Some functional aspects of air-jet nebulizers

Abstract

Mass median aerodynamic diameters (MMADs, /xm), flow rates (1 air/min) and mass outputs (ml/min and μ1/1 air) were determined over a range of air pressures (psig) for a variety of clinically used nebulizers. Nebulizers were also modified to determine the rate at which fluid was aspirated from the nebulizer cup into the nebulizer jets (ml/min and μ1/1 air). Only a small fraction of this fluid emerges as aerosol that can be inhaled. The ratio of mass output to fluid uptake from the reservoir is termed mechanical efficiency. The stability of the enzyme lactate dehydrogenase (LDH) to nebulization at two different starting volumes was examined in the 3-jet Collison nebulizer. The results are discussed with respect to the measured operating characteristics. In all the nebulizers tested, MMADs were inversely related (e.g. 6.4 to 3.8 μm at 10 and 40 psig, respectively, for the ‘Cirrus’ nebulizer), whereas mass outputs (e.g. 0.11 to 0.35 ml/min at 10 and 40 psig for the ‘Misty’) and flow rates (e.g. 4.5 to 10.4 l/min at 10 and 40 psig for the Misty) were directly related, to the driving air pressure. MMADs and mass output are measures of how well nebulizers atomize liquid into droplets of respirable sizes. Aspiration rates varied substantially between the nebulizers and ranged from 9 ml/min (5 psig) to 28 ml/min (40 psig) for the DeVilbiss No. 45 whereas the Collison 3-jet nebulizer aspirated liquid at 118 ml/min (5 psig) to 333 ml/min (40 psig). All nebulizers exhibited an exponential decline in the aspiration rate of liquid expressed in terms of the throughput of air (ml/1 air). Rate constants ranged from 0.6 to 21.6 (× 10 −2 min/1 air) in the DeVilbiss No. 45 and Airlife Misty, respectively. These values give an indication of how the aspiration of fluid is affected by the air flow. Nebulizer mechanical efficiency was never > 2% (vents closed); this parameter is inversely related to the probable number of times that a drug will be aerosolized within the nebulizer. LDH was inactivated by nebulization. The degree of inactivation was greater at a 10 ml starting volume than at a 40 ml starting volume. The results indicate that monitoring the functional characteristics of nebulizers may be helpful in the assessment of drug stability.