In biological structures, application of a constant alternating low current results in an impedance to the spread of the current that is frequency dependent. The living organism consists of intra- and extracellular fluids that behave as electrical conductors, and cell membranes that act as electrical condensers. Body fluids and electrolytes are responsible for electrical conduction.

We studied body impedance and fluid loss during hemodialysis in 20 patients with chronic renal failure. Two pairs of electrodes with a thin layer of electrolyte gel were placed on the dorsal surfaces of the hands and feet. 50KHz 800uA current was applied via the distal electrodes of each pair and the voltage drop was measured by proximal electrodes and impedance was derived from Ohms law. We measured body impedance and fluid loss at every one hour during hemodialysis.

Body impedance value according to weight before hemodialysis changed in inveise ratio. During hemodialysis the rate of increase of body impedance according to body fluid loss revealed individual variance, but it was nearly constant in each individual patient.

The method assessing total body water by body impedance is very simple, safe, rapid and noninvasive. The change of total body water can be predicted with body impedance method in individual patient by continuous monitoring, and this method may be useful particularly in critically ill patient. Further validation of this method is required for clinical application.