AEROSOL DELIVERY DURING MECHANICAL VENTILATION - A PREDICTIVE INVITRO LUNG MODEL

Objective: to compare aerosol delivery from a standard (Bennett) nebulizer, a sub-micronic (Ultravent) nebulizer, a metered dose inhaler (MDI) plus small holding chamber, and an MDI plus large holding chamber, using a laboratory model simulating a mechanically ventilated patient. Method: saline solution (for the nebulizers) and fenoterol MDI's were radiolabelled and the model set up. After 15 minutes nebulization, or after single puffs from MDI, 1 minute static images were taken of the filter (representing the 'lungs'). Radioactivity in the filter was then counted and expressed as a percentage of the initial radioactivity in the nebulizer, or per puff from MDI. Counts were also made of the endotracheal tube (ETT) and chamber in a subgroup of studies. Results: radioactivity delivered to the filter from MDI plus large chamber (30.3 +/- 7.4%) (mean +/- 1 SD) and MDI plus small chamber (27.7 +/- 5.1%) was significantly greater than from Bennett (4.6 +/- 2.1%) and Ultravent (1.3 +/- 0.4%) nebulizers. In experiments using MDI plus small chamber there was 52.1 +/- 5.8% deposition into the chamber, and 5.9 +/-2.0% deposition onto the ETT. In experiments using Bennett nebulizer there was 2.3 +/-1.0% deposition to the ETT. Comparison of 'lung' inhalation from Bennett nebulizer between experiments with and without humidity was 4.6 +/- 2.1% and 7.8 +/- 0.5% respectively (p = 0.002). Conclusions: 1) Delivery of aerosol to the model lung is greater using MDI plus chamber than using nebulizer. 2) There is no significant difference in activity delivered to the filter between MDI using either size of chamber. 3) Much of the initial aerosol remains in the nebulizer or chamber, and less in the ETT. 5) Warming and humidification of the inspired gas reduces the amount of available aerosol towards the level of lung deposition seen in clinical studies. 4) Deposition to filter of both Technetium-99m sulphur colloid ((Tc-SC)-Tc-99m) and Technetium-99m pertechnetate ((TcO4)-Tc-99m) from a nebulized saline solution was reduced by approximately 50% when fenoterol was added to the solution. As this difference was similar for both compounds, proportional differences of deposition between the devices tested here remain unchanged. 6) Lung models such as the one described in this study receive approximately 3 times as much on the filter as deposits in the lungs in human studies, for all devices. This is likely due in part to impaction of all the aerosol on the absolute filter in the model, but exhalation of part of the inhaled aerosol in human studies. 7) Further efforts must be made to develop models that more closely mimic human physiology in mechanically ventilated patients.