Edelman Revisited: Concepts, Achievements, and Challenges

Abstract

The key message from the 1958 Edelman study states that combinations of external gains or losses of sodium, potassium and water leading to an increase of the fraction (total body sodium plus total body potassium) over total body water will raise the serum sodium concentration ([Na]_S), while external gains or losses leading to a decrease in this fraction will lower [Na]_S. A variety of studies have supported this concept and current quantitative methods for correcting dysnatremias, including formulas calculating the volume of saline needed for a change in [Na]_S are based on it. Not accounting for external losses of sodium, potassium and water during treatment and faulty values for body water inserted in the formulas predicting the change in [Na]_S affect the accuracy of these formulas. Newly described factors potentially affecting the change in [Na]_S during treatment of dysnatremias include the following: (a) exchanges during development or correction of dysnatremias between osmotically inactive sodium stored in tissues and osmotically active sodium in solution in body fluids; (b) chemical binding of part of body water to macromolecules which would decrease the amount of body water available for osmotic exchanges; and (c) genetic influences on the determination of sodium concentration in body fluids. The effects of these newer developments on the methods of treatment of dysnatremias are not well-established and will need extensive studying. Currently, monitoring of serum sodium concentration remains a critical step during treatment of dysnatremias.

Keywords: dysnatremia; hydrophilic compounds; hypernatremia; hyponatremia; osmotic sodium inactivation.

Figure 1

Effects of isolated decreases in total body sodium or total body potassium, and isolated increase in total body water on sodium concentration in serum water and on intracellular and extracellular volumes. Ordinates. sodium concentrations ([Na]). Abscissae: volumes. I: baselines state. II: state after an isolated change. ICV, intracellular volume; ECV, extracellular volume; ICV + ECV = TBW. Product of ordinate and abscissa: amount of monovalent cation in intracellular or extracellular compartment. II, A: decrease in TBNa, which results in hyponatremia, increase in ICV and decrease in ECV. B: decrease in TBK resulting in hyponatremia, decrease in ICV and increase in ECV. C: gain in TBW, which results in hyponatremia and increase in both ICV and ECV, so that their ratio remains the same as in the normonatremic state. Changes in [Na], ICV and ECV are dictated by the Edelman formula, the osmotic principle, and the principle of body water distribution. Hypernatremia secondary to isolated sodium gain is associated with ICV contraction and ECV expansion. Hypernatremia secondary to isolated water loss is associated with proportional ICV and ECV contraction, so that the ICV:ECV ratio is the same as in the normonatremic state. Osmolyte changes for preservation of the normonatremic ICV were not considered in this figure. In clinical practice, isolated changes in one of the components of the Edelman formula are infrequently encountered.

Figure 2

Potential influences on the magnitude of the change in sodium concentration in serum water when there is a change in the fraction (total body sodium plus total body potassium) over total body water. Δ(TBNa + TBK)/TBW: change in the fraction (total body sodium plus total body potassium) over total body water; Δ[Na]_SW: change in sodium concentration in serum water.