In vitro properties of pressurized metered dose inhalers with and without spacer devices.

Pressurized metered dose inhalers (pMDIs) have been used for the administration of drugs to patients with asthma since the late 1950s. However, it is only during the last few years that attention has been paid to how they work in detail. Studies have shown that the first dose, after the inhaler has been at rest for some hours, can be very variable. For suspension formulations, shaking the inhaler before actuation is important, since omitting shaking will affect the dose uniformity. The addition of a spacer device may increase the variability of the available dose. Factors affecting the available dose are, the electrostatic charge on the walls of the spacer device, the volume of the spacer device and the choice of material for the spacer device. Moreover, the fine particle dose, a measure of the effective deposition of inhaled drug in the airways, has been shown to be greater with a pMDI and a spacer device in comparison with a pMDI alone and is dependent on the choice of impactor inlet.