Assessment of the sensitivity of bioimpedance to volume changes in body water

The sensitivity of whole-body electrical impedance measurements to changes in the volume of total body water in 12 children undergoing haemodialysis has been assessed. The impedance (I) of each child was measured at 20-min intervals during dialysis using the standard four-electrode technique to apply a constant current (800 microA, 50 kHz) between the wrist and ankle on the non-fistula side of the patient. The ultrafiltration volume (U) was also recorded. A simple electrical model suggests that U = aHt2 ((1/I0)-(1/I)), where I0 is the whole body impedance 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 dialysis without ultrafiltration, whereas in 8 patients undergoing ultrafiltration and dialysis I increased. Linear regression analysis and the above equation gave a mean value for a = 0.566 1 Ohm/cm2 (coefficient of variation = 3%), (mean r = 0.97), values comparable to those values obtained from isotope dilution studies. Predicted fluid loss in 8 patients following a single dialysis session gave a mean overestimate 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 changes 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.