Are the total exchangeable sodium, total exchangeable potassium and total body water the only determinants of the plasma water sodium concentration?

The dysnatraemias are common electrolyte disorders encountered in hospitalized patients [1]. The pathophysiology and approach to management of these electrolyte disorders have been well characterized. It is well known that the total exchangeable sodium (Nae), total exchangeable potassium (Ke) and total body water (TBW) are the major determinants of the plasma water sodium concentration ([Na]pw) [2]. Changes in the mass balance of Na, K and water, therefore, determine the quantitative impact on the [Na]pw. Several formulas (sodium deficit equation, water deficit equation, Androgue–Madias equation and Barsoum– Levine equation) have been derived to help predict the changes in plasma Na concentration ([Na]p) following a therapeutic manoeuvre [3–8]. However, although Edelman et al. [2] demonstrated that the plasma water sodium concentration is equal to 1.11(NaeqKe)uTBW 25.6, the significance of the y-intercept ( 25.6) in this equation has never been evaluated. Indeed, previous analyses of the pathogenesis and treatment of the dysnatraemias have failed to consider the quantitative and biological significance of the y-intercept in this equation. In this article, we show quantitatively the necessity for the y-intercept and its physiologic and clinical significance. Our analysis demonstrates that there are several determinants of the y-intercept which independently alter the [Na]pw: the osmotically inactive exchangeable Na and K, the plasma water [K] ([K]pw) and the osmotically active non-Na and non-K osmoles. Importantly, changes in these parameters determine the magnitude of hyponatraemia in hyperglycaemia. Although the primary determinants of the [Na]pw are the Nae, Ke and the TBW, a quantitative understanding of the pathogenesis of the dysnatraemias requires that the determinants of the y-intercept in the Edelman equation are not ignored.