Use of aerosols to estimate mean air-space size in chronic obstructive pulmonary disease

Using in vivo measures of aerosol recovery (RC) as a function of breath-hold time (T) (Gebhart et al. J. Appl. Physiol. 51:465-476, 1981), we estimated the mean diameter (D) of the pulmonary air spaces in subjects diagnosed with chronic obstructive pulmonary disease (COPD) (n = 8) and in subjects with normal pulmonary function (n = 10). For each subject, RC (aerosol expired/aerosol inspired) decreased exponentially with t. Based on a model of the lung as a system of randomly oriented cylindrical tubes, the half time (t( 1/2 )) (i.e., the breath-hold time to reach 50% of RC with no breath hold) is proportional to a mean diameter (D) of air spaces filled with aerosol. Subjects with normal pulmonary function had a mean t( 1/2 ) = 6.5 ± 0.8 s, corresponding to a mean D = 0.36 ± 0.05 mm. On the other hand, subjects with COPD had a mean t( 1/2 ) = 12.7 ± 3.2 s, corresponding to a mean D = 0.70 ± 0.18 mm [i.e., twice as large (P < 0.01) as normal subjects]. Furthermore, D correlated significantly with diffusing capacity in the patients with COPD (r = -0.95, P < 0.001 for D vs. percent predicted diffusing capacity of CO) but not with any other measure of pulmonary function. In contrast, D varied only slightly in normals and did not correlate with any measure of pulmonary function. We conclude that in vivo measures of RC vs. t, in conjunction with other pulmonary function tests, may be a useful tool for identifying actual changes in pulmonary air-space sizes associated with pulmonary disease.