A new formula for predicting alterations in plasma sodium concentration in peritoneal dialysis.

Alterations in the plasma water sodium concentration ([Na+]pw) result from changes in the total exchangeable sodium (Na e), total exchangeable potassium (K e), and total body water (TBW). The empirical relationship between the [Na+]pw and Na e, K e, and TBW was originally demonstrated (Edelman IS, Leibman J, O'Meara MP, and Birkenfeld LW. J Clin Invest 37: 1236-1256, 1958), where [Na+]pw = 1.11(Na e + K e)/TBW - 25.6 (Eq. 1). Based on Eq. 1, alterations in the [Na+]pw can be predicted by considering changes in the mass balance of Na+, K+, and H2O. In accounting for the mass balance of Na+, K+, and H2O in patients on peritoneal dialysis, considerations must also be taken to determine the modulating effect of dialysate clearance of Na+ and K+ and fluid changes resulting from this therapeutic modality on the [Na+]pw. In this article, we derive a new formula for predicting alterations in the plasma Na+ concentration ([Na+]p) in patients on peritoneal dialysis, taking into consideration the empirical relationship between the [Na+]pw and Na e, K e, and TBW (Eq. 1) as well as changes in mass balance of Na+ + K+ and H2O.