Physiological Pharmaceutics

234 Physiological Pharmaceuticsinhalers such as the Autohaler ® have been designed to overcome this by triggering the valveas the patient breathes in 8 . The Mist-Assist ® inspiratory flow control device (IFCD, BallardMedical, Draper, UT) is a compact device (similar in size to a spacer) through which bothan MDI or medication from a nebulizer can be administered. By use of a floating ball withinthe inspiratory chamber, it provides visual and auditory (clicking sound) feedback tooptimize timing of medication delivery and rate of inspiratory flow. Most important, theinspiratory flow rate (and therefore inspiratory resistance) can be adjusted on the device.This inspiratory flow control enhances laminar flow of particles and gas and increases thelung deposition.Dry powder inhalersThe environmental concerns surrounding the use of chlorofluorocarbons have led to aresurgence of interest in dry powder inhaler devices. Early dry powder inhalers such as theRotahaler ® used individual capsules of micronized drug which were difficult to handle.Modern devices use blister packs (e.g. Diskus ® ) or reservoirs (e.g. Turbuhaler ® ) (Figure10.7). The dry powder inhalers rely on inspiration to withdraw drug from the inhaler to thelung and hence the effect of inhalation flow rate through various devices has beenextensively studied. The major problem to be overcome with these devices is to ensure thatthe finely micronized drug is thoroughly dispersed in the airstream. It has beenrecommended that patients inhale as rapidly as possible from these devices in order toprovide the maximum force to disperse the powder 9 . The quantity of drug and depositionpattern varies enormously depending on the device 10 , for example the Turbuhaler ® producessignificantly greater lung delivery of salbutamol than the Diskus ® . Vidgren and coworkers 11demonstrated by gamma scintigraphy that a typical dry powder formulation of sodiumcromoglycate suffers losses of 44% in the mouth and 40% in the actuator nozzle itself.Figure 10.7 A simplified view of the Turbuhaler, a typical dry powder device