Comparison of a respiratory suspension aerosolized by an air-jet and an ultrasonic nebulizer

In the absence of USP standards and performance monographs, this research sought to determine if differences in the aerosolization mechanism (air-jet vs. ultrasonic) affected droplet and insoluble particle deposition of a nebulized model respiratory suspension. Five milliliters of a model suspension containing 0.1% w/v fluorescein (to estimate droplet deposition) and known quantities of 1, 3, and 6 microns latex spheres (representing insoluble drug particles) was aerosolized from an air-jet and an ultrasonic nebulizer. Nebulized output was collected in a modified Andersen impactor. Samples were analyzed spectrophotometrically (490.5 nm) and by a Coulter Counter to estimate droplet and sphere deposition, respectively. The distribution of droplets throughout the modified impactor for both nebulizers suggested that both the air-jet and the ultrasonic nebulizer produced droplets (0.4 to 10 microns in aerodynamic diameter) large enough to incorporate 1, 3, and 6 microns insoluble spheres. However, Coulter Counter analysis of the sphere distribution revealed that while the air-jet nebulized output contained spheres of all sizes, this was not true for the ultrasonic nebulizer. In the ultrasonic nebulizer, 99% of the spheres (irrespective of size) were not aerosolized and were recovered from the nebulizer reservoir at the aerosolization end point. The results highlight the importance of evaluating performance of a respiratory suspension in combination with a specific nebulizer. When conducting in vitro inertial deposition testing of a respiratory suspension, it is inappropriate to assume that deposition trends of droplets will predict the deposition of the insoluble dispersed phase.