Helium and sulfur hexafluoride bolus washin in short-term microgravity

We performed single-breath washout (SBW) tests in which He and sulfur hexaf luoride (SF6) were inspired throughout the vital capacity inspirations or w ere inhaled as discrete boluses at different points in the inspiration. Tes ts were performed in normal gravity (1 G) and in up to 27 s of microgravity (mu G) during parabolic flight. The phase III slope of the SEW could be ac curately reconstructed from individual bolus tests when allowance for airwa ys closure was made. Bolus tests showed that most of the SEW phase III slop e results from events during inspiration at lung volumes below closing capa city and near total lung capacity, as does the SF6-He phase III slope diffe rence. Similarly, the difference between 1 G and mu G in phase III slopes f or both gases was entirely accounted for by gravity-dependent events at hig h and low lung volumes. Phase IV height was always larger for SF6 than for He, suggesting at least some airway closure in close proximity to airways t hat remain open at residual volume. These results help explain previous stu dies in mu G, which show large changes in gas mixing in vital capacity mane uvers but only small effects in tidal volume breaths.