Factors influencing the size distribution of aerosols from jet nebulisers

The size distribution of saline and bronchodilator (terbutaline) aerosol droplets generated from four widely used jet nebulisers (Acorn, Upmist, Turret, and Inspiron Mini-neb) has been measured with a Malvern 2200 Laser Particle Sizer. The mass median diameter of aerosol droplets generated by each nebuliser was strongly influenced by the driving flow rate of compressed air. By increasing the flow rate from 4 to 8 1 min-1 mass median diameters were halved (p less than 0.01) and there was an increase in the mass of aerosol within the optimum respirable range (less than 5 micron). To achieve this range the following individual flow rates were required: Turret 4 1 min-1, Acorn and Upmist 6 1 min-1, and Inspiron Mini-neb 8 1 min-1. A significant inverse relation (p less than 0.001) was found between mass median diameter and the geometric standard deviation, indicating that the aerosols were smaller but more heterodisperse at high flow rates. Changes in drug concentration had little effect on aerosol size. In 72% of the nebulisations followed to dryness there was no significant change in mass median diameter during the course of nebulisation and in the remainder it was less than 1.3 micron.