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OPTIMUM ELECTROLYTE COMPOSITION OF A DIALYSIS SOLUTION

Department of Nephrology, Lund University, University Hospital of Lund, Lund, Sweden

Correspondence to: Bengt Rippe, Department of Nephrology, Lund University, University Hospital of Lund, S-211, 85 Lund, Sweden.
Bengt.Rippe{at}med.lu.se

In patients undergoing peritoneal dialysis (PD) for end-stage renal failure, the optimum electrolyte composition of a dialysis solution is that which best serves the homeostatic needs of the body. Comparing the transperitoneal removal of electrolytes by conventional PD solutions (CPDSs) with that by normal kidneys, it is evident that peritoneal removal is in the lower range of what can be considered "normal." Given the electrolyte composition of CPDSs and a total dwell volume of 4 exchanges of 2 L each, approximately 90 mmol NaCl, 40 mmol K⁺, 10 - 15 mmol HPO₄^- and 1 - 2 mmol Ca²⁺ can be removed daily [plus 1 L ultrafiltration (UF)]. Na⁺, Ca²⁺, and Mg²⁺ are supplied in CPDSs in concentrations close to their plasma concentrations, which makes their removal almost entirely dependent on UF. In UF failure (UFF), plasma levels of the foregoing ions will tend to rise, producing a higher diffusion gradient to compensate for their defective UF removal. Peritoneal removal of HCO₃^-, HPO₄^-, and K⁺ are usually quite efficient because of the zero CPDS concentrations of these ions. Approximately 150 mmol HCO₃^- is lost daily with CPDSs, compensated for by the addition of 30 - 40 mmol/L lactate, or, with the use of multi-compartment bags, bicarbonate instead. However, a mixture of bicarbonate and lactate should be preferred as a buffer, to avoid intracellular acidosis from high levels of pCO₂ in the dialysis fluid. For patients on continuous ambulatory peritoneal dialysis (CAPD) without UFF and with some residual renal function, PD fluid concentrations of Na⁺ 130 - 133 mmol/L, Ca²⁺ 1.25 - 1.35 mmol/L, and Mg²⁺ 0.25 - 0.3 mmol/L seem appropriate. With reduced UF after a few years of PD, the removal of fluid and electrolytes often becomes deficient. Dietary salt restriction can be prescribed, but it is hard to implement. The use of low-Na⁺ solution (LNa) is a potential alternative. The reduction in osmolality resulting from Na⁺ removal in LNa should preferably be compensated by the addition of glucose (G). In a recent study, a regimen including 1 LNa exchange daily (Na⁺ 115 mmol/L) in a G-compensated solution showed very promising effects on blood pressure and fluid status. However, large-scale randomized controlled studies have to be performed to definitively settle the role of LNa in volume-overloaded patients.

KEY WORDS: Low-sodium solutions; bicarbonate solutions; calcium; mathematical modeling; three-pore model.

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