A First-Order Design Requirement to Prevent Edema in Mechanical Counter-Pressure Space Suit Garments

Abstract

ABSTRACTIntroductionMechanical counter-pressure (MCP) space suits may provide enhanced mobility relative to gas-pressure space suits. One challenge to realizing operational MCP suits is the potential for edema caused by spatial variations in the applied body-surface pressure (dP). We determined a first-order requirement for these variations.MethodsDarcy’s law relates volume flux, of fluid from capillaries to the interstitial space, to transmural hydraulic and osmotic pressure differences. Albumin and fibrinogen levels determine, to first order, the capillary oncotic pressure (COP). We estimated dP, neglecting hydrostatic pressure differences, by equating the volume flux under MCP and under normal with the volume flux under abnormal variations in COP; then we compared these estimates to results from MCP garment studies.ResultsNormal COP varies from 20-32 mm Hg; with constant hydraulic conductivity, dP≈12 mm Hg. In nephrotic syndrome, COP may drop to 11 mm Hg, yielding dP≈15 mm Hg relative to mid-normal COP. Previous studies found dPmax =151 mm Hg (MCP glove; finger and hand dorsum relative to palm), dPmax=51 mm Hg (MCP arm; finger, hand dorsum, and wrist relative to arm), and dP=52, 90 and 239 mm Hg (three MCP lower leg garments).ConclusionsMCP garments with dPmax≤12 mm Hg are unlikely to produce edema or restrict capillary blood flow; however, garments with dPmax>12 mm Hg will not necessarily produce edema. For example, the hydrostatic pressure gradient at the feet in 1g can range from 70-90 mm Hg. Current garment prototypes do not meet our conservative design requirement.