Modeling human orthostatic responses on the Moon and on Mars

Since manned missions to the Moon and Mars are planned, we conducted active standing tests with lunar, Martian, terrestrial, and 1.8 loads of inertial resistance (+G_z) modeled through defined parabolic flight maneuvers. We hypothesized that the cardiovascular response to active standing is proportional to the +G_z load.

During partial-+G_z parabolic flights, 14 healthy test subjects performed active stand-up maneuvers under 1 +G_z, lunar (0.16 +G_z), Martian (0.38 +G_z), and hyper inertial resistance (1.8 +G_z) while heart rate and finger blood pressure were continuously monitored. We quantified amplitudes and timing of orthostatic response immediately following standing up.

The maximum early heart rate increase was 21 (SD ± 10) bpm with lunar, 23 (± 11) bpm with Martian, 34 (± 17) bpm with terrestrial +G_z, and 40 (± 11) bpm hyper +G_z. The time to maximum heart rate increased gradually with increasing loads of inertial resistance. The transient blood pressure reduction was most pronounced with hyper +G_z but did not differ significantly between lunar and Martian +G_z. The mean arterial pressure nadir was reached significantly later with Martian and lunar compared to 1 +G_z. Paradoxically, the time for blood pressure to recover was shortest with terrestrial +G_z.

While load of inertial resistance directly affects the magnitude of the transient blood pressure reduction and heart rate response to active standing, blood pressure stabilization is most rapidly attained during terrestrial +G_z. The observation might suggest that the human cardiovascular system is tuned to cope with orthostatic stress on earth.