Assessment of changes in total body water in patients undergoing renal dialysis using bioelectrical impedance analysis.

This study aimed to assess whether changes in fluid can be accurately predicted from the change in whole-body impedance with an appropriate correction for the conductor length (i.e. height). Eight studies to measure changes in impedance were performed on six patients during dialysis. The measured loss of fluid from the beginning to the end of dialysis (1174+/-706ml) agreed well with the change in body weight (1188+/-768kg). The regression lines of the plot of change in height(2)/impedance versus loss of fluid for each subject had correlation coefficients ranging from 0.880-0.999, but there was a four-fold variation in the slopes of the different curves. The mean +/- SD change in height(2)/impedance during the removal of 0.5, 1 and 2l of water was 1.75+/-0.96, 3.41+/-1.55 and 6.72+/-2.91 m(2)/ohms respectively. The total change in body water, calculated by a variety of prediction equations, overestimated the loss of fluid by a mean of between 86 and 100%. The lack of a fixed relationship between changes in impedance and changes in body water precludes the use of the impedance technique as a means of accurately assessing small fluid changes in this patient group.