Disorders of Water Balance: Hypernatremia

Abstract

Hypernatremia is defined as a serum or plasma [Na+] >{\thinspace}145 mEq/L and hyperosmolality (serum osmolality >{\thinspace}295 mOsm/kg H2O). Serum [Na+] is determined by total body Na+, K+, and water content: Serum [Na+] = (Nae+{\thinspace}+Ke+)/TBW, where Nae+ and Ke+ are total exchangeable quantities of these cations, and TBW is total body water. Therefore, hypernatremia can develop by a deficit in total body water and/or a gain of Na+. In a healthy individual, an increase in serum [Na+] and associated hyperosmolality create thirst, and water intake lowers serum [Na+] to a normal level (Chap. 11). Hypernatremia develops when patients cannot experience or respond to thirst, and have no access to water.