Diameter of inferior caval vein (VCD) and bioelectrical impedance analysis (BIA) for the analysis of hydration status in patients on hemodialysis

Background: Vena cava diameter (VCD) measurement is an accepted method to evaluate hydration status in patients on hemodialysis. Bioelectrical impedance analysis (BIA) is a less laborious method to assess hydration variables and more suitable for routine patient care. However, BIA has not yet been validated in dialysis patients. We investigated whether BIA can replace VCD in patients on hemodialysis. Methods: In 20 stable hemodialysis patients [age (+/- SD): 47 +/- 17 yrs, dialysis duration (+/- SD): 76 +/- 59 months] hydration status was evaluated by VCD. Impedance variables such as resistance, reactance and phase angle were provided by BIA. They were used to calculate intracellular water (ICW), extracellular water (ECW) and total body water (TBW). Results: VCD did not correlate with TBW-BIA, but correlated with ECW/TBW (r = 0.46; p < 0.025), ECW/m(2) (r = 0.42; p < 0.005) and ICW/ECW (r = -0.49; p < 0.005). Hemodialysis decreased TBW with 2.7 +/- 1.91. The difference in ECW before and after dialysis (8.9 +/- 1.3 and 7.1 +/- 1.41, respectively) was significant (p = 0.001). The same did not hold true for ICW(13.3 +/- 1.4 and 13.1 +/- 1.41). Major underhydration (n = 9; VCD <6.5 mm/m(2)) revealed sharp limits for ICW/ECW (>1.80) and ECW/TBW (< 0.35), whereas these BIA-variables were significantly (p < 0.005) different from those in minor underhydration (n = 8; 6.6 < VCD < 8.0 mm/m(2)), normohydration (n = 15; 8 < VCD <11.5 mm/m(2)) and overhydration (n = 7; VCD >11.5 mm/m(2)). Conclusion: BIA can replace VCD only in major underhydration (VCD <6.5 mm/m(2)). Fluid loss during hemodialysis is caused by a decrease of ECW, compatible with the postulation that excess fluid volume is carried by the extracellular compartment.