Aerosol Delivery During Mechanical Ventilation
By Dean R. Hess, PhD, RRT
Inhaled drugs are commonly used during mechanical ventilation. The physiologic effects of bronchodilators delivered by nebulizer or pressurized metered dose inhaler (pMDI) are virtually equivalent.1, 2 Choice of device is typically based on clinician preference rather than clear superiority of one approach over the other. Dry powder inhalers, however, cannot be used in the ventilator circuit.
Nebulizers
The pneumatic nebulizer uses a pressurized gas flow to generate an aerosol.3 An important characteristic of nebulizer performance is the respirable dose, determined by its mass output and the size of the droplets. The droplet size should be 2-5 mm for airway deposition and 1-2 mm for parenchymal deposition. Other important characteristics of nebulizer performance include nebulization time, cost, ease of use, and requirements for cleaning and sterilization. Nebulizer output increases when the fill volume is increased—a fill volume of 4-5 mL is recommended. Increased nebulizer flow also increases output and decreases the particle size—a flow of 8 L/min is recommended. An alternative to the pneumatic nebulizer is the ultrasonic nebulizer. Although these are less commonly used than pneumatic nebulizers (related to their cost and complexity), they offer potential advantages over the pneumatic nebulizer during mechanical ventilation because no additional gas flow is delivered into the circuit.
Metered Dose Inhalers
A pMDI consists of a pressurized canister containing a micronized powder or solution of drug that is suspended in a mixture of propellants and other components. This mixture is released through a metering valve and stem that fits into an actuator boot. The volume emitted from the pMDI is 15-20 mL following volatilization of the propellant. The dose is loaded into the metering chamber by shaking. Several features of pMDI performance are not commonly appreciated by clinicians. The pMDI can be used as often as every 15 to 30 s without affecting its performance. A new pMDI, or one that has not been recently used, should be actuated several times before use to properly prime the metering chamber. The pMDI should not be used beyond the labeled number of actuations, as the delivered dose is unpredictable at this point despite the ability to actuate the device.
Aerosol Delivery During Mechanical Ventilation
Many factors affect aerosol delivery during mechanical ventilation (see Table).4-7 Deposition in the circuit and endotracheal tube may reduce the amount of aerosol delivered to the lower respiratory tract. Circuit humidity increases particle size and decreases aerosol deposition in the lower respiratory tract by about 40%. Nebulizer placement 30 cm from the endotracheal tube is more efficient than placement at the Y-piece. Operating the nebulizer only during the inspiratory phase is more efficient than aerosol generation throughout the respiratory cycle. Disadvantages of nebulizers during mechanical ventilation include circuit contamination, decreased ability of the patient to trigger the ventilator, and increased tidal volume and airway pressure.
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Table |
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Factors Affecting Aerosol Delivery During Mechanical Ventilation |
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Nebulizer or MDI |
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• Endotracheal tube size |
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• Humidification of the inspired gas |
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Nebulizer |
|
• Position of nebulizer placement in the circuit |
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• Type of nebulizer and fill volume |
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• Treatment time |
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• Duty cycle (I:E ratio) |
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• Pressure control vs volume control ventilation |
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• Ventilator brand |
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MDI |
|
• Type of actuator |
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• Timing of actuation |
A special actuator is needed to adapt the pMDI into the ventilator circuit. Size, shape, and design of these actuators affect drug delivery to the patient. A pMDI with a chamber results in a 4-6 fold greater delivery of aerosol than pMDI actuation into a connector that lacks a chamber. When using a pMDI during mechanical ventilation, it is important to synchronize actuation with the initiation of inspiratory airflow to optimize drug delivery.8
Delivery of a large tidal volume, use of an end-inspiratory pause, use of a slow inspiratory flow, and use of pressure control vs volume control ventilation affect aerosol delivery by nebulizer but not by pMDI.9-13 Thus, a more consistent dose may be delivered when using the pMDI in mechanically ventilated patients. In vitro modeling has reported a 50% increase in deposition of albuterol from a pMDI during mechanical ventilation with heliox.14 Performance of a nebulizer, however, may be adversely affected by the use of heliox.15
Although the nebulizer is less efficient than the pMDI during mechanical ventilation, the nebulizer delivers a greater dose to the lower respiratory tract.16 Nebulizers and pMDI produce similar therapeutic effects in mechanically ventilated patients. Use of pMDI for routine bronchodilator therapy in ventilator-supported patients is preferred due to the problems associated with use of nebulizers (contamination, triggering difficulty, increased pressure and volume delivery). When a pMDI is used with an inline spacer, the ventilator circuit does not need to be disconnected with each treatment, reducing risk for ventilator-associated pneumonia.
Aerosol therapy can also be administered during noninvasive positive pressure ventilation (NPPV) using nebulizer or pMDI. With NPPV an inline nebulizer17-19 or pMDI20 provides an aerosol bronchodilator dose that is therapeutic. However, aerosol delivery with NPPV has not been studied as extensively as during invasive ventilatory support.
Summary
Aerosols can be delivered effectively during mechanical ventilation using either a nebulizer or pMDI. The pMDI is the most efficient method of aerosol delivery, whereas the greatest absolute amount of drug delivery is with the nebulizer.
Dr. Hess is Assistant Professor of Anesthesia, Harvard Medical School, Assistant Director of Respiratory Care, Massachusetts General Hospital, Cambridge, MA.
References
1. Cates CJ. The Cochrane Database or Systematic Reviews 1999.
2. Dolovich MA, et al. Respir Care. 2000;45:589-593.
3. Hess DR. Respir Care. 2000;45:609-622.
4. Dhand R, Tobin MJ. Eur Respir J. 1996;9:585-595.
5. Dhand R, Tobin MJ. Am J Respir Crit Care Med. 1997; 156:3-10.
6. Hess D. Minerva Anestesiol. 2002;68:321-325.
7. Fink JB, et al. Respir Care. 1999;44:53-69.
8. Diot P, et al. Am J Respir Crit Care Med. 1995; 152:1391-1394.
9. Hess DR, et al. Intensive Care Medicine. 2003;29: 1145-1150.
10. Mouloudi E, et al. Intensive Care Med. 1999;25: 1215-1221.
11. Mouloudi E, et al. Eur Respir J. 1998;12:165-169.
12. Mouloudi E, et al. Eur Respir J. 2000;16:263-268.
13. Mouloudi E, et al. Intensive Care Med. 2001;27:42-46.
14. Good M, et al. Am J Respir Crit Care Med. 2001; 163: 109-114.
15. Hess DR, et al. Chest. 1999;115:184-189.
16. Marik P, et al. Chest. 1999;115:1653-1657.
17. Chatmongkolchart S, et al. Crit Care Med. 2002; 30:2515-1519.
18. Fauroux B, et al. Am J Respir Crit Care Med. 2000; 162:2265-2271.
19. Pollack CV, et al. Ann Emerg Med. 1995;26:552-557.
20. Nava S, et al. Intensive Care Med. 2001;27:1627-1635.
Inhaled drugs are commonly used during mechanical ventilation. The physiologic effects of bronchodilators delivered by nebulizer or pressurized metered dose inhaler (pMDI) are virtually equivalent.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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