advanced pulmonary mechanics during mechanical ventilation

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Advanced Pulmonary Mechanics during Mechanical Ventilation

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Page 1: Advanced Pulmonary Mechanics during Mechanical Ventilation

Advanced Pulmonary Mechanics during Mechanical Ventilation

Page 2: Advanced Pulmonary Mechanics during Mechanical Ventilation

Points of Discussion

Basics Abnormalities Equation of motion Dynamic Compliance Pressure-volume loop Flow-volume loop Work of breathing Lower and upper

inflection points Hysterexis Intrathoracic

Pressures

Air-leak Air trapping Increased airway

resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension

Page 3: Advanced Pulmonary Mechanics during Mechanical Ventilation

Tube + Spring Model

Elastic Forces

Resistive Forces

Page 4: Advanced Pulmonary Mechanics during Mechanical Ventilation

Static and Dynamic Pressures

time

Pressure

PEEP

PIP

Pplat

Alveolar Distending (recoil) Pressure difference (Pdis)

Flow-Resistive Pressure difference (Pres)

Page 5: Advanced Pulmonary Mechanics during Mechanical Ventilation

Resistive and Elastive Forces

DYNAMIC CHARACTERISTICS:dP = dV / Cdyn

DYNAMIC CHARACTERISTICS:dP = dV / Cdyn

RESISTANCE:dPresistive = R x Flow

RESISTANCE:dPresistive = R x Flow

STATIC COMPLIANCE:dPdistensive = dV / Cst

STATIC COMPLIANCE:dPdistensive = dV / Cst

dP = dPresist. + dP dist.

dP = R x Flow + dV / C st

Page 6: Advanced Pulmonary Mechanics during Mechanical Ventilation

Assessment of static P-V curveSuper-syringe method:

Stepwise inflation from a big syringe with multiply occlusions at each volumes to record recoil pressureTime consumingCumbersome to

performDifficult to standardizePatient must be

paralysed, connected to a special equipment

Great risk of oxygen desaturation

Volume

Pressure

Page 7: Advanced Pulmonary Mechanics during Mechanical Ventilation

Assessment of static P-V curveSlow Flow Single Inflation Method

Slow (5-10 lpm) inspiratory flow with large Vt and ZEEP

The inspiratory curve of the dynamic P-V loop closely approximates the static curve

The flow-resistive pressure component could be subtracted

Easy to perform, fast and relatively comfortable

Servillo: AJRCCM 1997

Lu: AJRCCM 1999 Pressure

Volume

LPIflex

UPIflex

inspir

ationStat

ic c

urve

Page 8: Advanced Pulmonary Mechanics during Mechanical Ventilation

FRC and PV Loop

FRC

VO

LU

ME

TLC

Negative Positive0

DISTENDING PRESSURE

Normal Compliance

FRC

Page 9: Advanced Pulmonary Mechanics during Mechanical Ventilation

Components of Pressure-Volume Loop

Components of Pressure-Volume Loop

Volume Volume (mL)(mL)

Insp

irat

ion

Expirat

ion

PIP

VT

PPawaw (cm H (cm H22O)O)

Page 10: Advanced Pulmonary Mechanics during Mechanical Ventilation

Pressure-Volume Loop(Type of Breath)

Pressure-Volume Loop(Type of Breath)

Controlled Assisted Spontaneous

Vol (m

l)

Paw

(cm H2O)

I: InspirationE: Expiration

I

E

E

E

II

Page 11: Advanced Pulmonary Mechanics during Mechanical Ventilation

PEEP and P-V LoopPEEP and P-V Loop

Volume (mL)

VT

PIPPIPPaw (cm H2O)

PEEPEEP P

Page 12: Advanced Pulmonary Mechanics during Mechanical Ventilation

Inflection PointsInflection Points

Pressure (cm H2O)

Volume (mL)

Upper Inflection Point

Lower Inflection Point

Upper Inflection Point: Represents pressure resulting in regional overdistension

Lower Inflection Point: Represents minimal pressure for adequate alveolar recruitment

Page 13: Advanced Pulmonary Mechanics during Mechanical Ventilation

Decreased ComplianceDecreased Compliance

Volu

me(m

l)

Pressure (cm HPressure (cm H22O)O)

NormalNormalPatientPatient

Page 14: Advanced Pulmonary Mechanics during Mechanical Ventilation

Lung Compliance Changes and the P-V Loop

Lung Compliance Changes and the P-V Loop

Volume (mL)Volume (mL)

PIP levels

Preset VT

PPawaw (cm H (cm H22O)O)

NormalNormal

Volume Targeted Ventilation

DecreasedDecreased

IncreasedIncreased

Page 15: Advanced Pulmonary Mechanics during Mechanical Ventilation

Lung Compliance Changes and the P-V Loop

Lung Compliance Changes and the P-V Loop

Volume (mL)Volume (mL)

Preset PIP

VT

levels

PPawaw (cm H (cm H22O)O)

NormalNormal

Pre

ssu

re T

arg

ete

d

Ven

tilatio

n

IncreasedIncreased

DecreasedDecreased

Page 16: Advanced Pulmonary Mechanics during Mechanical Ventilation

HysteresisHysteresis

Volume (ml)

Pressure (cm Pressure (cm HH22O)O)

Abnormal Hysteresis

Normal Hysteresis

Page 17: Advanced Pulmonary Mechanics during Mechanical Ventilation

Flow-Volume LoopFlow-Volume Loop

Volume (ml)Volume (ml)

Inspiration

Expiration

Flo

w (

L/m

in)

Flo

w (

L/m

in)

PEFR

FRC

PIFR

VT

Page 18: Advanced Pulmonary Mechanics during Mechanical Ventilation

Positive Pressure Ventilation: The Equation of Motion

In a passive subject, airway pressure represents the entire pressure (P) applied across the respiratory system.

