Effect of head-down bedrest on blood/plasma density after intravenous fluid load

Using the mechanical oscillator technique, the mass density of antecubital venous blood and plasma samples was measured in 6 men before and after infusion of 22 ml/kg isotonic NaCl solution before, on day 7 during, and on day 3 after ten days of 6 degrees head-down bedrest. We studied 1) the distribution volumes of the infused NaCl solution, 2) the magnitude and time-course of induced fluid shifts to the extravascular spaces after rapid volume expansion, and 3) the protein concentration of the translocated fluid. The NaCl distribution volume was 16.7 +/- 2.9% body weight (BW) before, 15.0 +/- 3.0% BW during (P less than 0.05) and 13.0 +/- 1.3% BW after the head-down tilt (HDT) period (P less than 0.01). The volume of fluid shifted 120 min after infusion was not different in the control (12.9 +/- 7.4% plasma volume and 8.3 +/- 3.2% blood volume), during HDT (14.7 +/- 3.8% plasma volume and 7.4 +/- 5.6% blood volume), during HDT HDT conditions (14.6 +/- 2.2% plasma volume, and 8.2 +/- 1.3% blood volume, respectively). The density of the shifted fluid which reflects protein concentration was also unchanged (1004.2 +/- 4.2 g/l, 1002.6 +/- 4.3 g/l and 1003.9 +/- 8.2 g/l in the control, during, and after HDT periods, respectively). The mass density of red cells did not change with any condition. It is concluded that the distribution volume of isotonic saline solution decreases with diminished body mass. Densitometry provides a means of monitoring volume changes and protein shifts under the circumstances of our study.