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

1997; American Physiological Society; Volume: 82; Issue: 3 Linguagem: Inglês

10.1152/jappl.1997.82.3.859

8750-7587

Anne‐Marie Lauzon, G. Kim Prisk, Ann R. Elliott, Sylvia Verbanck, Manuel Paiva, John B. West,

Quantum, superfluid, helium dynamics

Lauzon, Anne-Marie, G. Kim Prisk, Ann R. Elliott, Sylvia Verbanck, Manuel Paiva, and John B. West. Paradoxical helium and sulfur hexafluoride single-breath washouts in short-term vs. sustained microgravity. J. Appl. Physiol. 82(3): 859–865, 1997.—During single-breath washouts in normal gravity (1 G), the phase III slope of sulfur hexafluoride (SF 6 ) 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 SF 6 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 (μG) the SF 6 -to-He slope difference is abolished. We repeated the protocol during short periods (27 s) of μG (parabolic flights). The subjects performed a vital-capacity inspiration and expiration of a gas containing 5% He-1.25% SF 6 -balance O 2 . As in sustained μG, the phase III slopes of He and SF 6 decreased. However, during short-term μG, the SF 6 -to-He slope difference increased from 0.17 ± 0.03%/l in 1 G to 0.29 ± 0.06%/l in μG, respectively. This is contrary to sustained μG, in which the SF 6 -to-He slope difference decreased from 0.25 ± 0.03%/l in 1 G to −0.01 ± 0.06%/l in μG. The increase in phase III slope difference in short-term μG was caused by a larger decrease of He phase III slope compared with that in sustained μG. This suggests that changes in peripheral gas mixing seen in sustained μG are mainly due to alterations in the diffusive-convective inhomogeneity of He that require >27 s of μG to occur. Changes in pulmonary blood volume distribution or cardiogenic mixing may explain the differences between the results found in short-term and sustained μG.