Sodium-free water clearance in hemodialysis.

Water shift between intra- and extracellular body compartments is under osmotic pressure driving forces. Assuming that the cellular membrane is impermeable to Na, any change in extracellular Na concentration induces an osmotic pressure difference between the two compartments and results in water transfer in order to equilibrate osmotic pressures between body compartments. Ultrafiltration per se2-4 does not induce any change in plasma Na concentration. Thus, no water transfer occurs between the body compartments. Hemodialysis performed with a Na concentration in the dialysate different from that of the plasma induces a water flux between intra- and extracellular spaces. When using a closed batch dialysate delivery system, it becomes possible to measure the Na mass balance and then to get an accurate pattern of Na and water transfer between the body and the batch. First I would like to describe the Na-free water clearance concept. Then I will show how the increase of Na-free water clearance explains the tolerance of BW and blood pressure to high Na concentration in the dialysate.