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

Anne-Marie Lauzon 1 , G. Kim Prisk 1 , Ann R. Elliott 1 , Sylvia Verbanck 2 , Manuel Paiva 3 , and John B. West 1 1 Department of Medicine, University of California, San Diego, La Jolla, California 92093-0931; 2 Department of Pneumology Akademisch Ziekenhuis, Vrije Universiteit Brussel, 1090 Brussels; and 3 Biomedical Physics Laboratory, Université Libre de Bruxelles, 1070 Brussels, Belgium Received 26 June 1996; accepted in final form 28 October 1996. 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. phase III slope; inhomogeneity; convection; diffusion 0161-7567/97 $5.00 Copyright © 1997 the American Physiological Society