Electrical admittance for filling of the heart during lower body negative pressure in humans

To evaluate whether electrical admittance of intracellular water is applica ble for monitoring filling of the heart, we determined the difference in in tracellular water in the thorax (Thorax(ICW)), measured as the reciprocal v alue of the electrical impedance for the thorax at 1.5 and 100 kHz during l ower body negative pressure (LBNP) in humans. Changes in Thorax(ICW) were c ompared with positron emission tomography-determined (CO)-O-15-labeled eryt hrocytes over the heart. During -40 mmHg LBNP, the blood volume of the hear t decreased by 21 +/- 3% as the erythrocyte volume was reduced by 20 +/- 2% and the plasma volume declined by 26 +/- 2% (P < 0.01; n = 8). Over the he art region, LBNP was also associated with a decrease in the technetium-labe led erythrocyte activity by 26 +/- 4% and, conversely, an increase over the lower leg by 92 +/- 5% (P < 0.01; n = 6). For 15 subjects, LBNP increased thoracic impedance by 3.3 +/- 0.3 Omega (1.5 kHz) and 3.0 +/- 0.4 Omega (10 0 kHz), whereas leg impedance decreased by 9.0 +/- 3.3 Omega (1.5 kHz) and 6.1 +/- 3 Omega (100 kHz; P < 0.01). Thorax(ICW) was reduced by 7.1 +/- 1.9 S.10(-4) (P < 0.01) and intracellular water in the leg tended to increase (from 37.8 +/- 4.6 to 40.9 +/- 5.0 S.10(-4); P = 0.08). The correlation bet ween Thorax,,, and heart erythrocyte volume was 0.84 (P < 0.05). The result s suggest that thoracic electrical admittance of intracellular water can be applied to evaluate changes in blood volume of the heart during LBNP in hu mans.