Improved Aerosol Delivery with Heliox During Mechanical Ventilation
Abstract & Commentary
Synopsis: At appropriate flow rates and concentrations, heliox in the ventilator circuit may improve aerosol delivery in mechanically ventilated patients with severe airway obstruction.
Source: Goode ML, et al. Am J Respir Crit Care Med. 2001;163:109-114.
In mechanically ventilated patients with airway obstruction, heliox (helium-O2) gas mixtures reduce airway resistance and improve ventilation. However, the influence of heliox on aerosol delivery is unknown. Accordingly, Goode and associates determined the effect of various heliox mixtures on albuterol delivery from metered-dose inhalers (MDIs) and jet nebulizers in an in vitro model of mechanical ventilation. Albuterol delivery from a MDI was increased when the ventilator circuit contained 80% helium and 20% oxygen (heliox 80/20) vs. O2: 46.7 ± 3.3 vs. 30.2 ± 1.3 (SE)% of the nominal dose (P < .001). The difference was mainly due to decreased drug deposition in the spacer chamber, which was a mean of 39.2% and 55.2%, respectively (P < .001). Nebulizer efficiency at a flow of 6 L/min was 5 times lower with heliox 80/20 than O2, and the amount of nebulized drug was correlated with gas density (r = .94, P < .0001). When the nebulizer was operated with O2, greater albuterol delivery was achieved when the ventilator circuit contained heliox rather than O2.
Comment by Dean R. Hess, PhD, RRT
A gas mixture of helium and oxygen has a lower density than air. As such, heliox can decrease the resistive work of breathing. In ambulatory patients with asthma, some investigators have reported that inhalation of heliox decreased dyspnea and pulsus paradoxus and improved pulmonary gas exchange. However, others have reported no benefit of heliox in this patient population. In spontaneously breathing patients with severe chronic obstructive pulmonary disease (COPD), inhalation of heliox has been reported to reduce PaCO2 and auto-PEEP. In mechanically ventilated patients with severe asthma, inhalation of heliox has been reported to decrease airway resistance and improve PaCO2. Beneficial effects of heliox combined with noninvasive ventilation have recently been reported in patients with acute exacerbations of COPD. Although breathing heliox improves aerosol delivery into the lungs of stable asthmatics, this has not been shown to be of benefit when used in the treatment of patients with acute bronchospasm. Compared with air, data reported from my laboratory demonstrated that operating a nebulizer with heliox decreased both the fraction of the nominal dose at the mouthpiece of the nebulizer and the aerosol’s respirable mass. When a nebulizer is used with heliox, the flow used to power the nebulizer must be 12-15 L/min (compared to 6-8 L/min with air or O2) to provide an appropriate nebulizer output.
In-line nebulizers and MDIs are used for bronchodilator therapy in mechanically ventilated patients. Generally, the efficiency of these devices in delivering aerosols to the lower respiratory tract is less in mechanically ventilated patients than in ambulatory patients. Methods for enhancing aerosol delivery in such patients could improve clinical benefit and reduce costs. The influence of heliox on aerosol delivery during adult mechanical ventilation has not been previously reported. The results of this lung model study suggest that a heliox mixture can substantially enhance or reduce the efficiency of bronchodilator delivery during mechanical ventilation, depending on specific circumstances. Delivery of albuterol from a MDI to the lower respiratory tract was enhanced when the ventilator circuit contained heliox mixtures as opposed to air or O2. On the other hand, when a nebulizer was operated with heliox instead of O2, albuterol delivery was reduced. The greatest delivery of aerosol to the lower respiratory tract was achieved by operating the nebulizer with O2 and entraining the aerosol into a ventilator circuit containing heliox.
This was a lung model study. As such, it does not answer the more important question. That is, does the increased aerosol delivery with heliox enhance bronchodilation in mechanically ventilated patients? The answer to this question is unclear at the present time. Although some studies have reported benefit with the use of heliox for acute asthma, others have been equivocal. Although aerosol delivery into the lungs is improved with heliox in stable asthmatics, a benefit is yet to be reported in patients with acute bronchospasm.
With the available evidence, I would not recommend heliox therapy during mechanical ventilation for the sole purpose of improving aerosol delivery. On occasion, however, in my practice I use heliox during mechanical ventilation for patients with acute severe asthma. The data from this study are encouraging in that they suggest that aerosol therapy can be delivered effectively during heliox with mechanical ventilation. At the least, this study suggests that heliox will improve aerosol delivery to the distal endotracheal tube. It is also comforting to know that the nebulizer can be operated with oxygen during heliox therapy with mechanical ventilation, as continuous beta agonist therapy is also frequently part of the care for these patients. Using heliox to power the nebulizer is problematic in that it affects nebulizer performance and increases the amount of heliox that is needed.
Finally, it is important to appreciate that respiratory care equipment is designed to operate with air or oxygen. When a gas of different density, viscosity, and thermal conductivity (ie, heliox) is delivered with this equipment, the results can be unpredictable and potentially dangerous. For example, some ventilators will not function correctly if powered by heliox instead of air or oxygen. As data from my laboratory and this study report, the function of nebulizers is affected if they are powered by heliox. Before using any type of respiratory care equipment with heliox, the safety of this combination must be adequately tested in the laboratory. To the extent that it does just that, this paper provides a service to the clinician.
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