Edelman's equation is valid in acute hyponatremia in a porcine model: plasma sodium concentration is determined by external balances of water and cations

Abstract

Acute hyponatremia is a serious condition, which poses major challenges. Of particular importance is what determines plasma sodium concentration ([Na⁺]). Edelman introduced an explicit model to describe plasma [Na⁺] in a population as [Na⁺] = α·(exchangeable Na⁺+ exchangeable K⁺)/(total body water) − β. Evidence for the clinical utility of the model in the individual and in acute hyponatremia is sparse. We, therefore, investigated how the measured plasma [Na⁺] could be predicted in a porcine model of hyponatremia. Plasma [Na⁺] was estimated from in vivo-determined balances of water, Na⁺, and K⁺, according to Edelman's equation. Acute hyponatremia was induced with desmopressin acetate and infusion of a 2.5% glucose solution in anesthetized pigs. During 480 min, plasma [Na⁺] and osmolality were reduced from 136 (SD 2) to 120 mmol/l (SD 3) and from 284 (SD 4) to 252 mosmol/kgH₂O (SD 5), respectively. The following interpretations were made. First, Edelman's model, which, besides dilution, takes into account Na⁺and K⁺, fits plasma [Na⁺] significantly better than dilution alone. Second, a common value of α = 1.33 (SD 0.08) and β = −13.04 mmol/l (SD 7.68) for all pigs explains well the plasma [Na⁺] in the individual animal. Third, measured exchangeable Na⁺and calculated exchangeable Na⁺+ K⁺per weight in the pigs are close to Edelman's findings in humans, whereby the methods are cross-validated. In conclusion, plasma [Na⁺] can be explained in the individual animal by external balances, according to Edelman's construct in acute hyponatremia.