Aerosol Therapy for Mechanically Ventilated Patients

Done By : Bashaier A. Alyami

Introduction : Inhaled therapy has been routinely employed for over half acentury in ambulatory patients with respiratory disorders. In contrast, many barriers were previously thought to preclude effective aerosol delivery in mechanically ventilated patients. The major barriers were :1- the poor efficiency of aerosol-generating devices in ventilator circuits,2- inadequate understanding of the factors influencing aerosol delivery during mechanical ventilation3- mechanical ventilators that were not designed to optimize aerosol use.

Characteristics of Aerosol particles : 1. Particle size 2. Inertial Impaction 3.Gravitational Sedimentation 4.Diffusion

Devices used : - Nebulizers :

Nebulizer performance varies with diluent volume,gas flow, density, operating pressure, and nebulizermodel. During mechanical ventilation, nebulizers

producingaerosols with MMADs of 1–3 µm are more likelyto achieve deposition in the lower respiratory tract

sincelarger particles impact on the ventilator circuit and

endotracheal tube.

A common jet nebulizer system. An extrinsic gas flow, either from a compressed air source or from the ventilator passes through a Venturi and is accelerated leading to a pressure gradient thatcauses diluent/solubilized drug in the reservoir to be aerosolized and then entrained in another stream of gas (tidal volume) going to the patient.

A common ultrasonic nebulizer system . Ultrasonic waves of 1 MHz are applied to a reservoir of drug and diluent perturbing the liquid, leading to aerosolization and entrainment by the tidal Volume going to the patient.

Cont . - Metered Dose Inhaler :

The MDI canister contains a pressurized mixture of propellants, surfactants, preservatives, and flavoring agents, with ;1% of the total contents being active drug.

The velocity of the liquid spray leaving the MDI is ;15 m/s, falling by 50% within 0.1 s as a cloud develops and moves away from the actuator orifice.

Factors influencing lower respiratory tract deposition of aerosol :

* Ventilator/circuit-related factorsVentilator settings1. Inspiratory flow rate2. Respiratory rate3. Tidal volume4. Flow waveform5. Ventilator cycling-volume vs pressure6. Delivery by manual bag inflations

Cont .

* Circuit determinants1. Characteristics of the delivery devicea. Nebulizer1. Volume of fill2. Frequency selection for ultrasonic

devices3. Specifications of the nebulizer device

used, includingMMAD4. Flow rates for jet nebulization

Cont .

b. MDI1. Timing of the actuation2. Spacer device3. Actuator4. Intra-ETT catheters

Cont .

2. Amount of drug administered3. Humidification of inspired gases4. Where in circuit MDI/nebulizer is

administered5. Length and diameter of ventilator

tubing6. Diameter and length of the ETT7. Use of low-density gas (heliox)

Cont .

* Patient-determined factors :Airway determinants1. Bronchoconstriction2. Secretions3. Mucosal functionPatient's effects on gas flow1. Spontaneous respiratory pattern2. Generation of intrinsic PEEP

Differences during MV : 1- Breath Configurations

During controlled mechanical ventilation (CMV), the pattern and rate of inspiratory gas flow, as well as the rate and pattern of breathing, may differ from that during spontaneous respiration.

2 -The Airway :

The conduit between the aerosol device and the lower respiratory tract in mechanically ventilated patients is narrower than the oropharynx and has abrupt angles (eg, the 90-degree connector often used to connect the ventilator circuit wye to the ETT), which result in points of impaction and turbulence that are not found in the normal airway.

Cont .

While the ETT is narrower than the trachea, its smooth interior surface may create a more laminar-flow path than the structures of the glottis and be less of a barrier to aerosol delivery than the ventilator circuit.

3 -The Environment

Humidity has been shown to relate to an increase in particle size and reduced deposition during CMV, but no data exist to suggest that this reduction is unique to the ventilated patient.

4 -The Assessment :

The common method to assess patient response to bronchodilator administration is through changes in expiratory flow rates.

Most investigators have relied on changes in the inspiratory

airway resistance to quantitate a bronchodilator effect

in mechanically ventilated patients.

Aerosol Generating Devices :

Nebulizers : Alternatively, the air flow generated by a ventilator can be used to power the nebulizer during inspiration (intermittent operation). A separate line provides driving pressure and gas flow from the ventilator to a nebulizer connected in the ventilator circuit.

