Comparison between two solute equations and bioimpedance for estimation of body fluid volumes.

Background: The extracellular volume (ECV) and intracellular volume (ICV) estimated by bioimpedance analysis (BIA) deviates markedly from the textbook volumes of 20% and 40% of the body weight (BW). We estimated the transcellular exchange of water by calculating solute equilibriums after fluid challenges to examine whether the BIA or the textbook volumes are likely to be most correct.

Methods: Data was retrieved from 8 healthy male volunteers who received 25 mL/kg of Ringer's solution or 3-5 mL/kg of hypertonic (7.5%) saline over 30 min after the ECV and ICV had been estimated by BIA. The exchange of water between the ECV and the ICV was calculated according to a sodium equation and an osmolality equation. Simulations were performed, where deviating body fluid volumes were applied.

Results: The mean ECV measured with BIA was 24.9% of BW (p < 0.05 versus the "textbook" volume). Mean ICV measured with BIA was 22.3% of BW (p < 0.05). The sodium and osmolality equations correlated closely with respect to the translocation of water across the cell membrane (r = 0.86). By applying the "textbook" ECV, the sodium equation indicated that Ringer's solution exchanged negligible amounts of water, while hypertonic saline withdrew 1.4 L from the ICV to the ECV. By contrast, applying the BIA-derived ECV to the sodium equation implied that 3 L of water would be translocated from the ECV to the ICV once hypertonic saline was administered.

Conclusion: The "textbook" ECV and ICV volumes but not the BIA-derived volumes were consistent with the fluid shifts obtained by two solute equations.