Ventilator Aerosol Delivery

Delivery of aerosols to patients on ventilators has received much attention since the early 1990s. Some studies of early delivery techniques documented low (2.2%) deposition in the lung, with half retained in the nebulizer, and considerable intersubject variability [80]. While the primary focus of this discussion will be on aerosol delivery to ventilated patients, assessing efficacy of aerosol medications to ventilator patients is very difficult.

The complexity of delivering an aerosol to a patient via a mechanical ventilator is immediately apparent to anyone with even minimal familiarity with particle inhalation. For conventional aerosol therapy, it is generally accepted that good results require patient cooperation. As already noted, achieving high lung deposition is difficult even with the best of devices and very capable patients. With ventilators there is the problem of high particle losses in ventilator hardware before even getting near the endotracheal tube. Delivering a useful quantity to the lung is difficult unless, once again, we return to the concept of "overkill," starting with such a massive quantity that even a small fraction will do the job. A 50% reduction of delivery of aerosol to a ventilated patient was observed when the circuit was humidified, compared to dry [81]. One study attributed the very low delivery to reduced generation, which was a function of circuit humidity (but not temperature) and found (using a mechanical model) that increasing the size of the spacer to allow particles time to evaporate could increase delivery [82].

Evidence suggests that lung deposition of bronchodilators from MDIs via spacers is better than without any spacer and that delivery directly into an endotracheal tube is equivalent to that from a spacer [83].

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