Critical aspects of the function of inspiratory flow driven inhalers.

Powder inhalers rely on the patient's inspiratory flow for delivery and aerosolization of the medicament. Different inhalers offer different resistances to inspiration through the device. The flow rate attained by a patient depends on the effort expended and on the air flow resistance of the device; flow rate is thus a device-dependent variable. A comparison between powder inhalers should therefore take their air flow resistances into account. An empirical relation between the peak inspiratory flow rate through a device and the flow resistance of the device has been developed using results from healthy subjects and verified to a limited extent using data from asthmatic patients. The flow resistance was measured as the pressure drop at a fixed flow rate. The empirical relation can be used to predict the change in peak inspiratory flow rate resulting from a change in flow resistance. Matching flow rates, representing equal inspiratory efforts, may thus be calculated for different inhalation devices.