BODY-FLUID DISTRIBUTION IN MAN IN-SPACE AND EFFECT OF LOWER-BODY NEGATIVE-PRESSURE TREATMENT

The lack of hydrostatic forces in space eventually produces a fluid de ficit within the circulatory system. This deficit may alter the circul atory regulation patterns. The aim of the present study was to determi ne how much of this fluid deficit is attributable to interstitial flui d losses and to determine the effects of lower body negative pressure (LBNP) treatment on fluid distribution. The body fluid distribution of one subject was assessed before, during, and after weightlessness usi ng two electrical impedance methods: (a) standard quadripole impedance for the segments of upper torso, lower torso, thigh, and calf and (b) an electrical impedance tomography technique (applied potential tomog raphy) for a thigh cross-section. To assess the content of interstitia l free fluid a thigh cuff overlying the electrodes for applied potenti al tomography was inflated to suprasystolic values to ascertain how mu ch fluid can be squeezed out of blood vessels and tissue of skin and m uscle. After the first thigh cuff maneuver (CUFF I) the subject perfor med a cardiovascular stress test with LBNP to mimic the gravity-induce d blood shift to the lower part of the body. Then the compression mane uver was repeated (CUFF II). (a) This experimental sequence demonstrat ed a reduction in interstitial fluid in weightlessness of roughly 40% at the thigh. (b) The CUFF I and LBNP experiment demonstrated a reduce d ability to cope with blood pooling in microgravity. (c) The CUFF II experiment suggests that LBNP in microgravity can refill the interstit ial spaces and counteract the associated cardiovascular deterioration. The impedance measurements provided estimates of the contribution of different body sections to the observed body weight loss of more than 6 kg. The chest contributed nothing of significance, the lower torso m ore than 0.5 1, and both calves roughly 1.5 1. The thigh segments of b oth legs contributed between 1.5 1 and 2.0 1 with an interstitial free fluid reduction in muscle and skin by 40%.