HEMATOCRIT, PLASMA-VOLUME AND NORADRENALINE IN HUMANS DURING SIMULATED WEIGHTLESSNESS FOR 42 DAYS

Previous results from our laboratory demonstrate that changes in haema tocrit (Hct) and haemoglobin concentration (Hb) underestimate the rela tive (%) change in plasma volume (PV) in seated subjects during simula tion of weightlessness by water immersion. Therefore, we examined whet her changes in Hct and Hb would accurately reflect the changes in PV i n seven subjects during simulation of weightlessness by another model, 6 degrees head-down tilted bed rest (HDBR), for 42 days. Since we hav e previously observed unexpectedly high plasma levels of noradrenaline (NA) in astronauts during space flight, we also took the opportunity to measure this variable. The measurements were compared with those of the supine horizontal position before and after HDBR. During HDBR, PV measured by the Evans blue dye dilution technique decreased by 6.1+/- 2.8% (P<0.05) on day 2 and 9.6+/-2.2% (P<0.05) on the 42nd day compare d with that of the supine, horizontal position. Based on changes in Hc t and Hb, PV decreased similarly by 8.3+/-2.8 and 10.2+/-3.2% (P<0.05) respectively. There were no differences comparing the results of the two methods (P>0.05). Forearm venous plasma NA was unchanged during th e whole course of HDBR compared with that of the pre-HDBR supine posit ion. It is concluded that changes in Hct and Hb reliably reflect the c hanges in PV comparing prolonged HDBR with the pre- and post-HDBR hori zontal, supine position. Thus, changes in Hct and Hb might accurately reflect the change in PV during weightlessness in humans provided that the horizontal supine position is used as the ground-based reference. Furthermore, the results of this study, as well as of previous studie s from space, confirm that NA release is unchanged or even increased d uring weightlessness.