Droplets Produced by Medical Nebulizers: Discussion (Part 4)
Schoeffel et al found that small amounts of hypertonic (3.6 percent sodium chloride) aerosol caused bronchoconstriction in asthmatic subjects. Lewis and Tattersfield found that a number of asthmatic subjects had bronchoconstriction after inhaling a jet nebulized aerosol of isotonic saline solution, but not to an aerosol generated by a nebulizer heated to 37°C. This was explained in terms of a reduction in airway cooling by the warm aerosol, but it may have been due to a reduction in the concentrating effects that the nebulizer temperature fall imparted on the droplets. The possibility that initially isotonic, or even hypotonic, solutions may produce hypertonic aerosol droplets should therefore be accounted for when delivering therapeutic aerosols to patients with hyperreactive airways.
The necessity for nebulization to be reproducible for bronchial challenge testing, has led to some careful characterization of nebulizers; the nebulizer temperature change may, however, add to the variability in dose and site of delivery of challenge agents.
The size of the aerosol droplets falls with generation time in conjunction with the increase in concentration. The magnitude of this is variable but it is likely to affect the deposition pattern of aerosol within the lungs. Large differences in regional deposition as measured by “penetration index” on tomographic slices have been found in normal subjects inhaling aerosols with mass median aerodynamic diameters of 2.6 and 5.5 jim. The smaller droplet size showed a much higher relative deposition in the small airways and lung parenchyma. The change in droplet size, depending on the time since the start of generation and the humidity of dilution air (for example, from an initial droplet size of 4.2 urn with 100 percent relative humidity dilution air to 2.4 |xm after the equivalent of 4 min generation for the Up-Draft/Aerosol-One system) may be important in therapeutic, diagnostic or experimental applications when reproducibility of aerosol deposition or clinical response is important. It is, of course, very likely that some subsequent adjustment of droplet size will take place in the respiratory tract.