An electrical admittance based index of thoracic intracellular water during head-up tilt in humans

During 50 degrees head-up tilt (HUT), the number of erythrocytes within the thorax has been shown to be reduced by approximately 25% and this level is retained during a maintained tilt, whilst that in the thigh increases by a pproximately 70%. To evaluate whether the electrical admittance of intracel lular water (ICW) may be used to monitor this redistribution of red cells i n humans, we determined the regional difference in the reciprocal value of the impedance at 1.5 and 100 kHz for the thorax (thorax(ICW)) and for the l eg (leg(ICW)). In ten subjects all variables remained unchanged during head -down tilt but during HUT, presyncopal symptoms were induced in eight subje cts after a mean of 27 (SEM 7) min as mean heart rate dropped from 85 (SEM 4) to 66 (SEM 3) beats . min(-1), mean arterial blood pressure from SO (SEM 3) to 60 (SEM 5) mmHg, and mean oxygen saturation of venous blood from 76 (SEM 2)% to 73 (SEM 3)% (P < 0.05). The mean haematocrit increased from 50 (SEM 5)% to 52.5 (SEM 3.5)% (P < 0.01) and mean central venous pressure dec reased during tilting (from a mean of 1 (SEM 1) to a mean of -1 (SEM 1) mmH g; P < 0.05) and returned to value at rest during the maintained tilt. Mean thoracic impedances increased by 7.0 (SEM 1.0) <Omega> (1.5 kHz) and 5.4 ( SEM 1.2) Omega (100 kHz), and mean leg impedances decreased by 9.3 (SEM 1.2 ) Omega (1.5 kHz) and 3.1 (SEM 1.0) Omega (100 kHz) (P < 0.01). Mean thorax (ICW) decreased at 40<degrees> HUT and remained reduced by 11 (SEM 2) S . 1 0(-4) (P < 0.05) until the presyncopal symptoms developed, at which time it was lower by 16 (SEM 2) S . 10(-4) (P < 0.01). Mean leg(ICW) increased fro m 97 (SEM 15) to 99 (SEM 15) S . 10(-4) (P = 0.08) during HUT but decreased during maintained till (to 94 (SEM 15) S . 10(-4); P < 0.05). The results suggested that during HUT: the difference in electrical admittance at a hig h and a low frequency current reflects the reduced number of red cells with in the thorax.