanical Ventilation
32
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33triplehelix Mechanical Ventilation
33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
What is the Peak inspiratory pressure (PIP) and Plateau Pressure (Pplat)
The plateau pressure is the pressure applied (in positive pressure ventilation) to the small airways and alveoli It is believed that control of the plateau pressure is important as excessive stretch of alveoli has been implicated as the cause of ventilator induced lung injury
The peak pressure is the pressure measured by the ventilator in the major airways and it strongly reflects airways resistance For example in acute severe asthma there is a large gradient between the peak pressure (high) and the plateau pressure (normal) In pressure controlled ventilation the pressure limit is (usually) the plateau pressure due to the dispersion of gas in inspiration In volume control the pressure measured (the PAW) by the ventilator is the peak airway pressure which is really the pressure at the level of the major airways To know the real airway pressure the plateau pressure which is applied at alveolar level the volume breath must be made to simulate a pressure breath
An inspiratory hold (05 to 1 second) is applied and the airway pressure from the initial peak drops down to a plateau The hold represents a position of no flow
triplehelix Mechanical Ventilation
32
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33triplehelix Mechanical Ventilation
33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
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37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
triplehelix Mechanical Ventilation
33triplehelix Mechanical Ventilation
33
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
triplehelix Mechanical Ventilation
37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
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42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
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44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
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50
What is permissive hypercapnia
With acute respiratory distress syndrome (ARDS) a tidal volume of 6-8 mLkg is used with a rate of 10-12minute This reduced tidal volume allows for minimal volutrauma but may result in an elevated pCO2 (due to the relative decreased oxygen delivered) is called permissive hypercapnia but this elevation does not need to be corrected
Sighs
Uuml An adult patient breathing spontaneously will usually sigh about 6-8 timeshr to prevent microatelectasis and this has led some to propose that ventilators should deliver 15-2 times the amount of the preset tidal volume 6-8 timeshr to account for the sighs However such high quantity of volume delivery requires very high peak pressure that predisposes to barotrauma
Uuml Currently accounting for sighs is not recommended if the patient is receiving 10-12 mLkg or is on PEEP If the tidal volume used is lower the sigh adjustment can be used as long as the peak and plateau pressures are acceptable
Uuml Sighs are not generally used with ventilation of infants and young children
Connection to ventilators
There are various procedures and mechanical devices that provide protection against airway collapse air leakage and aspiration
Uuml Face mask - In resuscitation and for minor procedures under anaesthesia a face mask is often sufficient to achieve a seal against air leakage Airway patency of the unconscious patient is maintained either by manipulation of the jaw or by the use of nasopharyngeal or oropharyngeal airway These are designed to provide a passage of air to the pharynx through the nose or mouth respectively Poorly fitted masks often cause nasal bridge ulcers a problem for some patients Face masks are also used for non-invasive ventilation in conscious patients A full face mask does not however provide protection against aspiration
Uuml Laryngeal mask airway - The laryngeal mask airway (LMA) causes less pain and coughing than a tracheal tube However unlike tracheal tubes it does not seal
triplehelix Mechanical Ventilation
34
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
triplehelix Mechanical Ventilation
37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
against aspiration making careful individualised evaluation and patient selection mandatory
Uuml Tracheal intubation is often performed for mechanical ventilation of hours to weeks duration A tube is inserted through the nose (nasotracheal intubation) or mouth (orotracheal intubation) and advanced into the trachea In most cases tubes with inflatable cuffs are used for protection against leakage and aspiration Intubation with a cuffed tube is thought to provide the best protection against aspiration Tracheal tubes inevitably cause pain and coughing Therefore unless a patient is unconscious or anaesthetized for other reasons sedative drugs are usually given to provide tolerance of the tube Other disadvantages of tracheal intubation include damage to the mucosal lining of the nasopharynx or oropharynx and subglottic stenosis
