Disparity in Molar Conductivity Among Ions: Implications for Conductivity Monitoring in Hemodialysis

BACKGROUND: Conductivity monitoring during hemodialysis allows for online estimation of patient's natremia and enables the hemodialysis machine to automatically adjust the dialysate sodium. It is based on the premise that the conductivity is proportional to sodium concentration or, more precisely, to ionic osmolality. Given the actual disparity in molar conductivity among ions, this premise is not entirely justified, and it is important to consider its implications. METHODS: Explain why the disparity in molar conductivity: (1) results in a discrepancy between the plasma conductivity calculated from conductivity measurements in the dialysate or the ultrafiltrate and its actual value; (2) affects the accuracy of software designed to automatically adjust the dialysate sodium. RESULTS: (1) Despite a part of cations (primarily sodium) retained in plasma due to the Gibbs-Donnan effect, the ionic osmolalities of the dialysate at equilibrium with plasma and of the ultrafiltrate are virtually equal to that of plasma. (2) Plasma conductivity calculated from conductivity measurements in the dialysate or the ultrafiltrate is higher than its actual value. (3) Conductivity kinetic models carry a risk of inaccuracy. CONCLUSION: Since software designed to automatically adjust the dialysate sodium is based on the inaccurate assumption that conductivity is strictly proportional to ionic osmolality, corrective adjustments are necessary.