Cardiovascular oxygen transport in exercising humans in microgravity

The relationship between oxygen delivered (Qao2) and oxygen consumption (VO2) is remarkably constant in a variety of physiological conditions. However, this may not be the case in microgravity, because of: (1) a decrease in blood haemoglobin concentration (Hb); and (2) a downregulation of the baroreflex, which plays a major role in the cardiovascular response to exercise. The VO2, by a closed circuit method, heart rate, by electrocardiography, cardiac output (Q), by the one-step CO2 rebreathing method, Hb, by a photometric technique, and arterial oxygen saturation (Sa,°2), by infrared oxymetry, were measured at rest and at the steady-state of three sub-maximal cyclo-ergometric work loads (50, 75 and 100 W, respectively) on two subjects before (1 g) and during (0 g) a 6 month duration space flight on board the Russian Space Station MIR. Qa,o° was calculated as the product of Q x Sal2 X Hb x the physiological oxygen binding coefficient (1-34 ml g1). V02 was linearly related to power in all conditions. At 0 g, V02 did not change as a function of flight time. At any work load, it was significantly lower than at 1 g (-18-5 and -13-8% for subjects 1 and 2, respectively). Similarly, lower Q values at exercise, independent of flight time, were found at 0 g than at 1 g (-8-1 and -15-4%, respectively). The Q vs. VO2 relationship at exercise was the same at 0 g as at 1 g. In contrast, at rest Q was higher at 0 g than at 1 g, because a significant increase in stroke volume was only partially compensated for by a decrease in heart rate. As the heart rate vs. power relationships were slightly displaced downward at 0 g, no changes in stroke volume during exercise at 0 g were found. The concomitantly decreased (subject 1) or unchanged (subject 2) Hb at 0 g contributed to a significant decrease in Qa,O, (-17-6 and -11 2%). The relationships between QaO2 and VO2 were the same at 0 g as at 1 g in both subjects. These data reveal a tight coupling of cardiovascular response to exercise and peripheral metabolic demand.