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

Prisk, G. Kim, Ann R. Elliott, Harold J. B. Guy, SylviaVerbanck, Manuel Paiva, and John B. West. Multiple-breath washin of helium and sulfur hexafluoride in sustained microgravity.J. Appl. Physiol. 84(1): 244-252, 1998.We performed multiple-breath washouts ofN₂ and simultaneous washins of Heand SF₆ with fixed tidal volume(~1,250 ml) and preinspiratory lung volume (approximately thesubject's functional residual capacity in the standing position) infour normal subjects (mean age 40 yr) standing and supine in normalgravity (1 G) and during exposure to sustained microgravity (µG). Theprimary objective was to examine the influence of diffusive processeson the residual, nongravitational ventilatory inhomogeneity in the lungin µG. We calculated several indexes of convective ventilatoryinhomogeneity from each gas species. A normal degree of ventilatoryinhomogeneity was seen in the standing position at 1 G that was largelyunaltered in the supine position. When we compared the standingposition in 1 G with µG, there were reductions in phase III slope inall gases, consistent with a reduction in convection-dependentinhomogeneity in the lung in µG, although considerable convectiveinhomogeneity persisted in µG. The reductions in the indexes ofconvection-dependent inhomogeneity were greater for He than forSF₆, suggesting that the distancesbetween remaining nonuniformly ventilated compartments in µG wereshort enough for diffusion of He to be an effective mechanism to reducegas concentration differences between them.