Uuml Oesophageal obturator airway - commonly used by emergency medical technicians if they are not authorized to intubate It is a tube which is inserted into the oesophagus past the epiglottis Once it is inserted a bladder at the tip of the airway is inflated to block (obturate) the oesophagus and air or oxygen is delivered through a series of holes in the side of the tube
Uuml Cricothyrotomy - Patients who require emergency airway management in whom tracheal intubation has been unsuccessful may require an airway inserted through a surgical opening in the cricothyroid membrane This is similar to a tracheostomy but a cricothyrotomy is reserved for emergency access
Uuml Tracheostomy - When patients require mechanical ventilation for several weeks a tracheostomy may provide the most suitable access to the trachea A tracheostomy is a surgically created passage into the trachea Tracheostomy tubes are well tolerated and often do not necessitate any use of sedative drugs Tracheostomy tubes may be inserted early during treatment in patients with pre-existing severe respiratory disease or in any patient who is expected to be difficult to wean from mechanical ventilation ie patients who have little muscular reserve
triplehelix Mechanical Ventilation
35
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
triplehelix Mechanical Ventilation
37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Uuml Mouthpiece - Less common interface It does not provide protection against aspiration There are lip seal mouthpieces with flanges to help hold them in place if patient is unable
The other method of classifying mechanical ventilation is based on how to determine when to start giving a breath Similar to the termination classification noted above microprocessor control has resulted in a myriad of hybrid modes that combine features of the traditional classifications Note that most of the timing initiation classifications below can be combined with any of the termination classifications listed above
Uuml Assist Control (AC) In this mode the ventilator provides a mechanical breath with either a pre-set tidal volume or peak pressure every time the patient initiates a breath Traditional assist-control used only a pre-set tidal volume--when a preset peak pressure is used this is also sometimes termed Intermittent Positive Pressure Ventilation or IPPV However the initiation timing is the same--both provide a ventilator breath with every patient effort In most ventilators a back-up minimum breath rate can be set in the event that the patient becomes apnoeic Although a maximum rate is not usually set an alarm can be set if the ventilator cycles too frequently This can alert that the patient is tachypneic or that the ventilator may be auto-cycling (a problem that results when the ventilator interprets fluctuations in the circuit due to the last breath termination as a new breath initiation attempt)
Uuml Synchronized Intermittent Mandatory Ventilation (SIMV) In this mode the ventilator provides a pre-set mechanical breath (pressure or volume limited) every specified number of seconds (determined by dividing the respiratory rate into 60 - thus a respiratory rate of 12 results in a 5 second cycle time) Within that cycle time the ventilator waits for the patient to initiate a breath using either a pressure or flow sensor When the ventilator senses the first patient breathing attempt within the cycle it delivers the preset ventilator breath If the patient fails to initiate a breath the ventilator delivers a mechanical breath at the end of the breath cycle Additional spontaneous breaths after the first one within the breath cycle do not trigger another SIMV breath However SIMV may be combined with pressure support (see below) SIMV is frequently employed as a method of decreasing ventilatory support (weaning) by turning down the rate which requires the patient to take additional breaths beyond the SIMV triggered breath
Uuml Controlled Mechanical Ventilation (CMV) In this mode the ventilator provides a mechanical breath on a preset timing Patient respiratory efforts are ignored This is generally uncomfortable for children and adults who are conscious and is usually only used in an unconscious patient It may also be used in infants who often quickly adapt their breathing pattern to the ventilator timing
