Physiological Pharmaceutics

Pulmonary drug delivery 235NebulizersMedical nebulizers can be divided into two main groups, pneumatic and electric. Apneumatic generator operates from a pressurized gas source, while an electric generatorderives its power from an electric source. There are two types of pneumatic nebulizers (jetand hydrodynamic) and one electric generator (ultrasonic) presently used for medicalpurposes.The jet nebulizer is a system in which a high-velocity gas flow is directed over a tubethat is immersed in a water reservoir (Figure 10.8). The expansion of the driver gas decreasesthe pressure over the tube, which draws the formulation into the gas stream. The high shearrate in the jet stream then nebulizes it. The hydrodynamic nebulizer uses a system thatprepares a film of water for aerosol formation by flowing it over a hollow sphere. A smallorifice in the sphere expels gas at supersonic velocity. This high-velocity gas ruptures the thinfilm of water and produces a continuous dispersion of fine, liquid particles. A gas cylinderor compressor supplies the gas pressure. The ultrasonic nebulizer consists of a piezo-electriccrystal which produces high frequency sound waves in the liquid in the nebulizing unit. Thesurface waves produce small droplets (Faraday crispations) which are conducted away byan airstream for inhalation. All these devices produce relatively broad droplet sizedistributions in which a large fraction of coarse droplets are present. Consequently most usesome sort of baffle system in the airstream; coarse droplets impact on this and are returnedto the reservoir for re-nebulization, while the smaller particles avoid the baffle and arepassed to the patient.The properties of nebulizers vary widely; while all produce droplets with sizes in therange 1–10 µm, they vary significantly in droplet size distribution and pulmonarydeposition 12 13 . Despite this they have a number of advantages that is causing a renewal ofinterest in their use. Because MDIs and DPIs have a relatively high gas flow rate, they showhigh oropharyngeal impaction. This problem is reduced in nebulizers since the airflow canbe adjusted to suit the patient’s inhalation rate. Continuous nebulization can deliver verylarge quantities of drugs if necessary, from aqueous solutions without major formulationFigure 10.8 An air-driven nebulizer