Hypernatremia is a relatively common problem which stems from one of two basic mechanisms: either by administration of hypertonic sodium solutions or, in almost all cases, by loss of free water. Persistent hypernatremia does not occur in a normal subject due to the increase in osmolality stimulating both ADH release and more importantly, thirst. Thus, hypernatremia ocurs in those who cannot express thirst normally, infants and adults with impaired mental status. If a person is alert but not thirsty with a serum sodium >150, by definition, a hypothalamic lesion affecting the thirst center is present.
Various etiologies include the following:
- Insensible and sweat losses
- fever, exercise, exposure to high temperatures
- G.I. losses
- osmotic diarrheas
- Central or nephrogenic diabetes insipidus
- Osmotic diuresis
- glucose, mannitol
- Hypothalamic lesions
- tumors, granulomatous diseases (Sarcoidosis) or vascular diseases
To calculate the actual water deficit use the following formula:
Water Deficit = 0.6(Wt. in kg)(serum Na/140 -1)
So for our patient who weighed 70 kg and Serum Na=160:
- Total Water Deficit=0.6(70)(160/140 -1)
Overly rapid correction is potentially dangerous in hypernatremia. Within 1-3 days, the brain volume which is initially contracted, is largely restored due both to water movement from the CSF and uptake of solutes from cells. Rapidly correcting the sodium level may cause osmotic water movement into the brain. The resultant cerebral edema can then lead to seizures, permanent neurologic damage or death. This has been primarily described in children in which neurologic deficits occured when correction rate exceeded 0.7 meq/L/hour and no deficits occurred when corrected <0.5 meq/L/hour. In this specific example, given the patient is tachycardic and hypotensive, we would start with NS to replenish his intravascular space. Once his BP rises and HR improves, he can then be switched to D5W to replace his free water deficit.
So for our example, the 20 meq/L rise should be corrected over (20/0.5)=40 hours. Since the patient has a 6 liter water deficit, 6000 ml/40 hours = 150 ml/hour IV rate. Remember this doesn't include any continued free water losses which must also be replaced (approximately 40ml/h). Be sure to carefully monitor the serum sodium to confirm that this calculated rate gives you the the desired rate of correction.
- Rose BD. Clinical physiology of acid-base and electrolyte disorders, 4th ed, McGraw-Hill, New York, 1994, pp.698-709.
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