Factors determining the time course of VO2max decay during bedrest:: implications for VO2max limitation

The aim of this study was to characterize the time course of maximal oxygen consumption (VO2 (max)) changes during bedrests longer than 30 days, on the hypothesis that the decrease in VO2 max 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, VO2 max maximal cardiac output d (Q(max)) and maximal systemic O-2 delivery d (QaO(2max)) were measured. After all periods of HDT, VO2max, Q(max) and QaO(2max) were significantly lower than before. The VO2 max decreased less than Q(max) after the two shortest bedrests, but its per cent decay was about 10% larger than that of Q(max) after 90-day bedrest. The VO2 max decrease after 90-day bedrest was larger than after 42- and 14-day bedrests, where it was similar. The Q(max) and QaO(2max) declines after 90-day bedrest was equal to those after 14- and 42- day bedrest. The average daily rates of the VO2 max, Q(max) and QaO(2max) decay during bedrest were less if the bedrest duration were longer, with the exception of that of VO2max in the longest bedrest. The asymptotic VO2 max decay demonstrates the possibility that humans could keep working effectively even after an extremely long time in microgravity. Two components in the VO2 max 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.