Factors determining the time course of V'O2 max decay during bedrest: implications for V'O2 max limitation.

The aim of this study was to characterizethe time course of maximal oxygen consumption (V'O2 max) changes during bedrests longer than 30 days,on the hypothesis that the decrease in V'O2 max tends toasymptote. On a total of 26 subjects who participatedin one of three bedrest campaigns without countermeasures,lasting 14, 42 and 90 days, respectively,_VO2 max maximal cardiac output (Qmax) and maximalsystemic O2 delivery (QaO2max) were measured. Afte rall periods of HDT, V'O2 max; Q'max and Q'aO2max weresignificantly lower than before. The 'V O2 max decreasedless than 'Qmax after the two shortest bedrests, but itsper cent decay was about 10% larger than that of _Qmaxafter 90-day bedrest. The 'V O2 max decrease after90-day bedrest was larger than after 42- and 14-daybedrests, where it was similar. The 'Qmax and 'QaO2maxdeclines after 90-day bedrest was equal to those after14- and 42-day bedrest. The average daily rates of the_VO2 max; Q'max and Q'aO2max decay during bedrest wereless if the bedrest duration were longer, with theexception of that of V'O2 max in the longest bedrest. Theasymptotic V'O2 max decay demonstrates the possibilitythat humans could keep working effectively even afteran extremely long time in microgravity. Two componentsin the 'VO2 max decrease were identified, which wepostulate were related to cardiovascular deconditioningand to impairment of peripheral gas exchanges dueto a possible muscle function deterioration.