Decreases in maximal oxygen uptake following long-duration spaceflight: Role of convective and diffusive O2 transport mechanisms

We have previously predicted that the decrease in maximal oxygen uptake (Vo(2max)) that accompanies time in microgravity reflects decrements in both convective and diffusive O-2 transport to the mitochondria of the contracting myocytes. The aim of this investigation was therefore to quantify the relative changes in convective O-2 transport (Qo(2)) and O-2 diffusing capacity (Do(2)) following long-duration spaceflight. In nine astronauts, resting hemoglobin concentration ([Hb]), Vo(2max), maximal cardiac output (Q(Tmax)), and differences in arterial and venous O-2 contents (Ca-o2-Cv(o2)) were obtained retrospectively for International Space Station Increments 19-33 (April 2009-November 2012). Qo(2) and Do(2) were calculated from these variables via integration of Fick's Principle of Mass Conservation and Fick's Law of Diffusion. Vo(2max) significantly decreased from pre- to postflight (-53.9 +/- 45.5%, P = 0.008). The significant decrease in Q(Tmax) (-7.8 +/- 9.1%, P = 0.05), despite an unchanged [Hb], resulted in a significantly decreased Qo(2) (-11.4 +/- 10.5%, P = 0.02). Do(2) significantly decreased from pre- to postflight by -27.5 +/- 24.5% (P = 0.04), as did the peak Ca-o2-Cv(o2) (-9.2 +/- 7.5%, P = 0.007). With the use of linear regression analysis, changes in Vo(2max) were significantly correlated with changes in Do(2) (R-2 = 0.47; P = 0.04). These data suggest that spaceflight decreases both convective and diffusive O-2 transport. These results have practical implications for future long-duration space missions and highlight the need to resolve the specific mechanisms underlying these spaceflight-induced changes along the O-2 transport pathway. NEW & NOTEWORTHY Long-duration spaceflight elicited a significant decrease in maximal oxygen uptake. Given the adverse physiological adaptations to microgravity along the O-2 transport pathway that have been reported, an integrative approach to the determinants of postflight maximal oxygen uptake is needed. We demonstrate that both convective and diffusive oxygen transport are decreased following similar to 6 mo International Space Station missions.