Cardiovascular determinants of maximal oxygen consumption in upright and supine posture at the end of prolonged bed rest in humans

We tested the hypothesis that, after bed rest, maximal oxygen consumption (over(V, ̇)_{O2 max}) decreases more upright than supine, because of adequate cardiovascular response supine, but not upright. On 9 subjects, we determined over(V, ̇)_{O2 max} and maximal cardiac output (over(Q, ̇)) upright and supine, before and after (reambulation day upright, the following day supine) 35-day bed rest, by classical steady state protocol. Oxygen consumption, heart rate (f_H) and stroke volume (Q_st) were measured by a metabolic cart, electrocardiography and Modelflow from pulse pressure profiles, respectively. We computed over(Q, ̇) as f_H times Q_st, and systemic oxygen flow (over(Q, ̇) a_O2) as over(Q, .) times arterial oxygen concentration, obtained after haemoglobin and arterial oxygen saturation measurements. Before bed rest, all parameters at maximal exercise were similar upright and supine. After bed rest, over(V, ̇)_{O2 max} was lower (p <0.05) than before, both upright (-38.6%) and supine (-17.0%), being 30.8% higher supine than upright. Maximal Q_st decreased upright (-44.3%), but not supine (+3.7%), being 98.9% higher supine than upright. Maximal over(Q, ̇) decreased upright (-45.1%), but not supine (+9.0%), being higher supine than upright (+98.4%). Maximal over(Q, ̇) a_O2 decreased upright (-37.8%), but not supine (+14.8%), being higher (+74.8%) upright than supine. After bed rest, the cardiovascular response (i) did not affect over(V, ̇)_{O2 max} supine, (ii) partially explained the over(V, ̇)_{O2 max} decrease upright, and (iii) caused the over(V, ̇)_{O2 max} differences between postures. We speculate that impaired peripheral oxygen transfer and/or utilisation may explain the over(V, ̇)_{O2 max} decrease supine and the fraction of over(V, ̇)_{O2 max} decrease upright unexplained by cardiovascular responses.