Uuml Pressure Support Ventilation (PSV) When a patient attempts to breath spontaneously through an endotracheal tube the narrowed diameter of the airway results in higher resistance to airflow and thus a higher work of breathing PSV was developed as a method to decrease the work of breathing in-between ventilator mandated breaths by providing an elevated pressure triggered by spontaneous breathing that supports ventilation during inspiration Thus for example SIMV might be combined with PSV so that additional breaths beyond the SIMV programmed breaths are supported However while the SIMV mandated breaths have a preset volume or peak pressure the PSV breaths are designed to cut short when the inspiratory flow reaches a percentage of the peak inspiratory flow (eg 10-25) Also the peak pressure set for the PSV breaths is usually a lower pressure than that set for the full ventilator mandated breath PSV can be also be used as an independent mode However since there is generally no back-up rate in PSV appropriate apnoea alarms must be set on the ventilator
Uuml Continuous Positive Airway Pressure (CPAP) A continuous level of elevated pressure is provided through the patient circuit to maintain adequate oxygenation decrease the work of breathing and decrease the work of the heart (such as in left-sided heart failure - CHF) Note that no cycling of ventilator pressures occurs and the patient must initiate all breaths In addition no
triplehelix Mechanical Ventilation
36
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
triplehelix Mechanical Ventilation
37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
additional pressure above the CPAP pressure is provided during those breaths CPAP may be used invasively through an endotracheal tube or tracheostomy or non-invasively with a face mask or nasal prongs
Uuml Positive End Expiratory Pressure (PEEP) is functionally the same as CPAP but refers to the use of an elevated pressure during the expiratory phase of the ventilatory cycle After delivery of the set amount of breath by the ventilator the patient then exhales passively The volume of gas remaining in the lung after a normal expiration is termed the functional residual capacity (FRC) The FRC is primarily determined by the elastic qualities of the lung and the chest wall In many lung diseases the FRC is reduced due to collapse of the unstable alveoli leading to a decreased surface area for gas exchange and intrapulmonary shunting with wasted oxygen inspired Adding PEEP can reduce the work of breathing (at low levels) and help preserve FRC
Complications of Mechanical Ventilation Multiple direct complications of mechanical ventilation have been described
Other indirectly associated complications of mechanical ventilation include critical illness polyneuropathy acalculous cholecystitis and venous thromboembolism Three of these barotrauma ventilator-induced lung injury and altered hemodynamics will be discussed below as both direct and indirect complications have important practical implications
Pulmonary
Barotrauma (eg pneumothorax pneumomediastinum systemic gas embolism etc)
Ventilator-induced lung injury (ie volutrauma atelec-trauma biotrauma)
Oxygen toxicity
Ventilator-associated pneumoni
Tracheal stenosis
Cardiac
Reduced cardiac outputhypotension
Right ventricular ischemia
Propagation of right-to-left interatrial shunt
Gastrointestinal
Ileus
Gastrointestinal hemorrhage
Renal
triplehelix Mechanical Ventilation
37
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Fluid retention
Hyponatremia
Cerebrovascular
Increased intracranial pressure
BAROTRAUMA Uuml Barotrauma is the term for the specific complications of extra-alveolar air such as
pneumothorax or pneumomediastinum thought to occur from alveolar rupture into the adjacent bronchovascular interstitium
Uuml It is less common than in earlier days of positive pressure ventilation likely because of attention to patient ventilator synchrony and high peak airway pressures
Uuml In rare instances the air extravasates into blood vessels with resultant air emboli in the brain heart or skin causing changes in mental status cardiac arrhythmias and livedo reticularis
VENTILATOR-INDUCED LU NG INJURY (VILI)
Uuml Intriguingly although VILI is synergistic with preexistent lung injury positive-pressure
ventilation of even the normal lung can produce pathological hyaline membrane changes
indistinguishable from ARDS Studies in the early 1970s introduced the concept of
barotrauma or pressure-induced lung injurydemonstrating that high inflation pressures injured
the lung Subsequent studies showed that the injurious variable was the transpulmonary pressure
distending the lung rather than peak alveolar pressure (ie alveolar pressure minus pleural
pressure) or more simply end-inspiratory volume This in turn led to the current VILI concept
of volume-induced lung injury or volutrauma with its implication that patients with decreased
chest wall compliance from abdominal distention or other restrictive causes may be relatively
protected from high airway pressures on the ventilator
Uuml A multicenter NIH-sponsored ARDS trial which demonstrated improved mortality using a low (6
mLkg ideal body weight) compared to a high tidal volume strategy (12 mLkg ideal body
weight) supports this volutrauma idea
Uuml The lung in patients with ARDS is heterogeneously affected with the dependent consolidated
