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-2 spaceflight that in sustained microgravity (microG) the SF6-to-He slope difference is abolished. We repeated the protocol during short periods (27 s) of microG (parabolic flights). The subjects performed a vital-capacity inspiration and expiration of a gas containing 5% He-1.25% SF6-balance O2. As in sustained microG, the phase III slopes of He and SF6 decreased. However, during short-term microG, the SF6-to-He slope difference increased from 0.17 +/- 0.03%/l in 1 G to 0.29 +/- 0.06%/l in microG, respectively. This is contrary to sustained microG, 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 microG. The increase in phase III slope difference in short-term microG was caused by a larger decrease of He phase III slope compared with that in sustained microG. This suggests that changes in peripheral gas mixing seen in sustained microG are mainly due to alterations in the diffusive-convective inhomogeneity of He that require > 27 s of microG to occur. Changes in pulmonary blood volume distribution or cardiogenic mixing may explain the differences between the results found in short-term and sustained microG.