Do all dry powder inhalers show the same pharmaceutical performance?

Important measures of pharmaceutical performance of dry powder inhalers include total emitted dose (TED, the quantity of drug emitted from the device when used at a fixed flow rate, typically 60 l/min or the flow rate achieved at a pressure drop of 4 kPa) and fine particle mass (FPM, the quantity of drug with small particles, typically with an aerodynamic diameter of less than 5 microns emitted from the device at a fixed flow rate). There is a wide range of dry powder inhalers available. The complexity of design decisions means that dry powder inhalers are very diverse, with identical designs unlikely. It is not surprising therefore that evidence from in vitro studies shows considerable variation in pharmaceutical performance of dry powder inhalers. Although neither TED nor FPM can accurately predict in vivo performance, it may be expected that highly variable or inhalation flow sensitive in vitro behaviour will be reflected in variable performance when used by patients.