MULTIPLE-BREATH WASHIN OF HELIUM AND SULFUR-HEXAFLUORIDE IN SUSTAINEDMICROGRAVITY

We performed multiple-breath washouts of Nz and simultaneous washins o f He and SF6 with fixed tidal volume (similar to 1,250 mi) and preinsp iratory lung volume (approximately the subject's functional residual c apacity in the standing position) in four normal subjects (mean age 40 yr) standing and supine in normal gravity (1 G) and during exposure t o sustained microgravity mu G). The primary objective was to examine t he influence of diffusive processes on the residual, nongravitational ventilatory inhomogeneity in the lung in mu G. We calculated several i ndexes 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. Whe n we compared the standing position in 1 G with mu G, there were reduc tions in phase III slope in all gases, consistent with a reduction in convection-dependent inhomogeneity in the lung in mu G, although consi derable convective inhomogeneity persisted in mu G The reductions in t he indexes of convection-dependent inhomogeneity were greater for He t han for SF6, suggesting that the distances between remaining nonunifor mly ventilated compartments in mu G were short enough for diffusion of He to be an effective mechanism to reduce gas concentration differenc es between them.