The venous pump does not affect the indifference point for electrical impedance in humans

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We have used regional electrical impedance at 2.5 and 100 kHz over nine body sections (two thoracic, one abdominal, two thigh, two around the knee, and two lower leg) in eight subjects to determine the volume indifference point defined as the level at which fluid volume remained constant independent of body position changes in space. Passive head-up tilt and tilt with activation of the venous muscle pump of the legs were performed in 10° increments from 0 to 60° over 6 min. The impedance changes in relation to 0° were similar for the two frequencies. Over the thorax it increased in proportion to the head-up tilt angle by a mean of 3.8 (range 1.9 to 9.3) Ω (100 kHz) at 60° (P < 0.05), while the abdominal impedance did not change significantly. Over the thigh it decreased with increasing head-up tilt angle by a mean maximum of − 2.3 (range − 9.4 to − 0.4) Ω and over the lower leg by a mean of − 2.7 (range − 6.0 to − 0.8) Ω. There were only marginal changes around the knee, mean − 1.5 (range − 2.3 to − 0.2) Ω (P < 0.05), and no change around the ankle indicating that little or no fluid was accumulated in these regions. Changes in impedance during passive and active head-up tilt did not differ significantly in any but one position: between the greater trochanter and the mid thigh, where during passive tilt it decreased by a mean of − 4.8 (range − 9.4 to − 1.9) Ω, and with activation of the venous pump by a mean of only − 1.2 (range − 1.9 to − 0.4)Ω (P < 0.05). These results indicated that the vascular volume indifference point was positioned between the navel and iliac crest both during the passive and active head-up tilts although during the passive tilt, apparently more fluid was accumulated in the vessels of the thigh.