Dispersion of 0.5- to 2-mu m aerosol in mu G and hypergravity as a probe of convective inhomogeneity in the lung

We used aerosol boluses to study convective gas mixing in the lung of four healthy subjects on the ground (1 G) and during short periods of microgravi ty (mu G) and hypergravity (similar to 1.6 G). Boluses of 0.5-, 1-, and 2-m u m-diameter particles were inhaled at different points in an inspiration f rom residual volume to 1 liter above functional residual capacity. The volu me of air inhaled after the bolus [the penetration volume (V-p)] ranged fro m 150 to 1,500 mi. Aerosol concentration and flow rate were continuously me asured at the mouth. The dispersion, deposition, and position of the bolus in the expired gas were calculated from these data. For each particle size, both bolus dispersion and deposition increased with V-p and were gravity d ependent, with the largest dispersion and deposition occurring for the larg est G level. Whereas intrinsic particle motions (diffusion, sedimentation, inertia) did not influence dispersion at shallow depths, we found that sedi mentation significantly affected dispersion in the distal part of the lung (V-p >500 ml). For 0.5-mu m-diameter particles for which sedimentation velo city is low, the differences between dispersion in mu G and 1 G likely refl ect the differences in gravitational convective inhomogeneity of ventilatio n between mu G and 1 G.