INHOMOGENEITY OF PULMONARY VENTILATION DURING SUSTAINED MICROGRAVITY AS DETERMINED BY SINGLE-BREATH WASHOUTS

Gravity is known to cause inhomogeneity of ventilation. Nongravitation al factors are also recognized, but their relative contribution is not understood. We therefore studied ventilatory inhomogeneity during sus tained microgravity during the 9-day flight of Spacelab SLS-1. All sev en crew members performed single-breath nitrogen washouts. They inspir ed a vital capacity breath of 100% oxygen with a bolus of argon at the start of inspiration, and the inspiratory and expiratory flow rates w ere controlled at 0.5 l/s. Control measurements in normal gravity (1 G ) were made pre- and postflight in the standing and supine position. C ompared with the standing l-G measurements, there was a marked decreas e in ventilatory inhomogeneity during microgravity, as evidenced by th e significant reductions in cardiogenic oscillations, slope of phase I II, and height of phase IV for nitrogen and argon. However, argon phas e IV volume was not reduced, and considerable ventilatory inhomogeneit y remained. For example, the heights of the cardiogenic oscillations d uring microgravity for nitrogen and argon were 44 and 24%, respectivel y, of their values at 1 G, whereas the slopes of phase III for nitroge n and argon were 78 and 29%, respectively, of those at 1 G. The presen ce of a phase IV in microgravity is strong evidence that airway closur e still occurs in the absence of gravity. The results were qualitative ly similar to those found previously during short periods of 0 G in pa rabolic flight.