lung not participating in gas exchange Only the relatively nondiseased compliant portion of the
lung is vulnerable to overdistention by the delivered tidal volume This has led practitioners to
theorize that pressure ventilation with a uniform pressure ceiling in all lung units is less injurious
to the relatively normal lung than volume ventilation which directs volume along the path of least
resistance primarily to the nondiseased lung
triplehelix Mechanical Ventilation
38
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Uuml Despite these claims however no well-designed randomized controlled trial has shown a
difference in outcome between pressure- and volume-targeted ventilation in patients with ARDS
Uuml Finally perhaps the most interesting recent concept of VILI is that of biotrauma the idea that
VILI may lead to multiple organ dysfunction syndrome by leakage of both stretch-induced
injurious lung cytokines and bacteria into the systemic circulation Lower tidal volumes have been
shown to generate fewer cytokines and are associated with less extrapulmonary organ
dysfunction
Uuml This phenomenon may explain why most patients with ARDS die of extrapulmonary
complications rather than from respiratory failure itself
ALTERED HEMODYNAMICS Uuml Positive-pressure ventilation and PEEP both cause hypotension by reducing
cardiac output with a blood pressure drop that is most dramatic immediately following endotracheal intubation
Uuml PEEP decreases venous return and thus cardiac output primarily by compressing the inferior vena cava By increasing pulmonary vascular resistance and right ventricular afterload high levels of PEEP may also
not decrease right ventricular systolic function particularly for patients with underlying right ventricular dysfunction or right coronary artery disease
not aggravate right ventricular ischemia and
not propagate right-to-left interatrial shunting through a patent foramen ovale PEEP reduces left ventricular afterload and thereby may occasionally lead to improved left ventricular function and cardiac output in patients with dilated cardiomyopathy
Uuml Because of this therapeutic effect of the ventilator both occult left ventricular ischemia and left ventricular systolic dysfunction may occasionally complicate ventilator weaning
When to Withdraw Mechanical Ventilation
Uuml Withdrawal from mechanical ventilationmdashalso known as weaningmdashshould not be
delayed unnecessarily nor should it be done prematurely Patients should have their ventilation considered for withdrawal if they are able to support their own ventilation and oxygenation and this should be assessed continuously
Uuml There are several objective parameters to look for when considering withdrawal but there is no specific criteria that generalizes to all patients
Uuml The best measure of when a patient may be extubated is the Rapid Shallow Breathing Index (RSBI) (Tobin Index)
triplehelix Mechanical Ventilation
39
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Uuml This is calculated by dividing the respiratory rate by the tidal volume in liters(RRTV) A rapid shallow breathing index of less than 100 is considered ideal for extubation Certainly other measures such as patients mental status should be considered
WEANING STRATEGY Uuml First most patients (75) do not need to be weaned from the ventilator a
graduated reduction of support is unnecessary particularly for postoperative patients
Uuml The majority can simply be extubated after their first successful attempt at spontaneous breathing For patients who fail their initial attempts at spontaneous breathing synchronized intermittent mandatory ventilation (SIMV) weaning appears to be inferior and may even prolong the duration of mechanical ventilation
Uuml Next most of the long list of classic weaning parameters (eg maximum inspiratory pressure respiratory rate vital capacity) are poorly predictive of successful liberation The clinical gestalt of even experienced practitioners is often poorly predictive of successful liberation
Uuml Lastly the duration of ventilation can be reduced by using validated clinical parameters such as the rapid-shallow breathing index (RSBI) in a protocol-directed approach
Uuml Most experts agree that the process of liberation should involve an initial achievement of clinical criteria of readiness (eg SaO2 90 with FIO2 05 and PEEP 5 cm H2O no or low-dose vasopressors mental status at least easily arousable some indication of improvement in the initial cause of respiratory failure minute ventilation ideally lt12ndash15 Lmin) followed by some form of initial spontaneous breathing trial (SBT)
Uuml The initial trial may be by a T-piece (breathing without any assistance through the endotracheal tube with a flowing gas source) PSV or CPAP (ie PEEP without inspiratory pressure assistance)
Uuml Initial SBT failure should be followed by reassessment and a more gradual weaning mode
triplehelix Mechanical Ventilation
40
