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

Abstract

We tested the hypothesis that, after bed rest, maximal oxygen consumption ( VO₂max ) decreases more upright than supine, because of adequate cardiovascular response supine, but not upright. On 9 subjects, we determined VO₂max and maximal cardiac output (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 Q as f(H) times Q(st), and systemic oxygen flow ( QaO₂) as 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, VO₂max 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 decreased upright (-45.1%), but not supine (+9.0%), being higher supine than upright (+98.4%). Maximal QaO₂ 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 VO₂max supine, (ii) partially explained the VO₂max decrease upright, and (iii) caused the VO₂max differences between postures. We speculate that impaired peripheral oxygen transfer and/or utilisation may explain the VO₂max decrease supine and the fraction of VO₂max decrease upright unexplained by cardiovascular responses.

Energetics of Muscular Exercise

“…Direct experimental assessment of the parameters of Eq. (4.13) confirmed that F Q in normoxia is between 0.65 and 0.76 (Bringard et al 2010;Turner et al 1993). …”

“…This component in fact was already visible, after the same time, in the study by Trappe et al (2006) in space and in supine posture after bed rest. Bringard et al (2010) found a 44 % reduction of stroke volume at maximal exercise in upright posture after 35-day bed rest as compared to the value before bed rest with no modification in maximal heart rate: the consequence was a 45 % decrease in maximal _ Q. This results, associated with a 13 % increase in CaO 2 due to higher haemoglobin concentration, originated a 38 % decrease in _ Q aO 2max .…”

“…It is kept implicit that these statements apply to _ V O 2max measurements in upright posture shortly after the end of the bed rest period. In fact, this is not so during bed rest (or space flight), or in supine posture after bed rest, since very small changes, if any, in _ V O 2max were found in these conditions (Bringard et al 2010;Greenleaf et al 1989;Levine et al 1996;Trappe et al 2006).…”

“…Moreover, in this experimental model, only the duration of the bed rest period varies among studies, allowing a precise evaluation of the time courses of the functional modifications induced by bed rest. _ V O 2max decreases after bed rest (Bringard et al 2010;Capelli et al 2006;Convertino et al 1982;Ferretti et al 1997a;Greenleaf et al 1989;Kashihara et al 1994;Saltin et al 1968;Stremel et al 1976;Trappe et al 2006), even of very short duration (Smorawinski et al 2001). The size of the _ V O 2max decrease is larger the longer is the bed rest duration, being fast in the first days, and progressively slower as bed rest proceeds.…”

“…4.9, _ V O 2max was measured at least three days after the end of bed rest, at a time when recovery of cardiovascular function was already taking place (Spaak et al 2005), so that the amplitude of the rapid component of the _ V O 2max decline might have been underestimated, especially when the bed rest duration was short. Only exceptions were the studies by Bringard et al (2010) and Lee et al (2007Lee et al ( , 2009, with measurements carried out on the day of reambulation. The results from these studies, for which the bed rest duration was 35 and 30 days, respectively, compared well with those from other studies, perhaps because there was already a significant impact of the slow component of the _ V O 2max decline, the recovery of which is expected to be slower than that of the rapid cardiovascular component.…”