The work required to deliver a tidal breath (Wb) = tidal volume (VT) x airway pressure

The pressure (P) associated with the delivery of a tidal breath is defined by the simplified equation of motion of the respiratory system (lungs & chest wall):

P = VT/CR+ VT/Ti x RR + PEEP total

Where CR = compliance of the respiratory system, Ti = inspiratory time and VT/Ti = Flow, RR = resistance of the respiratory system and PEEP total = the alveolar pressure at the end of expiration = external PEEP + auto (or intrinsic) PEEP, if any. Auto PEEP = PEEP total – P extrinsic (PEEP dialed in the ventilator) adds to the inspiratory pressure one needs to

generate a tidal breath.

P elastic P resistive P elastic

Page 19: Advanced Pulmonary Mechanics during Mechanical Ventilation

Work of BreathingWork of Breathing

A: Resistive Work B: Elastic Work A: Resistive Work B: Elastic Work

Pressure (cm H2O)

Volume (ml)

BBBB

AAAA

Page 20: Advanced Pulmonary Mechanics during Mechanical Ventilation

Work of BreathingWOB is a major source of caloric expenditure and

oxygen consumptionAppr. 70% to overcome elastic forces, 30% flow-

resistive workPatient work is a one of the most sensitive indicator of

ventilator dependencyComparison of Ventilator and Patient work is a useful

indicator during weaning processWOB may be altered by changes in compliance,

resistance, patient effort, level of support, PEEP, improper Ti, demand system sensitivity, mode setting

Elevated WOB may contraindicate the weaning process

Page 21: Advanced Pulmonary Mechanics during Mechanical Ventilation

WOB Measurements

PA

B C

D

E

VWOB = ∫0

ti P x Vdt

Elasic work: ABCAResistive work

Inspiratory: ADCAExpiratory: ACEA

Page 22: Advanced Pulmonary Mechanics during Mechanical Ventilation

Work of Breathing Measurements

WOB = ∫0

ti P x Vdt

Paw: Ventilator Work: The physical force required to move gas into the lung, represents the total work of the resp. system (patient + ventilator)

Peso: Patient Work: done by respiratory muscles, represents the pulmonary work of breathing

Paw-Ptr: Imposed Work by the Endotracheal tube

Page 23: Advanced Pulmonary Mechanics during Mechanical Ventilation

P-V Loop and WOB

P

V

P

V

P

VNormal ComplianceNormal Resistance

Normal ComplianceIncreased Resistance

Decreased ComplianceNormal Resistance

Page 24: Advanced Pulmonary Mechanics during Mechanical Ventilation

Work per breath is depicted as a pressure-volume area

Work per breath (Wbreath) = P x tidal volume (VT)

Wmin = wbreath x respiratory rate

Pressure Pressure Pressure

Vol

um

e

Vol

um

e

Vol

um

e

VT

WR = resistive work

WEL = elastic work

The total work of breathing can be partitioned between an elastic and resistive work. By analogy, the pressure needed to inflate a balloon through a straw varies; one needs to overcome the resistance of the straw and the elasticity of the balloon.

Work of Breathing

Page 25: Advanced Pulmonary Mechanics during Mechanical Ventilation

Intrinsic PEEP and Work of Breathing

Vol

um

e

VT

VT

FRCPressure

PEEPi

Dynamic Hyperinflation

PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation

When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP.

Page 26: Advanced Pulmonary Mechanics during Mechanical Ventilation

++

++

Ventilator

₊ ₊

The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive Patient)

Page 27: Advanced Pulmonary Mechanics during Mechanical Ventilation

The Work of Breathing can be Shared Between the Ventilator and the Patient

PAW

PES

patient machine

time

AC mode

The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space.

Paw = Airway pressure, Pes= esophageal pressure

Page 28: Advanced Pulmonary Mechanics during Mechanical Ventilation

Work of breath

Resistive Work

Elastic Work of Lung

Elastic Work of Chest

Paw

Pes

Volume

Work to inflate the chest wall

Inflation Deflation

Page 29: Advanced Pulmonary Mechanics during Mechanical Ventilation

Relationship Between the Set Pressure Support Level and the Patient’s Breathing Effort

Carrey et al. Chest. 1990;97:150.

The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator.

