The volume of inspiration is important, and drug delivery may be improved if the breath is initiated at functional residual capacity, that is, at the end of a relaxed expiration as opposed to asking the patient to breathe out completely to residual volume that may cause smaller airways to collapse . A breath hold at end inspiration of at least 5 s is needed, with drug delivery continuing to improve significantly for up to 10 s . Whether a pMDI is placed in the open mouth or held at two finger breadths in front of the mouth is of only minor importance, though it is the author’s experience that the latter technique reduces the likelihood of significant deposition on the tongue or hard palate in many patients.
While available CFC pMDIs are both practical and portable, allowing accurate and consistent dosing in most cases, certain characteristics of the devices require attention to assure adequate drug delivery. If an inhaler is not used for a week or more, the dose of medication may have escaped from the metering chamber and a full dose may not be obtained on initial actuation . This phenomenon is of increasing importance in light of current recommendations to use short-acting beta-agonists on an as-needed basis . Patients should be instructed to prime the device by actuating it once or twice before such intermittent use. Storage position of pMDIs, even when storage is not prolonged, may alter the quantity of drug in the first actuation . This may not be noticeable to the patient, however, because the amount of propellant, the greater part of the plume, remains the same. Finally, as with all inhalation devices, the proportion of medication deposited in the lungs varies with the rate of inspiration; breathing in too quickly after activation of pMDIs substantially reduces the dose delivered to the lungs .