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1 Biomedical Physics Laboratory, Université Libre de Bruxelles, 1070 Brussels, Belgium; 2 Department of Medicine, University of California, San Diego, La Jolla, California 92093-0931; and 3 Department of Pneumology, Akademisch Ziekenhuis, Vrije Universiteit Brussel, 1090 Brussels, Belgium
We performed
single-breath washout (SBW) tests in which He and sulfur hexafluoride
(SF6) were inspired throughout
the vital capacity inspirations or were inhaled as discrete boluses at
different points in the inspiration. Tests were performed in normal
gravity (1 G) and in up to 27 s of microgravity (µG) during parabolic flight. The phase III slope of the SBW could be accurately
reconstructed from individual bolus tests when allowance for airways
closure was made. Bolus tests showed that most of the SBW phase III
slope results from events during inspiration at lung volumes below
closing capacity and near total lung capacity, as does the
SF6-He phase III slope difference.
Similarly, the difference between 1 G and µG in phase III slopes for
both gases was entirely accounted for by gravity-dependent events at
high and low lung volumes. Phase IV height was always larger for
SF6 than for He, suggesting at least some airway closure in close proximity to airways that remain open at residual volume. These results help explain previous studies in
µG, which show large changes in gas mixing in vital capacity maneuvers but only small effects in tidal volume breaths.
single-breath washout; phase III slope; phase IV height; closing volume; closing capacity; gas mixing
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