However, the driving pressure provided by most ventilators to the nebulizer (<15 psi) is much lower than that provided by compressed air or oxygen sources commonly available in the hospital (≥ 50 psi).

Position and Method of Connecting the Aerosol Generator in the Ventilator Circuit :

Placement of a nebulizer at a distance of 30 cm from the endotracheal tube is more efficient than placement between the patient Y and the endotracheal tube because the ventilator tubing acts as a spacer for the aerosol to accumulate between inspirations.

Addition of a spacer between the nebulizer and the endotracheal tube further modestly increases aerosol delivery.

Cont .

Metered Dose Inhaler :

Both in vitro and in vivo studies have found that the combination of an MDI and a chamber device results in a four- to- six-fold greater delivery of aerosol than MDI actuation into a connector attached directly to the endotracheal tube or into an in-line device that lacks a chamber.

Collapsible spacer chamber

Non-collapsible spacer chamber

Aerosol Particle Size :

Deposition in the lower respiratory tract of mechanically ventilated patients is likely to be more efficient with devices that generate aerosols with a MMAD of 1–3 µm.

Cont .

Endotracheal Tube : Smaller the size of ETT, greater the

particle impaction (esp in pediatric ETT)

Cont .

Heating and Humidity of inhaled gas :- Greater aerosol deposition in the

ventilator circuit and ETT with heated and humidified gas

- Both diminishes pulmonary deposition of aerosols ~40% .

Density of the Inhaled Gas : Inhalation of a less dense gas (ie, helium-

oxygen [heliox]), decreases the turbulence associated with high inspiratory flow rates during mechanical ventilation.

Preliminary reports indicate up to 50% increase in deposition of albuterol from an MDI during CMV of a simulated adult patient when breathing heliox compared to that while breathing air or oxygen.

Ventilator settings and Modes :

For efficient aerosol delivery to the lower respiratory tract, the V T of the ventilator-delivered breath must be larger than the volume of the ventilator tubing and endotracheal tube.

V T of ≥ 500 mL in adults are associated with adequate aerosol delivery (see Table 1), 19,25 but the higher pressures required to deliver a larger V T can be detrimental to the lungs.

Actuation of an MDI into a cylindrical spacer synchronized with inspiration resulted in ~ 30% greater efficiency of aerosol delivery compared actuation during expiration.

- Albuterol deposition was up to 23% higher in vitro during simulated spontaneous breaths (continuous positive airway pressure) than with controlled breaths of equivalent V T .

Administration of the Aerosol Therapy :

Facing disadvantages :

Nebulizers : 1. Contamination and VAP

Use of aerosol was one of the independent factor associated with VAP

Need to be cleaned and disinfected to minimize the risk

2. Difficulty triggering In patient on PS mode, a –ve airway

pressure must be generated before the ventilator deliver a breath

A continuous-flow nebulizer between the patient and the sensor in the ventilator makes it more difficult for the patient to generate the –ve pressure

May lead to under-ventilation of the patients

3. Damage to expiratory transducer In some ventilator brand only

4. Variable rate and particle size (depends on the brand)

5. Operational efficiency of nebulizer changes with the pressure of the driving gas and with different fill volumes

6. FiO2 change

7. Increase tidal volume and/ or airway pressure

8. Cost Time consuming (prepare the

drug, disinfection…). Purchasing the aerosol generating

device.

Advantages of MDIs :

Decreased costReliability of dosingEase of administration

Less personnel timeFreedom from contamination

The ventilator circuit need not be disconnected

Reduce VAP

Options of inhaled drug delivery during NIPPV

Remove patient from ventilator and administer drug by nebulizer or MDI

Administer nebulizer therapy inline with NIPPV

Administer MDI therapy inline with NIPPV

A 42-year-old intravenous drug user was transferred to the ward for noninvasive

respiratory support after discharge from the intensive care unit, where she had been treated

for fungal pneumonia and septicemia .

Bronchodilator use :

Based on the finding that aerosol deposition is lower in MV patients than in non-intubated patients higher dose of BD were

recommended

So, what is the precise dosing regimen ?

In general, significant BD effects occur after administration of 4 puffs albuterol with a MDI+spacer

2.5mg of albuterol with a standard nebulizer

Potential side effects were increased if administrated higher doses

Duration of action (e.g. Ventolin)Ambulatory patients: 4-6hrsMechanical ventilated: 2-4hrs vs 4-6hrs

Ventilated patients need more frequent administration of BD (short-acting)E.g. every 3-4 hrs

Thank you