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

Abstract

In summary, although Nae, Ke, and TBW are the major determinants of the [Na+]pw, we now demonstrate that additional physiologic parameters must also be considered. These parameters are the components of the γ-intercept in Edelman's equation and include the quantity of osmotically inactive exchangeable Na+ and K+, the [K+]pw, and the osmotically active, non-Na+ and non-K+ osmoles. Since total body water is passively distributed in proportion to osmotic activity, the fact that some of the Nae and Ke, are bound and are osmotically inactive must be taken into account in the determination of the [Na+]pw. Moreover, we show that the [K+]pw, and intracellular and extra-cellular osmotically active, non-Na+ and non-K+ osmoles also determine the distribution of water between the plasma, interstitial (extracellular) fluid, and intracellular spaces. In patients with hyperglycaemia, the correction factor of 1.6 results from the net change in the [K+]pw and concentration of extracellular osmotically active, non-Na+ and non-K+ osmoles. Since hyperglycaemia also induces changes in the mass balance of Na+, K+ and TBW, the plasma sodium concentration can be predicted from the following equation: [Na+]p=1.03(Nae+Ke)/TBW-23. 8-(1.6/100)([glucose]-120). By considering all of these factors, clinicians will have a more complete and quantitative understanding of the determinants of the [Na+]pw, and the pathogenesis and treatment of the dysnatraemias.