External Jet Nebulization and Measured Ventilator Performance–Is It Time to Change Things Up?

Many concerns exist about the delivery of medications by nebulization during mechanical ventilation.^1,2 These concerns include, but are not limited to, optimal technique to maximize pulmonary drug deposition, choice of nebulizer device, and effects of nebulization on mechanical ventilation itself.^3,4 In the project reported in this manuscript, Jayakumaran et al⁵ used an in vitro model to evaluate the effects of in-line jet nebulization on tidal volumes, peak inspiratory pressure, and time to minimum pressure in pressure controlled and volume controlled ventilation modes with several different ventilators. When evaluating tidal volumes, the effects of jet nebulizer delivery on both the exhaled tidal volume measured by the ventilator and those actually delivered to the lungs were determined. These values were compared between 2 jet nebulizer flows and also between ventilators with internal and external flow sensors. The authors found that the use of jet nebulizers at both their high- and low-flow settings significantly affected the tidal volumes measured by the ventilator and those actually delivered to the test lungs when using ventilators with internal flow sensors. The least change in actual delivered tidal volumes occurred during pressure control ventilation with these ventilators. Almost no changes of measured or delivered tidal volumes occurred when using ventilators with external flow sensors. Also, very few changes were noted in peak inspiratory pressures in any group.

These findings make sense based on the location of the flow sensors in each scenario. An internal flow sensor processes a delivered tidal volume upstream of the nebulizer and measures the exhaled tidal volume downstream from nebulizer, which would then include the addition of flow from the nebulizer. This additional flow registers as added exhaled tidal volume to the sensors. Basically, this added flow from the nebulizer comes from “nowhere” in the eyes of the sensor, other than from the patient. For ventilators with an external flow sensor, the sensor is at the Y-piece of the ventilator circuit, just proximal to the airway. In this setting, the added flow from the nebulizer passes the sensor prior to inhalation; this allows the sensor to process the added flow to inhalation; therefore, the sensor can include this flow it its calculations.

The authors conclude that, when delivering nebulized medications from jet nebulizers in line with ventilators that have internal flow sensors, clinicians should “…consider peak inspiratory pressure to estimate tidal volume changes.”⁵ The rationale was based upon their findings that peak inspiratory pressures “…reliably increased in parallel to changes in actual tidal volume.”⁵ These recommendations, while useful, are limited by the in vitro model they used and ignore some basic physiological principles. Changes in lung compliance and airway resistance can drastically alter the relationship between peak inspiratory pressure and delivered tidal volume. Our patients their lung compliance and their airway resistance are not static but rather quite dynamic. Changes in disease progression or simply buildup of secretions within an endotracheal tube can occur quickly and will shift the ability to predict changes in delivered tidal volume by changes in peak inspiratory pressure. Also, the breathing patterns simulated by the authors do not encompass the range of changes in breathing patterns and efforts by patients in vivo, which can also alter peak pressure–to-delivered tidal volume ratios. Finally, regardless of whether peak inspiratory pressures can be used to estimate actual delivered tidal volumes, any significant change in tidal volume due to nebulization is undesirable. Here the authors observed changes in delivered tidal volume of ≤ 30%, which should alarm any clinician. It is certainly a reason to consider the value of external flow sensors since almost no statistically significant change in delivered tidal volume was observed in any condition when using ventilators with these sensors.

An overall takeaway from the work of the authors may be the general limitations of jet nebulizers in this setting. Although they are still within the standard of care and routinely used throughout the world,^6,7 different nebulizer technologies exist that do not affect reported, nor actually delivered, tidal volumes as significantly during mechanical ventilation. For example, vibrating mesh and ultrasonic nebulizers introduce nearly negligible amounts of flow during aerosolization of medications and, therefore, are unlikely to have the effects seen in these experiments.⁸ These devices can be used in the same manner evaluated by the authors, and the findings reported here may suggest doing so. The findings of the authors are undoubtedly helpful to bedside clinicians delivering aerosolized medications via jet nebulizers during mechanical ventilation and may be even more helpful to the respiratory care community as we consider best practices moving forward.