Loss of total body water and extracellular water assessed by multifrequency impedance

Fat-free mass (FFM), total body water (TBW), extracellular water (ECW) and bioelectrical impedance at 15 selected frequencies, ranging from 1 kHz to 1350 kHz, were determined in 12 subjects before and 3 h after the use of a diuretic drug to induce loss of ECW. Weight loss and urine loss were recorded, and these were assumed to be equal to loss of FFM and TBW, respectively. From the excretion of sodium the amount of excreted ECW was calculated. Prediction formulae at all frequencies were developed for FFM, TBW and ECW from impedance data before the use of the diuretic drug. With these formulae the changes in FFM, TBW and ECW were predicted and compared with the losses of body weight, urine and ECW. At all frequencies FFM, TBW and ECW could be well predicted, with comparable values of explained variance and standard errors of estimate. However, the losses in FFM and TBW were significantly overestimated using the low-frequency impedance formulae. At higher frequencies the predicted losses were no longer significantly different from the weight and urine losses, respectively. The change in ECW was only adequately predicted at 1 kHz. Prediction formulae for ECW at higher frequencies underestimated the loss of ECW. It is concluded that multifrequency impedance offers the possibility of distinguishing between body fluid compartments and of assessing changes in body fluid compartments with a reasonable error.