SODIUM MODELING IN HEMODIAFILTRATION

A computer model was developed to simulate sodium and water kinetics during hemodiafiltration (HDF), acetate-free biofiltration (AFB) and hemodialysis (HD). Multiple regression analysis of the results of 3,240 simulated applications of the model (1,620 HDF, 1,080 AFB, 540 HD) showed that, during HDF and AFB, there is a close correlation (R2 = 0.92 and 0.91) between plasma water sodium concentration ([Na+P]) and a set of three variables: 1) the sodium gradient between plasma water and dialysate, 2) the sodium concentration of the substitution fluid and 3) ultrafiltration (UF) rate. With HD, a close correlation (R2 = 0.94) was found between changes in [Na+P] and combined changes in sodium gradient and the UF rate. On this basis, a regression equation was formulated for each procedure which allowed a reliable prediction of final [Na+P] to be made on the basis of knowledge of the imposed Na gradient, the programmed infusion (during HDF and AFB), and the U F rate. Clinical validation of the model was obtained in 12 patients: predicted final [Na+P] agreed well with the values measured by means of direct potentiometry (141.9 vs. 142.1 mEq/liter; P = NS), with a mean difference (-0.16 mEq/liter) and limits of agreement (+0.8 to -1.03 mEq/liter) fully acceptable for clinical purposes. During HD, isonatricity was maintained by imposing a mean sodium gradient of 6.8 mEq/ liter, whereas, during HDF and AFB, (HDF performed with infusion of 49 to 66 ml/min of a solution containing 140 to 145 mEq/liter of Na, and AFB with a solution containing 145 or 167 mEq/liter of Na infused at the rate of 28.5 to 41 ml/min) the mean gradients required to avoid (Na+P] changes were 9.5 and 14.6 mEq/liter, respectively. The sodium infused with the substitution fluid, and the Donnan effect related to the high imposed UF rates, led to progressive sodium retention, which had to be counterbalanced by diffusion, increasing the transmembrane sodium gradient, in order to reach the target sodium and water balance and reduce the risk of untoward cardiovascular side-effects.