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

We studied the phase relationships of the cardiogenic oscillations in the phase III portion of single-breath washouts (SEW) in normal gravity (1 G) and in sustained microgravity (mu G). The SEW consisted of a vital capacity inspiration of 5% He-1.25% sulfurhexafluoride-balance O-2, 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 (Nz) showed no change from 1 G (211 +/- 9 degrees) to mu G(163 +/- 7 degrees). Ar bolus and He were unaltered between 1 G (173 +/- 15 degrees) and mu G (211 +/- 25 degrees), showing that airway closure in mu G 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 degrees) and mu G(263 +/- 27 degrees), strongly suggesting that, in mu G, areas of high ventilation are associated with high ventilation-perfusion ratio ((V) over dot A/(Q) over dot). This widening of the range of (V) over dot A/(Q) over dot in mu G 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 of (V) over dot A/(Q) over dot in mu G, despite more homogeneous distributions of both ventilation and perfusion.