ASSESSMENT OF THE SENSITIVITY OF BIOIMPEDANCE TO VOLUME CHANGES IN BODY-WATER

The sensitivity of whole-body electrical impedance measurements to cha nges in the volume of total body water in 12 children undergoing haemo dialysis has been assessed. The impedance (I) of each child was measur ed at 20-min intervals during dialysis using the standard four-electro de technique to apply a constant current (800 mu A, 50 kHz) between th e wrist and ankle on the non-fistula side of the patient. The ultrafil tration volume (U) was also recorded. A simple electrical model sugges ts that U = aHt(2) ((1/I-o)-(1/I)), where I-o is the whole body impeda nce at the start of dialysis, a is a constant and Ht is patient height . No significant changes in I were measured on 4 patients undergoing d ialysis without ultrafiltration, whereas in 8 patients undergoing ultr afiltration and dialysis I increased. Linear regression analysis and t he above equation gave a mean value for a = 0.566 1 Ohm/cm(2) (coeffic ient of variation = 3%), (mean r = 0.97), values comparable to those v alues obtained from isotope dilution studies. Predicted fluid loss in 8 patients following a single dialysis session gave a mean overestimat e of 4.3% (limits of agreement 27.3% and -19.7%), although in 6 of the patients agreement was to within 6%. Changes in impedance reflect cha nges in total body water in children undergoing haemodialysis and are relatively insensitive to factors such as the possible differences in electrolyte levels between these patients.