Abstract

We tested the hypothesis that bioelectrical resistance and reactance are indices of total body water (TBW) and extracellular water (ECW), respectively. Infants less than 2500 g in birth weight were studied while less than 24 h old and at 4-7 d of age. TBW (n = 32) was measured by H2(18)O dilution and ECW (n = 34), by bromide dilution. Measurements were made by using an impedance plethysmograph from each of four electrode pairs. Bioelectrical resistance correlated inversely with TBW (r = -0.683). When body weight and crown-heel length were factored into the equation, the correlation improved (r = 0.953). Bioelectrical reactance correlated inversely with ECW (r = -0.707). When body weight and surface area were factored into the equation the correlation improved (r = 0.882). We conclude that bioelectrical resistance and reactance are good indices of TBW and ECW, respectively. However, the ability to resolve small changes in body-water compartments when confined to the intraluminal or other third spaces remains questionable, particularly for single subjects.