Gk. Prisk et al., VENTILATORY INHOMOGENEITY DETERMINED FROM MULTIPLE-BREATH WASHOUTS DURING SUSTAINED MICROGRAVITY ON SPACELAB SLS-1, Journal of applied physiology, 78(2), 1995, pp. 597-607
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.