Cardiovascular response to submaximal exercise in sustained microgravity

Cardiac output (Q̇), heart rate (HR), blood pressure, and oxygen consumption (V̇O₂) 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 V̇O₂ 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 combined all microgravity measurements to compare in-flight values with erect or supine control values. At rest, Q̇ in flight was 126% of Q̇ erect (P < 0.01) but was not different from Q̇ supine, and HR in flight was 81% of HR erect (P < 0.01) and 91% of HR supine (P < 0.05). Thus resting 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 diastolic pressure were lower in flight than erect (P < 0.05). Exercise values were considered as functions of V̇O₂. The increase in Q̇ with V̇O₂ in flight was less than that at 1 G (slope 3.5 vs. 6 l · min^-1 · l^-1 · min^-1). SV in flight fell with increasing V̇O₂, whereas SV erect rose and SV supine remained constant. The blood pressure response to exercise was not different in flight from erect or supine. We conclude that true microgravity causes a cardiovascular response different from that seen during any of its putative simulations.