Determination of extracellular fluid volume using impedance measurements

Currently, accurate determination of body fluid compartments depends on the sodium bromide method (NaBr), an invasive measurement requiring venipuncture with infusion of a foreign substance. Impedance (Z) measurements may provide a practical noninvasive alternative for estimating fluid compartments in sick, premature neonates. To validate the impedance method, we compared it with the NaBr technique in nine anesthetized rabbits. Electrodes were placed for impedance measurement. Vascular catheters were inserted into the femoral artery and vein. Baseline impedance data were collected at 1.0 kHz and blood samples were drawn for NaBr standard assay. Using conventional assay techniques for determination of extracellular fluid volume (ECFV), we correlated NaBr data with impedance measurements. A linear relationship between ECFV by NaBr assay and the previously developed impedance equation rho L2/Z1.0 was established using regression analysis. A correlation value of r = .95 was obtained. These data suggest the potential for impedance to estimate ECFV.