While there is little doubt as to the efficacy of drug delivery with both the CFC and non-CFC propelled pMDIs and the newer multidose dry powder inhalation devices when they are used properly, inadequate technique, with subopti-mal drug delivery, is very common. Inadequate technique has been reported among anywhere from 12% to 89% of subjects . Among patients who do acquire this skill, up to one-fifth regress to previous bad habits or develop new ones . The only way to discern improper inhalation technique is by asking patients to demonstrate their inhaler use at every opportunity.
Once the basic technique has been mastered, the principal determinants of how much medication is delivered to the airways are particle size, the rate of inspiration, the volume of air inspired and the duration of breath hold at the end of inspiration . The particle size is set by the design of the device, though it may be modified somewhat by changes in temperature and humidity, significantly so at extremes of temperature and humidity. The rate of inspiration can drastically change the amount of medication delivered to the airways, and this can pose a problem, especially for some DPIs of older design (eg Rotahaler, Diskhaler, Glaxo Well-come plc, London, United Kingdom) which require very high inspiratory flows (100 L/s or more) for adequate drug delivery . The requirement for such an inspiratory effort limits the usefulness of such devices in the very young or old, or in patients with severe airflow limitation.