Effects of prolonged bed rest on cardiovascular oxygen transport during submaximal exercise in humans

The hypothesis was tested that prolonged bed rest impairs O2 transport during exercise, which implies a lowering of cardiac output Q̇(c) and O2 delivery (Q̇(a)O2). The following parameters were determined in five males at rest and at the steady-state of the 100-W exercise before (B) and after (A) 42-day bed rest with head-down tilt at -6°: O2 consumption (VO2), by a standard open-circuit method; Q̇(c), by the pressure pulse contour method, heart rate (f(c)), stroke volume (Q(h)), arterial O2 saturation, blood haemoglobin concentration ([Hb]) arterial O2 concentration (C(a)O2), and Q̇(a)O2. The VO2 was the same in A and in B, as was the resting f(c). The f(c) at 100 W was higher in A than in B (+17.5%). The Q(h) was markedly reduced (-27.7% and -22.2% at rest and 100 W, respectively). The Q̇(c) was lower in A than in B [-27.6% and -7.8% (NS) at rest and 100 W, respectively]. The C(a)O2 was lower in A than in B because of the reduction in [Hb]. Thus also Q̇(a)O2 was lower in A than in B (-32,0% and -11.9% at rest and at 100 W, respectively). The present. results would suggest a down-regulation of the O2 transport system after bed rest.