Fluid volumes determination by impedance spectroscopy and hematocrit monitoring: application to pediatric hemodialysis.

A method for extracting fluid volumes from multifrequency bioimpedance, which takes into account the body geometry and the presence of nonconducting elements, was tested on 12 young dialyzed patients against correlations for total body water volumes (TBW) from Watson et al. and Humes et al. Our calculations of TBW from impedance were found to overestimate Humes' values by 0.25 L (0.8%) postdialysis and by 2.08 L (6.5%) predialysis. Extracellular water (ECW) was found to contribute an average of 93% of ultrafiltered volume. Intracellular water volume (ICW) determination from impedance was found to be too imprecise to predict its variation during dialysis; therefore, ICW variations were calculated as the difference between ultrafiltration and ECW changes. The continuous recording of hematocrit by an optical device monitored changes in plasma and interstitial volumes. In most cases, ultrafiltration was compensated mainly by a contribution from interstitial fluid, and the drop in plasma volume was generally moderate.