Sodium removal and plasma tonicity balance are not different in hemodialysis and hemodiafiltration using high-flux membranes

The clinical benefits of on-line hemodiafiltration (HDF) versus high-flux membranes hemodialysis (hf-HD) are still debated. In fact, although a superiority of one treatment over the other, especially in terms of mortality, did not emerge from the analysis of clinical trials, improved intradialytic vascular stability and cardiovascular mortality have been observed in patients undergoing HDF rather than hf-HD; the lower removal of sodium (Na⁺) during HDF seems to play a major role. The plasma concentration of Na⁺ is the major determinant of plasma tonicity, which, by determining the flow of water between the intracellular and the extracellular compartment, contributes to the vascular refilling process and the maintenance of blood pressure during the hemodialysis treatment. Plasma tonicity also depends on plasma glucose concentration, especially in patients with diabetes mellitus with hyperglycaemia at the start of hemodialysis treatment.

We evaluated the removal of Na⁺ and plasma tonicity balance during a 2-week period by performing 2–3 consecutive sessions of hf-HD followed by 2–3 consecutive sessions of HDF, or vice versa, in 47 patients (40% diabetics) on chronic hemodialysis. Identical parameters were used in all dialytic sessions.

Na⁺ removal per session was − 224 ± 144 mmol and − 219 ± 152 mmol, respectively, in hf-HD and in HDF (p = 0.79). The plasma tonicity balance per session was − 575 ± 310 mOsm and − 563 ± 328 mOsm, respectively, in hf-HD and in HDF (p = 0.75).

The removal of Na⁺ and plasma tonicity balance did not differ between hf-HD and HDF. This observation suggests that factors other than those assessed in our study might explain the improved cardiovascular stability reported in HDF.