Cardiovascular Responses to Standing in Earth and Simulated Moon and Mars Gravity Environments

Abstract

INTRODUCTION: Basic knowledge of cardiovascular responses to reduced gravity are needed prior to exploration class space missions. Current paradigms of hypogravity, including parabolic flight, water immersion, head-up tilt (HUT) and body suspension models limit motion, simulation time, or tasks subjects can perform. The G-Trainer (Alter-G, Inc) is a combination treadmill and lower body positive pressure (LBPP) device that can be used to simulate hypogravity conditions, with and without concurrent treadmill exercise. PURPOSE: We compared cardiovascular responses, within the same subjects, to unloading conditions via LBPP and HUT at Moon (20% body weight (BW) vs 9.5° HUT), Mars (40% BW vs 22° HUT) and Earth (100% BW vs 80° HUT) weights. We hypothesized that indexes of healthy human segmental (thorax, abdomen, upper and lower leg) fluid volume shifts and regulatory (blood pressure (BP), heart rate (HR) and vascular resistance (VR)) responses to standing would be reduced when body ground reaction force was reduced by LBPP in a manner similar to that observed during HUT. METHODS: HR, BP, ECG, segmental fluid shifts, echocardiographic measures of diastolic (passive filling) and systolic cardiac (stroke volume, SV) function, LBPP and ground reaction force were recorded from 10 men & 10 women during 20 minute sessions while subjects were supine and standing at 100%, 40%, and 20% BW in the G-Trainer and while supine and tilted at 80°, 22°, 9.5° head-up. RESULTS (MIXED MODEL, P<0.05): Compared to supine, the shift of fluid from the chest to the abdomen was greater at 100% BW than at reduced weights, in both LBPP and HUT models. Similarly, the increase in HR and the decrease in SV was greater at 100% BW than at reduced weights, in both LBPP and HUT models. However, mean BP and VR were increased in response to LBPP but not in response to HUT. CONCLUSION: Body weight unloading via both LBPP and HUT resulted in fluid shifts and cardiovascular changes similar to those anticipated in actual reduced gravity environments. Similarities between the two models indicate that either can be used for modeling cardiovascular responses to standing on the Moon and Mars. The LBPP model (G-Trainer) has the advantage of offering dynamic activity at reduced body weight. Differences in BP need to be addressed. Supported by KY NASA EPSCoR 52611.