Aerosol delivery from a metered-dose inhaler during mechanical ventilation: An in vitro model

Successful bronchodilator therapy with a metered-dose inhaler (MDI) in intubated, mechanically ventilated patients requires adequate delivery of aerosol to the lower respiratory tract. We determined the effect of ventilator mode, inspiratory flow pattern, humidity, and spontaneous respiratory effort on albuterol delivery in a model of the trachea and bronchi. The model was ventilated through an endotracheal tube during controlled mechanical ventilation (CMV), assist control (AC), pressure support (PS), and continuous positive airway pressure (CPAP), separately with a dry and humidified ventilator circuit. Delivery of albuterol administered by a MDI and spacer on filters placed at the ends of the bronchi was measured by spectrophotometry (246 nm). Under dry conditions and with a frequency of 10 breaths/min, albuterol delivery with CMV (VT, 800 ml; 30.3 ± 3.4%), AC (VT, 800 ml; 31.9 ± 1.3%), PS 10 cm H₂O (VT, 700 ml; 28.8 ± 4.5%), or PS 20 cm H₂O (VT, 800 ml; 30.9 ± 1.8%) was lower than that observed with simulated spontaneous breaths with CPAP (VT, 800 ml; 39.2 ± 1.4%) (p < 0.01 for all modes). Delivery was greater under dry (28.8 to 39%) than under humidified conditions (15.9 to 20.2%) (p < 0.005 in all modes). Albuterol delivery showed a linear correlation with both inspiratory time and duty cycle (r > 0.91). Lower respiratory tract delivery of aerosol from a MDI varied from 4.9 to 39.2%. We conclude that in addition to other known factors such as dose, type of spacer, and its position the technique of administering MDIs in mechanically ventilated patients markedly influences lower respiratory tract aerosol delivery.