CARDIOVASCULAR-RESPONSE TO SUBMAXIMAL EXERCISE IN SUSTAINED MICROGRAVITY

Cardiac output (Q), heart rate (HR), blood pressure, and oxygen consum ption (Vo(2)) were measured repeatedly both at rest and at two levels of exercise in six subjects during microgravity exposure. Exercise was at 30 and 60% of the workload producing the individual's maximal Vo(2 ) in 1 G. Three of the subjects were on a 9-day flight, Spacelab Life Sciences-1, and three were on a 15-day flight, Spacelab Life Sciences- 2. We found no temporal differences during the flights. Thus we have c ombined all microgravity measure: ments to compare in-fight values wit h erect or supine control values. At rest, Q in flight was 126% of Q e rect (P < 0.01) but was not different from Q supine. and HR in flight was 81% of HR erect (P < 0.01) and 91% of HR supine (P < 0.05), Thus r esting stroke volume (SV in flight was 155% of SV erect (P < 0.01) and 109% SV supine (P < 0.05). Resting mean arterial blood pressure and d iastolic pressure were lower in flight than erect (P < 0.05), Exercise values were considered as functions of Vo(2). The increase in Q with Vo(2) in flight was less than that at 1 G (slope 3.5 vs. 6 1 . min(-1) . l(-1). min(-1). SV in flight fell with Increasing Vo(2), whereas SV erect rose and SV supine remained constant. The blood pressure respons e to exercise was not different in flight from erector supine. We conc lude that true microgravity causes a cardiovascular response different from that seen during any of its putative simulations.