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 ((V) over dot(O2max)) decreases more upright than supine, because of adequate cardiovascular response supine, but not upright. On 9 subjects, we determined (V) over dot(O2max) and maximal cardiac output ((Q) over dot) 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 (Q) over dot as f(H) times Q(st), and systemic oxygen flow ((Q) over dota(O2)) as (Q) over dot 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, (V) over dot(O2max) 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 (Q) over dot decreased upright (-45.1%), but not supine (+9.0%), being higher supine than upright (+98.4%). Maximal (Q) over dota(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 (V) over dot(O2max) supine, (ii) partially explained the (V) over dot(O2 max) decrease upright, and (iii) caused the (V) over dot(O2 max) differences between postures. We speculate that impaired peripheral oxygen transfer and/or utilisation may explain the (V) over dot(O2max) decrease supine and the fraction of (V) over dot(O2max) decrease upright unexplained by cardiovascular responses. (C) 2010 Elsevier B.V. All rights reserved.