Paradoxical helium and sulfur hexafluoride single-breath washouts in short-term vs. sustained microgravity

During single-breath washouts in normal gravity (1 G), the phase III slope of sulfur hexafluoride (SF6) is steeper than that of helium (He). Two mechanisms can account for this: 1) the higher diffusivity of He enhances its homogeneous distribution; and 2) the lower diffusivity of SF6 results in a more peripheral location of the diffusion front, where airway asymmetry is larger. These mechanisms were thought to be gravity independent. However, we showed during the Spacelab Life Sciences-a spaceflight that in sustained microgravity (mu G) the SF6-to-He slope difference is abolished. We repeated the protocol during short periods (27 s) of mu G (parabolic flights). The subjects performed a vital-capacity inspiration and expiration of a gas containing 5% He-1.25% SF6-balance O-2. As in sustained mu G, the phase III slopes of He and SF6 decreased. However, during short-term mu G, the SF6-to-He slope difference increased from 0.17 +/- 0.03%/l in 1 G to 0.29 +/- 0.06%/l in mu G, respectively. This is contrary to sustained mu G, in which the SF6-to-He slope difference decreased from 0.25 +/- 0.03%/l in 1 G to -0.01 +/- 0.06%/l in mu G. The increase in phase III slope difference in short-term mu G was caused by a larger decrease of He phase III slope compared with that in sustained mu G. This suggests that changes in peripheral gas mixing seen in sustained mu G are mainly due to alterations in the diffusive-convective inhomogeneity of Be that require >27 s of mu G to occur. Changes in pulmonary blood volume distribution or cardiogenic mixing may explain the differences between the results found in short-term and sustained mu G.