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Criteria for Assessing Tolerance of a Spontaneous Breathing Trial
Uuml Acceptable oxygenation SaO2 90 or PaO2 60 mm Hg on FIO2 05
Uuml Acceptable ventilation increase in PaCO2 10 mm Hg or pH decrease 010
Uuml Acceptable respiratory rate respiratory rate 35 breathsmin or lt 50 increase in rate
Uuml Acceptable hemodynamics heart rate 140 beatsmin or an increase 20 of baseline systolic blood pressure 80 mm Hg and 160 mm Hg or change of lt 20 from baseline
Uuml RSBI (ratetidal volume) lt 100 breathsminL
Uuml No signs of elevated work of breathing such as thoracoabdominal paradox or excessive use of accessory muscles
Uuml No signs of distress such as diaphoresis or agitation
Perhaps the best-validated and easiest index to assess readiness to wean is the RSBI which is the respiratory rate divided by tidal volume in liters When the RSBI is less than 100 as measured for 1 min on T-piece the patient is likely to achieve successful extubation
Uuml For those patients who fail the initial SBT three different methods for subsequent weaning are generally used SIMV PSV and T-piece The latter two have gained predominant support in the literature as effective weaning modes and the former should be abandoned as a primary weaning strategy
Uuml SIMV involves a stepwise reduction of the mandatory respiratory rate (eg a decrease by two breathsmin twice daily) until the patient can tolerate a minimal rate ( 4 breathsmin) with the remainder of breaths unassisted
Uuml PSV typically involves the stepwise reduction in PS level (eg by 2 cm H2O twice daily) until a minimal PS level is reached (eg 5 cm H2O) This low pressure is thought to approximate the work of breathing off the ventilator by overcoming the additional work of breathing imposed by the endotracheal tube PSV was the most effective mode when compared to SIMV and T-piece in a large randomized trial and is used in many centers
Uuml T-piece involves unassisted spontaneous breathing by disconnection of the endotracheal tube from the ventilator circuit and reconnection to a flowing oxygen-enriched source of gas It was the most effective weaning mode in a second large randomized controlled trial Although progressive increases in T-
triplehelix Mechanical Ventilation
41
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
piece trial duration and daily number (eg increasing from a few minutes twice a day to two or more hours several times daily) are commonly used and in fact may be necessary in chronically ventilator-dependent patients once-daily T-piece trials have been shown to be as effective as multiple daily trials for patients without complications
Uuml In sum for the patient who fails an initial attempt at spontaneous breathing several strategies all using SBTs are available for progressive withdrawal from the ventilator daily SBTs (T-piece CPAP or PS) T-piece CPAP or PS trials of increasing duration or gradual decreases in PS level In certain cases an argument can be made for the superiority of one of these methods For example patients with COPD may occasionally have difficulty tolerating PSV Second because patients with congestive heart failure (CHF) may derive enough assistance in cardiac performance from positive pressure ventilation it is arguably most predictive to perform their SBTs on T-piece
THE DIFFICULT-TO-WEAN PATIENT Uuml Failure to wean commonly reflects an imbalance of excessive respiratory
workload and insufficient respiratory muscle strength or endurance with a typical pattern of tachypnea and shallow tidal volumes
Uuml Depending on the clinical scenario occult coronary ischemia occult left ventricular dysfunctionpulmonary edema PEEPi concretions narrowing the endotracheal tube and critical illness myopathypolyneuropathy should be considered
Uuml Most patients particularly those with diaphragmatic dysfunction congestive heart failure emphysema morbid obesity or abdominal distention should be positioned upright during weaning
Uuml Electrolytes affecting muscle function including potassium magnesium and phosphate should be repleted and correction of metabolic acidosis should be considered particularly if excessive minute ventilation persists
Uuml Lastly in patients with COPD exacerbation who fail SBTs extubation straight to noninvasive positive pressure ventilation [ie bilevel positive airway pressure (BiPAP)] can be tried In such patients studies show improved weaning rates less nosocomial pneumonia and decreased mortality
triplehelix Mechanical Ventilation
42
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Causes of Failure to Wean from Mechanical Ventilation
Respiratory muscle weakness
Electrolyte depletion (eg hypokalemia hypomagnesemia) malnutrition
Critical illness polyneuropathy
Inadequate rest
Prolonged paralysis or myopathy (eg neuromuscular agents corticosteroids aminoglycosides)
Decreased ventilatory drive
Hypothyroidism
Excessive administration of sedatives or opiates
Increased ventilatory load
