Continuous monitoring of changes in blood volume and limb volume during lower body negative pressure.

Lower body negative pressure (LBNP) induces venous pooling in the legs and a decrease of blood volume (BV). The present study was designed to investigate the dynamic changes in BV and limb volume during LBNP. We made continuous measurements of blood density (ρb) during LBNP at two different levels (-15 and -30 mmHg) in eight healthy male volunteers. Blood was withdrawn continuously from the antecubital vein to measure ρb during an experimental period of 40 min (a 10-min control period, a 10-min LBNP and a 20-min recovery period). The density was measured by the mechanical oscillator technique. The changes of circumference in the upper arm and calf were measured by strain gauge plethysmography. ρb decreased rapidly in the early phase of LBNP followed by an increase, as expected. The peak reduction (-0.35 ± 0.04 g/l) of ρb at -15 mmHg did not differ (P=0.47) from that (-0.42 ± 0.05 g/l) at -30 mmHg. The time (2.9 ± 0.3 min) from onset of LBNP to the peak reduction of ρb was not significantly different between - 15 and -30 mmHg (P =0.50). After LBNP, a further ρb increase was continued for 2.5±0.2 min at both intensities, followed by the return toward control levels. The maximal increase (1.34 ± 0.07 g/i ) of ρb after LBNP at -30 mmHg was greater (P<0.001) than that (0.57 ± 0.06 g/l) at -15 mmHg. LBNP decreased the arm circumference and increased the calf circumference in an intensity-dependent manner. These results suggest that LBNP induces an intensity-dependent decrease in BV with no difference in the time peak reduction occurring after LBNP. We found no significant difference between the intensities in the transient hemodilution effect during LBNP. It is suggested that LBNP also induces an intensity-dependent mobilization of blood from the upper portion to the lower portion of the body.