Figure 46

Solute sieving. A, Dialysate sodium concentration is initially reduced and tends to return to baseline later during a long dwell exchange of 6 to 8 hours. B, Dialysate sodium concentration decreases, particularly when using 4.25% dextrose dialysis solution, because of the sieving phenomenon. Removal of water during ultrafiltration unaccompanied by sodium, in proportion to its extracellular concentration, is called sodium sieving [7,12]. The peritoneum offers greater resistance to the movement of solutes than does water. This probably relates to approximately half the ultrafiltrate being generated by solute-free water movement through aquaporins channels. Therefore, ultrafiltrate is hypotonic compared with plasma. Dialysate chloride is also reduced below simple Gibbs-Donnan equilibrium, particularly during hypertonic exchanges. Patients with a low peritoneal membrane transport type tend to reduce dialysate sodium concentration more than do other patients. Therefore, during a short dwell exchange of 2 to 4 hours, net electrolyte removal per liter of ultrafiltrate is well below the extracelluar fluid concentration. As a result, severe hypernatremia, excessive thirst, and hypertension may develop. This hindrance can be overcome by lowering the dialysate sodium concentration to 132 mEq/L. In patients who use cyclers with short dwell exchanges and who generate large ultrafiltration volumes, lower sodium concentrations may need to be used (such as 118 mEq/L for 2.5% glucose solutions or 109 mEq/L for 4.25% solutions). In continuous ambulatory peritoneal dialysis with long dwell exchanges of 6 to 8 hours, significant sieving usually does not occur, whereas in automated peritoneal dialysis with short dwell exchanges, sieving may occur. Sieving predisposes patients to thirst and less than optimum blood pressure control, especially in those who have low-normal serum sodium levels, those with low peritoneal membrane transporter rates, or both. (From Nolph and coworkers [10]; with permission.)

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