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OBJECTIVE To determine whether the segmental multifrequency bioelectrical impedance analysis may improve the prediction for intraoperative changes in extracellular water volume (deltaECW) compared with whole body multifrequency bioelectrical impedance analysis in abdominal surgical patients. DESIGN Prospective, consecutive sample. SETTING Surgical operative patients in a university-affiliated city hospital. PATIENTS Thirty patients who underwent elective gastrointestinal surgery. INTERVENTIONS Multifrequency bioelectrical impedance analysis was conducted preoperatively (before the induction of anesthesia) and postoperatively (after recovery from anesthesia). Resistance values fitted at zero frequency (R0) in the whole body and in each body segment (arm, trunk, and leg) were determined by performing nonlinear curve-fitting and subsequent extrapolation. DeltaECW values were estimated from the whole body resistance between wrist and ankle using two different prediction formulas. In segmental multifrequency bioelectrical impedance analysis, however, ECW was obtained as the sum of each body segment (arms, trunk, and legs) using the equation newly derived from the cell suspension theory. DeltaECW estimated from both measurements were compared with net fluid balances during surgery. MEASUREMENTS AND MAIN RESULTS R0 in whole body and all body segments significantly decreased after surgery (p < .0001). The most striking decrease in post/preoperative ratios was found in the R0 in the trunk. The post/preoperative ratio of the R0 value in the trunk was significantly lower than the post/preoperative ratio of the R0 value in the leg (p = .0007). DeltaECW from segmental multifrequency bioelectrical impedance analysis was similar to net fluid balance (r2 = .80, bias = -0.03 L), whereas whole body multifrequency bioelectrical impedance analysis resulted in considerable underestimations of deltaECW (r2 = .50, .51, bias = 0.95, 0.53 L). CONCLUSIONS The difference in the prediction of deltaECW between whole body and segmental multifrequency bioelectrical impedance analysis may be explained by the significant decrease in the resistance of the trunk, which contributed only minimally to the whole body resistance. Segmental multifrequency bioelectrical impedance analysis provides a better approach to predict ECW changes in critically ill patients with nonuniform fluid distribution.