Hyponatremia

Hyponatremia is defined as a plasma [Na⁺] of 0.25-0.3 ml/kg/h provides a greater margin of safety and can be titrated based on subsequent laboratory data.

■ Alternatively, hypertonic saline can be given in 100 mL boluses (up to three doses as needed).

■ Since no equation or algorithm can adequately predict dynamic fluctuations in water balance, it is absolutely critical to frequently recheck laboratory data to ensure correction at an appropriate rate and adjust fluid administration.

■ Desmopressin acetate (DDAVP) can also be given to prevent overcorrection of hyponatremia, particularly in patients who may have a reversible cause of ADH secretion.

î In asymptomatic hyponatremia, treatment should be targeted to the cause of the disorder.

■ Hypovolemic hyponatremia. In patients with acute hypovolemic hyponatremia, isotonic saline can be used to restore the intravascular volume. Because ADH is stimulated by the volume depletion, fluid resuscitation will decrease ADH secretion and facilitate renal elimination of water.

■ Hypervolemic hyponatremia. Hyponatremia in congestive heart failure (CHF) and cirrhosis often reflects the severity of the underlying disease. The hyponatremia itself is typically asymptomatic. Definitive treatment requires management of the underlying condition, although restriction of water intake can attenuate the hyponatremia.

■ SIADH. In addition to the correction of contributing factors (pneumonia, drugs, etc.), water restriction, solute tablets, and diuretics can also be used.

? Water restriction. This is typically the first-line treatment for SIADH. The amount of fluid restriction depends on the amount of water eliminated by the kidney. A useful guide to the necessary degree of fluid restriction is as follows:

If (Urine Na⁺ + Urine K⁺)∕Serum Na⁺ 1, the patient has a negative renal free water clearance, and any amount of ingested water may be retained. In such situations, adjunctive therapy is required.

? A high dietary solute load (using salt or urea tablets) can be extremely helpful, particularly since water restriction can be challenging for patients. The obligate water loss that accompanies the excretion of the high dietary solute load helps to alleviate the water retention in SIADH.

? Loop diuretics impair the urinary concentrating mechanism and can enhance free water excretion.

? Vasopressin antagonists promote a water diuresis and may be useful in the therapy of SIADH. Both IV (conivaptan) and oral (tolvaptan) preparations are approved for the treatment of euvolemic hyponatremia. However, given the risks of overcorrection, these agents should be initiated in a closely monitored inpatient setting.