Sulfurhexafluoride single breath washouts in short-term versus
sustained microgravity.
Lauzon, Anne-Marie, G. Kim Prisk, Ann R. Elliott, Sylvia Verbanck,
Manuel Paiva, and John B. West.
Department of Medicine, University of California at San Diego, La
Jolla, CA 92093, Department of Pneumology Akademisch Ziekenhuis,
Vrije Universiteit Brussel, and Biomedical Physics Laboratory,
Universit[theta] Libre de Bruxelles, Brussels 1070, Belgium
APStracts 3:0503A, 1996.
When normal humans perform single breath washouts in normal gravity
(1G), the phase III slope of SF6 is steeper than that of He. Two
mechanisms can account for this: 1) the higher diffusivity of He
enhances its homogeneous distribution, 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 SLS-2 spaceflight
that in sustained microgravity (G) the SF6-He slope difference is
abolished. We repeated the protocol during short periods (27s) of G
generated by parabolic flight profiles aboard a NASA research
aircraft. Eight seated subjects inspired a gas containing 5%He,
1.25%SF6, in O2 at a constant flow rate, from residual volume (RV) to
total lung capacity, and then expired to RV at the same flow rate. As
was the case during sustained G, the phase III slopes of He and SF6
decreased in short-term G. However, during short-term G the SF6-He
slope difference increased from 0.17 0.03 %/l in 1G to 0.29 0.06 %/l
in G. This is in sharp contrast to sustained G where the SF6-He slope
difference fell from 0.25 0.03 %/l in 1G to -0.01 0.06%/l in G. The
increase in the phase III slope difference in short-term G was caused
by a larger decrease of He phase III slope compared to sustained G.
These results suggest that changes in peripheral gas mixing seen in
sustained G are mainly due to alterations in the diffusive-convective
inhomogeneity of He, and that these changes take more than 27s of G
to occur. It is possible that the 1.8 Gz period preceding the G
period in the parabolic flights affected the pattern of gas mixing.
We suggest that changes in pulmonary blood volume distribution or
cardiogenic mixing may explain the differences between the results
found in short-term and sustained G, but the exact mechanism remains
unclear.
Received 26 June 1996; accepted in final form 28 October 1996.
APS Manuscript Number A603-6.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 13 November 1996