Cardiogenic oscillation phase relationships during single-breath tests performed in microgravity.

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Abstract
Lauzon, Anne-Marie, Ann R. Elliott, Manuel Paiva, John B. West, and G. Kim Prisk. Cardiogenic oscillation phase relationships during single-breath tests performed in microgravity. J. Appl. Physiol. 84(2): 661–668, 1998.—We studied the phase relationships of the cardiogenic oscillations in the phase III portion of single-breath washouts (SBW) in normal gravity (1 G) and in sustained microgravity (μG). The SBW consisted of a vital capacity inspiration of 5% He-1.25% sulfurhexafluoride-balance O2, preceded at residual volume by a 150-ml Ar bolus. Pairs of gas signals, all of which still showed cardiogenic oscillations, were cross-correlated, and their phase difference was expressed as an angle. Phase relationships between inspired gases (e.g., He) and resident gas (N2) showed no change from 1 G (211 ± 9°) to μG (163 ± 7°). Ar bolus and He were unaltered between 1 G (173 ± 15°) and μG (211 ± 25°), showing that airway closure in μG remains in regions of high specific ventilation and suggesting that airway closure results from lung regions reaching low regional volume near residual volume. In contrast, CO2 reversed phase with He between 1 G (332 ± 6°) and μG (263 ± 27°), strongly suggesting that, in μG, areas of high ventilation are associated with high ventilation-perfusion ratio (V˙a/Q˙). This widening of the range ofV˙a/Q˙in μG may explain previous measurements (G. K. Prisk, A. R. Elliott, H. J. B. Guy, J. M. Kosonen, and J. B. West. J. Appl. Physiol. 79: 1290–1298, 1995) of an overall unaltered range ofV˙a/Q˙in μG, despite more homogeneous distributions of both ventilation and perfusion.