Role of potassium in hypokalemia-induced hyponatremia: lessons learned from the Edelman equation.

It is well known that changes in the mass balance of K+ can lead to an alteration in the plasma water sodium concentration ([Na+]pw). We have recently shown that based on the Edelman equation, the [Na+]pw is determined by the total exchangeable Na+ (Nae), total exchangeable K+ (Ke), total body water (TBW), osmotically inactive Nae and Ke, plasma water [K+], intracellular and extracellular osmotically active non-Na+ and non-K+ osmoles, and plasma osmotically active non-Na+ and non-K+ osmoles. In light of these findings, a re-analysis of the role of K+ in modulating the [Na+]pw is required in understanding the pathophysiology of hypokalemia-induced hyponatremia. In this article, we characterize the complex role of K+ in the pathogenesis of hypokalemia-induced hyponatremia using a three-compartment model and the known parameters in the Edelman equation. Our analysis indicates that K+ modulates the [Na+]pw by changing Ke in addition to the parameters in the y-intercept of the Edelman equation. Moreover, the magnitude of potassium-induced changes in the [Na+]pw is determined by the pathophysiologic mechanisms by which changes in Ke occur.