The use of a metered dose inhaler (MDI) with an adapter or spacer designed to deliver medication to ventilated patients has been found to produce effects similar to those achieved with small volume nebulizers. For 9 commonly used MDI adapters or spacers, we measured and compared their production of available respirable volume (ARV) of medication. A simulated ventilator circuit was configured to determine ARV as measured by a laser particle-size counter. The adapters or spacers were grouped in three categories: chamber, in-line, and elbow styles. The devices were actuated into the circuit, and ARV was measured 36 separate times with various combinations of medication and medication canister. We found significant differences in ARV between devices, device categories, and medications as compared by ANOVA testing. In this study, chamber-style MDI devices produced greater respirable volume than the in-line- or the elbow-style devices. Of the chamber-style devices, the cone-shaped chamber device provided more respirable volume than did the other two chamber devices. We also found differences among the medications, although we did not anticipate such differences. The possible factors to explain medication differences were not investigated in this study. Because ARV differs according to the design of the individual device used, the choice of device may alter the respirable volume of the medication delivered and, therefore, the effectiveness of the prescribed medication. We conclude that to achieve the desired effects of the medication, practitioners may need to customize the number of MDI actuations according to the device in use.