PLASMA-VOLUME WITH ALTERNATING TILTING - EFFECT OF FLUID INGESTION

The present study determines the effect of repeated 70 degrees head-up tilt (HUT) on plasma volume (PV) shifts by measuring blood density (ED), plasma density (PD), and hematocrit (Hct). Eight men (18-26 yr) underwent a predrink period with two supine (P1 and P3) and two HUT (P2 and P4) phases of 45 min each. At the end of P4 they drank 10 ml/kg body wt of isotonic (290 mosmol/kg) sodium chloride (Iso) or hypotonic (<10 mosmol/kg) unsweetened tea (Hypo) or nothing [control (Con)]. The following periods continued the supine (P5, P7)/upright (P6) sequence. ED and PD were measured from ear lobe blood; they were different (P < 0.05) between Con, Hypo, and Iso P6 and P7. The density of fluid that moved between intra- and extravascular compartments was 1,008.2 +/- 0.4 g/l and did not differ with test situations. In Con (P3, P5, P7), supine PV steadily decreased compared with P1 (P < 0.05). PV in P1, P2, and P3 of all treatments averaged 120 +/- 1, 101 +/- 1, and 115 +/- 1%, respectively, of PV in P4. Tilt-induced PV shifts ranged from -9.7 to -16.7% compared with PV during the respective previous phases. After drinking, PV increased (P < 0.05) above Con values at the end of P7 by 12.9% with Iso and by 6.6% with Hypo. Progressive hemoconcentration occurred in the nondrink supine periods; isotonic saline ingestion increased supine PV to Con level but did not stop or reverse the decrease of upright hemoconcentration. Upright blood and PD were not altered with repeated HUT, which resulted in decreased fluid loss in consecutive upright periods. This indicated that with repeated tilting, upright capillary pressure balance is achieved with attenuated PV loss.