ABSTRACT
The delivery device largely influences the deposition of drug via the emitted particle size and velocity of the aerosol, as described above. Consequently it is important that the device emits a plume of particles in the 2-5 µm size band. A number of formulation factors may conspire to prevent this; for example the particles suspended in MDI propellants are generally aggregated, and it is assumed that they are disaggregated efficiently by the shear forces in the actuator. Poor formulation, for example a poor choice of surfactant in the suspension, may cause the particles to be irreversibly aggregated. A similar problem occurs in dry powder devices, in which the fine drug powder is often cohesive and may not readily disperse. In the Turbuhaler® (AstraZeneca) the particles are broken up by a spiral in the mouthpiece producing a high resistance to the patient’s inspiratory flow28. The Turbuhaler®
produces twice as many particles with diameters less than 4.7 micrometers than does the MDI with spacer. The Diskus® (GlaxoWellcome) is also a dry powder inhaler, but this device has a low resistance to inspiratory flow. Thus, it is a less efficient producer of respirable particles under 4.7 micrometers than an MDI plus a metal spacer device, or the Turbuhaler®.
