Inhalation aids used with pMDIs can be divided into holding chambers with a larger volume and spacers which are not as bulky. Both kinds of inhalation aids facilitate the coordination between actuation and inhalation essential for the correct use of a pMDI. Additionally inhalation aids reduce oropharyngeal deposition frequently observed, when aerosol is released directly with high pressure from metered-dose inhalers.
In contrast to spacers holding chambers feature an inspiratory valve which inhibits the direct emission of aerosol from and exhalation into the hollow space. Due to intermediate storage of aerosol in the inhalation aid propellant evaporates resulting in the generation of smaller particles. Thereby bioavailability of the aerosol and intrabronchial deposition of particles is increased.
However filling a holding chamber with more than a single puff from the inhaler or delay of inspiration reduces the amount of breathable particles. Using a spacer still requires some coordination because inhaler actuation must occur during the inhalation process.
Voshaar T. Therapie mit Aerosolen. 2005. Uni-Med: Bremen