Factors determining the time course of V̇O2max decay during bedrest: Implications for V̇O2max limitation

The aim of this study was to characterize the time course of maximal oxygen consumption V̇O2max changes during bedrests longer than 30 days, on the hypothesis that the decrease in V̇O2max tends to asymptote. On a total of 26 subjects who participated in one of three bedrest campaigns without countermeasures, lasting 14, 42 and 90 days, respectively, V̇O2max maximal cardiac output Q̇max and maximal systemic O2 delivery Q̇aO2max were measured. After all periods of HDT, V̇O2max, Q̇max and Q̇aO2max were significantly lower than before. The V̇O2max decreased less than Q̇max after the two shortest bedrests, but its per cent decay was about 10% larger than that of Q̇max after 90-day bedrest. The V̇O2max decrease after 90-day bedrest was larger than after 42- and 14-day bedrests, where it was similar. The Q̇max and Q̇aO2max declines after 90-day bedrest was equal to those after 14- and 42-day bedrest. The average daily rates of the V̇O2max Q̇max Q̇aO2max decay during bedrest were less if the bedrest duration were longer, with the exception of that of V̇O2max in the longest bedrest. The asymptotic V̇O2max decay demonstrates the possibility that humans could keep working effectively even after an extremely long time in microgravity. Two components in the V̇O2max decrease were identified, which we postulate were related to cardiovascular deconditioning and to impairment of peripheral gas exchanges due to a possible muscle function deterioration. © Springer-Verlag 2006.