Increased resistance bronchospasm secretions plugged endotracheal tube
Decreased lung compliance pulmonary edema ARDS
Decreased chest wall compliance pleural effusions abdominal distention and obesity
Increased minute ventilation elevated dead space ventilation fever and overfeeding
Intrinsic PEEP
Cardiac problems
Left ventricular dysfunction
Coronary ischemia
triplehelix Mechanical Ventilation
43
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Daily evaluation (Do NOT wean if any one is present)
Uuml Dopamine infusion gt5mcgkgmin
Uuml Systolic BP lt90 mmHg
Uuml HR lt50 or gt 130 bpm
Uuml Temperature gt1004rsquoF
Uuml FiO2 gt50 or PEEP gt8 cm H20
Weaning assessment (Do NOT wean if any one is present)
Uuml RR gt35 breathsmin
Uuml Spontaneous tidal volume lt03 L
Uuml Rapid Shallow Breathing Index gt100
Uuml O2 saturation lt90
Uuml HR lt50 or gt130 bpm or increase from baseline gt20 bpm
Uuml Prominent accessory muscle use
triplehelix Mechanical Ventilation
44
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
A System for Analyzing Ventilator Waveforms
Step 1 -determine the CPAP level
This is the baseline position from which there is a downward deflection on at least beginning of inspiration and to which the airway pressure returns at the end of expiration
Step 2 Is the patient triggering
There will be a negative deflection into the CPAP line just before inspiration
triplehelix Mechanical Ventilation
45
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
triplehelix Mechanical Ventilation
46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Step 3 What is the shape of the pressure wave
If the curve has a flat top then the breath is pressure limited if it has a triangular or sharkrsquos fin top then it is not pressure limited and is a volume breath
Step 4 What is the flow pattern
If it is constant flow (square shaped) this must be volume controlled if decelerating it can be any mode
Step 5 Is the patient gas trapping
Expiratory flow does not return to baseline before inspiration commences (ie gas is trapped in the airways at end-expiration) This indicates auto-PEEP
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46
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
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Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Step 6 The patient is triggering ndash is this a pressure supported or SIMV or VAC breath
This is easy the pressure supported breath looks completely differently than the volume control or synchronized breath the PS breath has a decelerating flow pattern and has a flat topped airway pressure wave The synchronized breath has a triangular shaped pressure wave
triplehelix Mechanical Ventilation
47
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Step 7 The patient is triggering ndash is this pressure support or pressure control
The fundamental difference between pressure support and pressure control is the length of the breath ndash in PC the ventilator determined this (the inspired time) and all breaths have an equal ldquoirdquo time In PS the patient determined the duration of inspiration and this varies from breath to breath
triplehelix Mechanical Ventilation
48
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Step 8 Is the patient synchronizing with the ventilator
Each time the ventilator is triggered a breath should be delivered If the number of triggering episodes is greater than the number of breaths the patient is asynchronous with the ventilator Further if the peak flow rate of the ventilator is inadequate then the inspiratory flow will be scooped inwards and the patient appears to be fighting the ventilator
triplehelix Mechanical Ventilation
49
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
Patient-Ventilator Dysynchrony
Uuml Dysynchrony is a term which describes a patient fighting the ventilator If we
assume that this is not because the patient is undersedated and is rebelling against the endotracheal tube in the majority of cases failure to synchronize is due to inadequate flow delivery from the ventilator
Uuml If the flow of gas is inadequate the patient attempts to suck gas out of the ventilator ndash which is extremely unpleasant This only occurs in volume control modes
Uuml In pressure control flow is unlimited ndash the reason is that flow is related to the pressure gradient between the upper and lower airway ndash a deeper attempted inspiration makes the pressure in the alveoli more negative in relation to the upper airway (this is true also in normal individuals how else would you take a deep breath) and the pressure gradient is larger ndash and the flow greater
Uuml In volume limited ventilation this flexibility (which is physiological) does not exist Of course as with most problems in critical care a number of technological solutions have been developed the first was pressure augmentation
triplehelix Mechanical Ventilation
50
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