Factors determining the time course of ${\dot{V}}\hbox{O}_{2\max}$ decay during bedrest: implications for ${\dot{V}}\hbox{O}_{2\max}$ limitation

Abstract

The aim of this study was to characterize the time course of maximal oxygen consumption $({\dot{V}}\hbox{O}_{2\max})$ changes during bedrests longer than 30 days, on the hypothesis that the decrease in ${\dot{V}}\hbox{O}_{2\max}$ 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, ${\dot{V}}\hbox{O}_{2\max}$ maximal cardiac output $(\dot{Q}_{\rm max})$ and maximal systemic O₂ delivery $({\dot{Q}}\hbox{aO}_{2{\rm max}})$ were measured. After all periods of HDT, ${\dot{V}}\hbox{O}_{2\max},\;{\dot{Q}}_{\rm max}$ and ${\dot{Q}}\hbox{aO}_{2{\rm max}}$ were significantly lower than before. The ${\dot{V}}\hbox{O}_{2\max}$ decreased less than ${\dot{Q}}_{\rm max}$ after the two shortest bedrests, but its per cent decay was about 10% larger than that of ${\dot{Q}}_{\rm max}$ after 90-day bedrest. The ${\dot{V}}\hbox{O}_{2\max}$ decrease after 90-day bedrest was larger than after 42- and 14-day bedrests, where it was similar. The ${\dot{Q}}_{\rm max}$ and ${\dot{Q}}\hbox{aO}_{2{\rm max}}$ declines after 90-day bedrest was equal to those after 14- and 42-day bedrest. The average daily rates of the ${\dot{V}}\hbox{O}_{2\max}, \;{\dot{Q}}_{\rm max}$ and ${\dot{Q}}\hbox{aO}_{2{\rm max}}$ decay during bedrest were less if the bedrest duration were longer, with the exception of that of ${\dot{V}}\hbox{O}_{2\max}$ in the longest bedrest. The asymptotic ${\dot{V}}\hbox{O}_{2\max}$ decay demonstrates the possibility that humans could keep working effectively even after an extremely long time in microgravity. Two components in the ${\dot{V}}\hbox{O}_{2\max}$ 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.