Metabolic Cost of Simulated Egress while Wearing Compression Garments Used to Prevent Orthostatic Intolerance

Abstract

During Space Shuttle reentry and landing, astronauts wear an antigravity suit (AGS) consisting of inflatable bladders that compress the legs and abdomen to prevent venous pooling and orthostatic hypotension. AGS inflation may impair emergency egress from the Shuttle through increased metabolic cost of ambulation and consequent accumulation of carbon dioxide in the Advanced Crew Escape Suit helmet. A compression garment that provides similar levels of protection against orthostatic intolerance with a smaller impact on metabolic rate is desirable. PURPOSE: The purpose of this study was to evaluate the metabolic cost of walking in the AGS compared to commercially available, thigh-high compression stockings. METHODS: Ten healthy subjects (5 M, 5 F) walked on a treadmill at 1.56 m·s-1 and 0% grade for 5 min to simulate an unaided emergency egress from the Shuttle. Subjects walked in three conditions presented in random order: (1) wearing the AGS inflated to 1.5 psi (nominal compression), (2) wearing compression stockings, and (3) wearing normal exercise clothes (unsuited). Oxygen consumption (VO2), carbon dioxide production (VCO2), and ventilation (VE) were measured using a standard metabolic gas analysis system. Averages of min 3-5 for VO2, VCO2, and VE were compared using one-way repeated-measures ANOVA with Bonferroni correction applied to pairwise comparisons. RESULTS: VO2 of subjects wearing the AGS was 15±4% greater than that of unsuited subjects (p<0.001) and 12±2% greater than that of subjects wearing compression stockings (p=0.002). VCO2 was greater when walking in the AGS than when unsuited (16±3%, p<0.001) or wearing the compression stockings (11±2%, p<0.001). VE was greater when walking in the AGS than when unsuited (13±3%, p<0.001) or wearing the compression stockings (9±2%, p=0.002). There were no differences in VO2, VCO2, or VE between the unsuited and compression stockings conditions. CONCLUSION: The metabolic cost of walking when wearing compression stockings is not different from the metabolic cost in the unsuited condition and is substantially less than the metabolic cost when wearing the AGS. The lower metabolic cost of walking when wearing the stockings may increase the likelihood of a successful unaided emergency egress, particularly in an astronaut who is deconditioned due to space flight.