When and how fast should hypertonic saline be given?
If the ratio of the sum of urine potassium and sodium to plasma sodium is greater than 1 in a patient with SIADH, infusion of normal saline will likely lead to worsening hyponatremia as the NaCl is excreted in a smaller, more concentrated volume of urine and the remaining electrolyte-free water is then retained. Administration of hypertonic (3%) saline, however, can quickly raise the serum sodium concentration irrespective of etiology of hyponatremia. Rapid correction is warranted in severely symptomatic patients, particularly if the hyponatremia developed over less than 48 hours, since these patients are at highest risk for seizures and tentorial herniation. Even very small increases in serum sodium concentration can reduce the amount of cerebral edema present and markedly lower the risk of these life-threatening events. Elevation of the serum sodium concentration by 4 to 6 mEq/L acutely is sufficient to prevent these complications. This can be accomplished with up to three 100 mL boluses of hypertonic saline given over 10 minutes at a time. If emergent correction is not required, hypertonic saline can be administered by continuous infusion without initial boluses. A number of formulas have been proposed to model the rate of serum sodium concentration rise. However, they ignore factors such as ongoing water and electrolyte excretion that may influence the rate of rise of serum sodium concentration. Consequently, we favor use of a simple prediction formula: infusion of 1 mL/kg per hour of hypertonic saline with the expectation that it will raise the serum sodium concentration by approximately 1 mEq/L per hour. How much the serum sodium concentration should be increased depends on the rate of development of hyponatremia. With chronic hyponatremia, strict limits for overall rate of correction should be observed to avoid complications from overly rapid correction itself (see below). Any formula is only a crude estimation, so it is essential to measure serum sodium concentration frequently during the initial stages of treatment to assure that the rate of rise matches the goal.