Multiple-breath washin of helium and sulfur hexafluoride in sustained microgravity

We performed multiple-breath washouts of Nz and simultaneous washins of He and SF6 with fixed tidal volume (similar to 1,250 mi) and preinspiratory lung volume (approximately the subject's functional residual capacity in the standing position) in four normal subjects (mean age 40 yr) standing and supine in normal gravity (1 G) and during exposure to sustained microgravity mu G). The primary objective was to examine the influence of diffusive processes on the residual, nongravitational ventilatory inhomogeneity in the lung in mu G. We calculated several indexes of convective ventilatory inhomogeneity from each gas species. A normal degree of ventilatory inhomogeneity was seen in the standing position at 1 G that was largely unaltered in the supine position. When we compared the standing position in 1 G with mu G, there were reductions in phase III slope in all gases, consistent with a reduction in convection-dependent inhomogeneity in the lung in mu G, although considerable convective inhomogeneity persisted in mu G The reductions in the indexes of convection-dependent inhomogeneity were greater for He than for SF6, suggesting that the distances between remaining nonuniformly ventilated compartments in mu G were short enough for diffusion of He to be an effective mechanism to reduce gas concentration differences between them.