A simple quantitative approach to analyzing the generation of the dysnatremias

Although the dysnatremias are the most common electrolyte disorders in hospitalized patients, the complexity of the parameters normally used to explain their generation mechanistically is often bewildering to medical students and experts alike. A number of methods have been utilized clinically to analyze retrospectively and predict prospectively the pathogenesis of these disorders. These approaches include the measurements of plasma and urine osmolality, free water clearance, electrolyte free water clearance, and tonicity balance.

All previous analyses are problematic in that they fail to incorporate mathematically in a single equation the known factors that account quantitatively for changes in the plasma water sodium concentration. In this paper, we have derived a simple formula for use at the bedside based on all known factors that can generate the dysnatremias. The formula incorporates (1) the known empirical relationship between the plasma water Na+ concentration, total body water (TBW), and exchangeable Na+ (Na+(e)) and K+ (K+(e)); (2) changes in mass balance of H2O (VMB) and Na+ + K+ (EMB); and (3) the effect of hyperglycemia.

This new equation, unlike all previous qualitative and quantitative approaches, can account mathematically for the simultaneous effects of TBW, Na(e), K(e), EMB, VMB, and the plasma glucose on the plasma water sodium concentration. Clinical examples are provided that demonstrate the utility of this new equation in analyzing the pathogenesis of the dysnatremias.

The conceptual simplification resulting from the use of this formula should significantly improve the current approaches used in analyzing the generation of the dysnatremias.