Inspiratory vs expiratory aerosol deposition in excised dog lungs

The difference between inspiratory and expiratory aerosol deposition efficiency (D) in the lung was analyzed. In excised dog lungs, from minimal lung volume, measurements of D for single breaths of a 0.9 μm monodisperse aerosol were made by Tyndallometry for a tidal volume of 0.5 TLC, and breathing periods, T, of 4, 6 and 8 s. At each T, D was measured for two combinations of inspiratory and expiratory times (t_i and t_e): t_e > t_i and t_e < t_i. From measurements in six lungs we found that (1) D increased with increasing T and (2) D was significantly less for t_e > t_i than for t_e < t_i at T = 4 and 6 s. Thus, particles deposited primarily by time-dependent mechanisms and these particles were more likely to deposit during inspiration than expiration. Using both air and a helium-air mixture as the carrier gas, we measured D at (V_T,T) = (0.5TLC, 4 s) for the two combinations of t_i and t_e in an additional five lungs. Contrary to findings with air, we found that D was not different for the two combinations of t_i and t_e in helium-air. In addition, D was greater in helium-air than in air even though particle mobility was similar in the two gases. We propose that the results are best explained by differences in the nature of inspiratory and expiratory flow profiles. However differences in airspace geometry between inspiration and expiration may also play a role.