Patient Encounter 5 Estimate the Anticipated Change in Serum Sodium

Estimate the anticipated change in serum sodium concentration after the infusion of 1 L of 3% NaCl in a 75-kg male with a serum sodium of 123 mEq/L (123 mmol/L).

Hypernatremia is a serum sodium concentration greater than 145 mEq/L (145 mmol/L) and can occur in the absence of a sodium deficit (pure water loss) or in its presence (hypotonic fluid loss).19 The signs and symptoms of hypernatremia manifest with a serum sodium concentration of greater than 160 mEq/L (160 mmol/L) and are usually the same as those found in TBW depletion: thirst, mental slowing, and dry mucous membranes. Signs and symptoms become more profound as hypernatremia worsens, with the patient eventually demonstrating confusion, hallucinations, acute weight loss, decreased skin turgor, intracranial bleeding, and/or coma. Many coexisting disorders and medications may complicate the diagnosis.

The classic causes of hypernatremia are associated with TBW depletion. These include dehydration from loss of hypotonic fluid from the respiratory tract or skin, decreased water intake, osmotic diuresis (e.g., mannitol, available as generic), and diabetes insipidus (e.g., decreased ADH; phenytoin, available as generic; lithium, available as generic). Hypernatremia in hospitalized patients occurs secondary to inappropriate fluid management in patients at risk for increased free water losses and

impaired thirst or restricted water intake. Iatrogenic hypernatremia is occasionally caused by the administration of excessive hypertonic saline. Treatment of hyper-natremia includes calculation of the TBW deficit followed by the administration of hypotonic fluids as previously described. The fluid volume should be replaced over 48 to 72 hours depending on the severity of symptoms and the degree of hyperton-

icity. For asymptomatic patients, the rate of correction should not exceed 0.5 mEq/

L/h (0.5 mmol/L/h). One rule of thumb is to replace half the calculated TBW deficit

2 19

over 12 to 24 hours and the other half of the deficit over the next 24 to 48 hours. ' Excessively rapid correction of hypernatremia may lead to cerebral edema and death. Patient Encounters 6 and 7 reinforce the concepts of calculating TBW deficit and expected changes in serum sodium concentration with therapy.

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