Influence of Moisture Accumulation in Inline Spacer on Delivery of Aerosol Using Metered-Dose Inhaler During Mechanical Ventilation

BACKGROUND: A practitioner questioned whether moisture that collected in the ventilator circuit and spacer affected the delivery of aerosol from a pressurized metered-dose inhaler (pMDI). An in vitro model was used to quantify the impact of accumulated humidity in a pMDI spacer and ventilator over time.

METHODS: A ventilator with an adult heated-wire ventilator circuit and humidifier was set to deliver adult settings. An impactor was placed between the endotracheal tube and the test lung to determine drug mass and mass median aerodynamic diameter of the aerosol delivered. An AeroVent pMDI spacer was placed in the inspiratory limb of the ventilator circuit and left in an open position. Eight actuations of HFA albuterol pMDI (720 μg) was administered at 1, 2, and 3 hours after the heater had reached equilibrium at 37°C, and < 10 min after turning off the heater/humidifier. The spacer was dried and returned to the heated circuit for additional testing. Samples were analyzed via spectrophotometer. One-way analysis of variance was applied (P < .05).

RESULTS: The delivered drug as a percent of emitted dose (mean ± SD) was greater at hour one (23 ± 2.1) and with the dry spacer (21.8 ± 3.3%) than at hours 2 and 3 or with humidifier off (11.4 ± 3.8%, 12.3 ± 0.8%, and 12.7 ± 0.3%, respectively, P = .002). Mass median aerodynamic diameters with each comparison did not vary between conditions. Delivery efficiency was similar for the dry spacer and the spacer in the humidified circuit for one hour. However, once visible condensate occurred, drug delivery efficiency decreased by approximately 50%.

CONCLUSIONS: Aerosol delivery from a pMDI with spacer during mechanical ventilation was greater with a dry spacer and unchanged for the first hour after initiating heated humidification. Turning off the heated humidifier did not increase drug delivered.