Disorders of body water homeostasis in critical illness

Disorders of sodium and water homeostasis are among the most commonly encountered disturbances in the critical care setting, because many disease states cause defects in the complex mechanisms that control the intake and output of water and solute. Because body water is the primary determinant of extracellular fluid osmolality, disorders of body water balance can be categorized into hypoosmolar and hyperosmolar disorders depending on the presence of an excess or a deficiency of body water relative to body solute. Because the main constituent of plasma osmolality is sodium, hypoosmolar and hyperosmolar disease states are generally characterized by hyponatremia and hypernatremia, respectively. After a brief review of normal water metabolism, this article focuses on the diagnosis and treatment of hyponatremia and hypernatremia in the critical care setting.