Effect of tidal volume on the output and particle size distribution of hypertonic saline from an ultrasonic nebulizer

The purpose of the present study was to measure the output and particle size distribution of hypertonic saline from an ultrasonic nebulizer over the range of tidal volumes (VT) used in an epidemiological survey of asthma in children. A Starling pump was used to generate different VTs from 250 to 500 ml, at different respiratory rates of 12 and 20 breaths.min-1, to cover the normal range of respiratory pattern of children aged 7-14 yrs. The Starling pump was connected to a Timeter Compuneb Ultrasonic Nebulizer Model MP 500 with 24 cm of corrugated aerosol tubing, ID 2.2 cm, and a two-way non-rebreathing valve. Nebulizer output was measured by weighing the nebulizer chamber plus tubing and valve before and after each nebulization. To measure particle size distribution, a Malvern Particle Sizer M 3.0 (laser diffraction) was used, with a special adapter attached between valve and Starling pump. Nebulizer output increased linearly from 1.6 to 2.5 ml.min-1 for VT 250 to 500 ml, with little influence by the respiratory frequency. Furthermore, we found that in the range of VT = 300-500 ml the mass median aerodynamic diameter (MMAD) was 2.61 microns, with 100% of the particles less than 5 microns, whereas at VT = 150-250 ml the MMAD was 8.89 microns, with 90% larger than 5 microns. As output from this ultrasonic nebulizer correlates linearly with tidal volumes, size correction may not be necessary in hypertonic saline challenge tests in children. Above a VT of 300 ml, the MMAD is in the respirable range, with 100% of particles less than 5 microns.(ABSTRACT TRUNCATED AT 250 WORDS)