Hypergravity and dehydration-induced shifts of interstitial fluid in the skin monitored by ultrasound

Introduction: Spaceflight is associated with a shift of interstitial fluic toward the skin of the head and a fluid loss that can decrease superficial tissue thickness (TT). To collect the physiological data needed to develop techniques for monitoring the hydration status of astronauts during spaceflight, we investigated the changes in TT induced by a 12-h nil-by-mouth period and a 30-min period of +2 Gz hypergravity measured at forehead and tibia. Methods: There were 16 male volunteers who were twice subjected to 30 min of +2 Gz acceleration in a human centrifuge, once following an oral fluid load of 250 ml · h^-1 (procedure A), and once following a nil-by-mouth period of 12 h (procedure B). The TT at the forehead (TT-f) and tibia (TT-t) was measured before (t0), after 10 min (t10), and after 30 min (t30) of hypergravity using a miniature 10 MHz A-mode ultrasound device. The bodyweight, hematocrit (HCT), and plasma viscosity (PVS) were measured at t0 and t30. Results: The nil-by-mouth period induced a significant weight loss and increase in HCT and PVS. A significant increase was noted in TT-t, but not in TT-f (p < 0.05). Following both procedures, exposure to hypergravity produced declines in TT-f at t10 and t30 compared with baseline (p < 0.05), whereas TT-t was not influenced. Conclusions: Changes in interstitial fluid load of superficial tissues can be tracked by A-mode ultrasonography. Fluid loss-induced changes are best detected at the tibia, hypergravity-induced changes at the forehead.