Exercise-Associated Hyponatraemia

Abstract

In 1958, Edelman and colleagues empirically showed plasma sodium concentration ([Na⁺]p) to be primarily a function of the sum of exchangeable sodium and potassium (E) divided by total body water (TBW). Based on Edelman’s equation, Nguyen and Kurtz derived an equation to show how [Na⁺]p changes as a function of TBW, change in TBW (ΔTBW), and change in the sum of exchangeable sodium and potassium (ΔE). Using the Nguyen-Kurtz equation, the present study examines the sensitivity of [Na⁺]p to these parameters: [Na⁺]p is very sensitive to ΔTBW and moderately sensitive to ΔE, and is modulated by TBW. For example, for a person with 50L TBW, a net increase of 1L water lowers [Na⁺]p by 3.2 mEq/L, but for a person with 25L TBW it lowers [Na⁺]p by 6.3 mEq/L (assuming initial [Na⁺]p is 140 mEq/L). In each case, a loss of 159 mEq of sodium plus potassium (roughly equivalent to 1.5 teaspoons of table salt) would be required to produce the same effect as the net increase of 1L water. The present review demonstrates why fluid overload predominates over electrolyte loss in the aetiology of exercise-associated hyponatraemia (EAH), and why the excretion of electrolyte-dilute urine is highly effective in correcting EAH (nonetheless, loss of sodium and potassium is significant in long events in warm weather). Sports drinks will, if overconsumed, result in hyponatraemia. Administration of a sports drink to an athlete with fluid overload hyponatraemia further lowers [Na⁺]p and increases fluid overload. Administration of either a sports drink or normal (0.9%) saline increases fluid overload.