Identifying body fluid distribution by measuring electrical impedance.

The effects of critical illness on extracellular water (ECW) and total body water (TBW) were measured using (1) a multiple dilutional technique, and (2) whole body and regional bioelectrical impedance analysis (BIA) in a group of stable patients. Total body water and body resistance (R) were similar in patients when compared with normal healthy subjects (TBW: 45.1 +/- 4.5 vs. 46.2 +/- 3.4 L, p = 0.85; R: 518 +/- 42 vs. 500 +/- 22 omega, p = 0.70), and a significant relationship was present between these measurements (r = -0.87, p < 0.001). However, patients demonstrated an increase in ECW compared with controls (ECW: 18.6 +/- 1.3 vs. 14.7 +/- 1.1 L, p < 0.05). Expanded ECW values were associated with diminished electrical reactance (Xc) values (38 +/- 6 vs. 70 +/- 4 omega, p < 0.001) and these values were correlated (r = -0.67, p < 0.005). The ratio of Xc to R determined across the body and each of the segments was significantly lower in patients compared with controls (at least p < 0.005) and this ratio measured across a leg was the most sensitive predictor of health (Xc/R > or = 0.137) and disease (Xc/R < or = 0.101). Bioelectrical impedance analysis is a noninvasive and simple bedside technique that can be used to predict TBW and identify altered fluid distribution following critical illness.