An indifference point for electrical impedance in humans.

Regional electrical impedance was used over eight sections of the human body (two thoracic; one abdominal; two thigh; two around the knee; and one lower leg) to determine the volume indifference point during passive head-up tilt in eight subjects. Head-up tilt was performed in 10 degrees increments from 0 degree to 60 degrees over 6 min. Electrical impedance increased over the thorax in proportion to the head-up tilt angle, while abdominal impedance did not change significantly, and over the thigh and the lower leg it decreased with increasing head-up tilt angle. No change in electrical impedance was noted just above the knee, and electrical impedance just below the knee decreased only marginally. Results demonstrate minimal fluid accumulation around the knee during head-up tilt. Furthermore, in humans the electrical impedance and therefore probably the volume indifference point is positioned between the umbilicus and crista iliaca.