SPECIFIC VENTILATION DISTRIBUTION IN MICROGRAVITY

We studied the contribution of inter- and intraregional inhomogeneitie s of specific ventilation (Delta V/Vo) from the rebreathing inert gas trace in microgravity and on Earth. The rebreathing tests were carried out by four astronauts before, during, and after the 10-day Spacelab D-2 mission. Starting from functional residual capacity, the rebreathi ng maneuver consisted of eight reinspirations from a bag filled with 1 .8-2.2 liters of test gas mixtures containing similar to 5% argon. The rate of argon equilibration in the rebreathing bag, termed RB(eq), wa s quantified by determining the logarithm of the actual minus the equi librated argon concentrations normalized to the inspired minus the equ ilibrated argon concentrations. A compartmental model of the lung (S. Verbanck and M. Paiva. J. Appl. Physiol. 76: 445-454, 1994) was used t o validate the method for determining RB(eq) and to simulate the influ ence of intra- and interregional Delta V/Vo inhomogeneities on the RB( eq) curve. The comparison between the experimental Earth-based and mic rogravity RB(eq) curves and model simulations shows that gravity-indep endent Delta V/Vo inhomogeneity is at least as large as gravity-depend ent Delta V/Vo inhomogeneity.