VENTILATORY INHOMOGENEITY DETERMINED FROM MULTIPLE-BREATH WASHOUTS DURING SUSTAINED MICROGRAVITY ON SPACELAB SLS-1

We used multiple-breath Na washouts (MBNW) to study the inhomogeneity of ventilation in four normal humans (mean age 42.5 yr) before, during , and after 9 days of exposure to microgravity on Spacelab Life Scienc es-1. Subjects performed 20-breath MBNW at tidal volumes of similar to 700 ml and 12-breath MBNW at tidal volumes of similar to 1,250 ml. Si x indexes of ventilatory inhomogeneity were derived from data from 1) distribution of specific ventilation (SV) from mixed-expired and 2) en d-tidal N-2, 3) change of slope of N-2 washout (semilog plot) with tim e, 4) change of slope of normalized phase III of successive breaths, 5 ) anatomic dead space, and 6) Bohr dead space. Significant ventilatory inhomogeneity was seen in the standing position at normal gravity (1 G). When we compared standing 1 G with microgravity, the distributions of SV became slightly narrower, but the difference was not significan t. Also, there were no significant changes in the change of slope of t he Na washout, change of normalized phase III slopes, or the anatomic and Bohr dead spaces. By contrast, transition from the standing to sup ine position in 1 G resulted in significantly broader distributions of SV (P < 0.05) and significantly greater changes in the changes in slo pe of the N-2 washouts (P < 0.001), indicating more ventilatory inhomo geneity in that posture. Thus these techniques can detect relatively s mall changes in ventilatory inhomogeneity. We conclude that the primar y determinants of ventilatory inhomogeneity during tidal breathing in the upright posture are not gravitational in origin.