Hypernatremia is strictly defined as serum [Na+] greater than 150 meq/L. The most frequent cause of hypernatremia is a decrease in TBW secondary to either reduced intake or excessive loss. The main defense against hypernatremia is likely thirst. A 2 percent increase in plasma osmolality should stimulate thirst and thereby increase free water intake. The problem arises when patients, such as those in a coma or those who are bedridden, are unable to obtain adequate fluid. All hypernatremic states are hyperosmolar (TaMe,...23.:5).
TABLE 23-5 Etiologic Classification of Hypernatremia
Clinical manifestations are much more severe with acute rapidly developing hypernatremia than with chronic hypernatremia. In the elderly and in infants (two groups at great risk), the morbidity and mortality rates are high when [Na+] is greater than 160 meq/L. Symptoms usually occur when plasma osmolality exceeds 350 mosm/kg of water. Neurologic dysfunction predominates (Table.23-6).
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