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

Nguyen, Minhtri K., and Ira Kurtz. A new formula for predicting alterations in plasma sodium concentration in peritoneal dialysis. Am J Physiol Renal Physiol 288: F1113 - F1117, 2005; doi: 10.1152/ ajprenal. 00387.2004. - Alterations in the plasma water sodium concentration ([ Na (+)] pw) result from changes in the total exchangeable sodium ( Na-e), total exchangeable potassium ( Ke), and total body water ( TBW). The empirical relationship between the [ Na (+)] pw and Nae, Ke, 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 Nae, Ke, and TBW ( Eq. 1) as well as changes in mass balance of Na+ + K (+) and H2O.