Evaluation of plasma sodium concentration during hemodialysis by computerization of dialysate conductivity

Kinetics modeling based on sodium mass balance and changes in conductivity at the dialysate outlet compared to that at the dialysate inlet, led to predicting the plasma water conductivity of the blood inlet. Conductivity transducers, with temperature compensation, located at the dialysate inlet and outlet ports of the dialyzer and connected to a conductimeter, were interfaced with a portable computer for data acquisition. In 38 dialysis sessions, a close relationship was found between estimated plasma water conductivity (CdBi) and measured plasma sodium concentration [Na]Bi, according to the formula: CdBi = 0.101[Na]Bi + 0.37 (n = 38; r = 0.922). The study shows that plasma sodium concentration at the dialyzer inlet could be evaluated, with a standard error of +/- 1.5 mEq/L, by continuous measurement of conductivity gradient between dialysate inlet and outlet.