Page 30: Advanced Pulmonary Mechanics during Mechanical Ventilation

Partitioning of the Workload Between the Ventilator and the Patient

How the work of breathing partitions between the patient and the ventilatordepends on:

• Mode of ventilation (e.g., in assist control most of the work is usually done by the ventilator)• Patient effort and synchrony with the mode of ventilation• Specific settings of a given mode (e.g., level of pressure in PS and set rate in SIMV)

Page 31: Advanced Pulmonary Mechanics during Mechanical Ventilation

Respiratory Mechanics in ARF*

Reduced range of volume

excursion: Low

compliance

Flattering at low and high

volumes: Lower and

upper inflection points

*Bigatello: Br J Anaest 1996

Volume

Pressure

NORMAL

ARDS

Page 32: Advanced Pulmonary Mechanics during Mechanical Ventilation

Lung Protective Strategy

1. Set PEEP above the lower Pflex to keep the lung open and avoid alveolar collapse

2. Apply small Vt to minimize stretching forces

3. Set Pplat below the upper Pflex to avoid regional overdistension

Volume

Pressure

Page 33: Advanced Pulmonary Mechanics during Mechanical Ventilation

Abnormalities Air-leak Air trapping Increased airway resistance Inadequate flow support Inadequate sensitivity Atelectasis Inadequate PEEP Airway obstruction Over-distension

Page 34: Advanced Pulmonary Mechanics during Mechanical Ventilation

Air LeakAir Leak

Volume (ml)

Pressure (cm H2O)

Air Leak

Page 35: Advanced Pulmonary Mechanics during Mechanical Ventilation

Air LeakAir LeakInspiration

Expiration

Volume (ml)

Flow Flow (L/min)(L/min)

Air Leak in mL

NormalAbnormal

Page 36: Advanced Pulmonary Mechanics during Mechanical Ventilation

Air LeakVolume (mL)

Time (sec)

Page 37: Advanced Pulmonary Mechanics during Mechanical Ventilation

Air TrappingAir Trapping

Inspiration

Expiration

NormalNormalPatientPatient

Time (sec)

Flo

w (

L/m

in)

Air TrappingAuto-PEEP

}

Page 38: Advanced Pulmonary Mechanics during Mechanical Ventilation

Air TrappingAir TrappingInspiration

Expiration

Volume (ml)Volume (ml)

Flow (L/min)

Does not returnDoes not returnto baselineto baseline

NormalAbnormal

Page 39: Advanced Pulmonary Mechanics during Mechanical Ventilation

Response to Bronchodilator

Response to Bronchodilator

Before

Time (sec)

Flo

w (

L/m

in)

PEFR

After

Long TE

Higher PEFR

Shorter TE

Page 40: Advanced Pulmonary Mechanics during Mechanical Ventilation

Increased Airway Resistance

Increased Airway Resistance

Inspiration

Expiration

VolumeVolume (ml) (ml)

Flow (L/min)

Decreased PEFR

NormalNormalAbnormal

“Scooped out” pattern

Page 41: Advanced Pulmonary Mechanics during Mechanical Ventilation

Increased RawIncreased Raw

Pressure (cm Pressure (cm HH22O)O)

Higher PTA

Norm

al S

lope

Norm

al S

lope

Vol (mL)Vol (mL)

Lower S

lope

Lower S

lope

Page 42: Advanced Pulmonary Mechanics during Mechanical Ventilation

Airway Secretions/Water in the Circuit

Airway Secretions/Water in the Circuit

InspirationInspiration

ExpirationExpiration

Volume (ml)Volume (ml)

Flow Flow (L/min)(L/min)

NormalNormalAbnormalAbnormal

Page 43: Advanced Pulmonary Mechanics during Mechanical Ventilation

F

VV

F

After SuctionBefore Suction

Airway Obstruction

Page 44: Advanced Pulmonary Mechanics during Mechanical Ventilation

Optimising PEEP

V

P

PEEP: 3 cmH2O

V

P

PEEP: 8 cmH2O

Page 45: Advanced Pulmonary Mechanics during Mechanical Ventilation

Inadequate SensitivityInadequate Sensitivity

Volume Volume (mL)(mL)

PPawaw (cm H (cm H22O)O)Increased WOB

Page 46: Advanced Pulmonary Mechanics during Mechanical Ventilation

Replaced FRC

P

V

Lost FRC

V

P

Atelectasis

Page 47: Advanced Pulmonary Mechanics during Mechanical Ventilation

OverdistensionOverdistension

Volu

me (

ml)

Pressure (cm HPressure (cm H22O)O)

With little or no change in VTWith little or no change in VT

Paw risesPaw

rises

NormalAbnormal

Page 48: Advanced Pulmonary Mechanics during Mechanical Ventilation

Overdistension

Overdistension occurs when the volume limit of some components of the lung has been exceeded

Abrupt decrease in compliance at the termination of inspiration

Results in a terminal “Beaking” of the P/V Loop

Volume

Pressure

Page 49: Advanced Pulmonary Mechanics during Mechanical Ventilation

0.8 Pmax Pmax

Overdistension Index

Volume

Pressure

Cdyn

C20

Page 50: Advanced Pulmonary Mechanics during Mechanical Ventilation

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