Hypernatremic Disorders in the Intensive Care Unit

Abstract

Hypernatremia, defined as plasma sodium concentration >145 mEq/L, is frequently encountered in critically ill patients admitted to the intensive care unit (ICU). Hypernatremia indicates a decrease in total body water relative to sodium and is invariably associated with plasma hyperosmolality though total body sodium content may be normal, decreased, or increased. Hypernatremia usually occurs as a result of impaired thirst or access to water, with or without increased water losses from renal and extrarenal sources. Critically ill patients in ICU are at high risk of hypernatremia because of their inability to control free water intake as a result of sedation, intubation, change in mental status, and fluid restriction for various other reasons. In addition, excessive fluid losses from various renal or nonrenal sources and treatment with sodium containing fluids are commonly encountered in this population, predisposing them to hypernatremia. The consequences of hypernatremia result from osmotic movement of water across the cell membrane, leading to primarily intracellular and variable degree of extracellular volume depletion. The clinical features depend on severity and rapidity of hypernatremia development with abnormal cognitive and neuromuscular function in many cases and potential risk of hemorrhagic complications or death from vascular stretching and rupture in advanced cases. The management of hypernatremia focuses on judicious replacement of free water deficit to restore normal plasma osmolality as well as identification and correction of underlying causes of hypernatremia. Electrolyte-free water replacement is the preferred therapy though electrolyte (sodium) containing hypotonic fluids can also be used in some circumstances. Oral free water replacement guided by thirst is ideal though parenteral fluid replacement is usually necessary in critically ill ICU patients. Various calculations for estimating free water deficit are available and any can be used to guide initial fluid replacement therapy. Rate of correction depends on rapidity of hypernatremia development, though frequent monitoring of plasma sodium levels is essential to ensure appropriate response and to adjust the rate of fluid replacement to prevent the risk of cerebral edema from rapid correction of chronic hypernatremia. Free water requirements should be routinely assessed in ICU patients and judicious electrolyte and free water replacement prescribed for those at risk of hypernatremia.