Applied potential tomography shows differential changes in fluid content of leg tissue layers in microgravity

Absence of hydrostatic forces in the human cardiocirculatory system normally leads to an overall body fluid deficit. It was hypothesized that this is mainly due to a loss of interstitial fluid. An experiment was performed on board the Russian MIR station. Cuffs were positioned around both thighs and inflated up to suprasystolic values. This maneuver took place just before and after immediately a lower body negative pressure session (LBNP). The redistribution of fluids underneath the cuffs was assessed by means of cross-sectional impedance tomography (Applied Potential Tomography, APT). A microgravity induced loss of interstitial fluid was measured in all layers of the observed cross-section. The APT-readings changed significantly (SD approximately +/- .9) from 3.0 at 1g to 1.7 at 0g for the outer layer and from 2.7 at 1g to 2.0 at 0g for the middle layer (expressed in arbitrary units). The LBNP maneuver was able to fill the interstitial space but only at levels higher than -15 mmHg LBNP. This suggests that the superficial tissues in the legs are as much affected as the deeper ones by changing g-conditions and LBNP can be used to counteract interstitial fluid loss in this area.