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Estimation of the sodium deficit in patients with hyponatremia

INTRODUCTION

Hyponatremia represents a relative excess of water in relation to sodium. It can be induced by a marked increase in water intake (primary polydipsia) or, in the great majority of cases, by impaired water excretion resulting from advanced renal failure or from persistent release of antidiuretic hormone (ADH) induced by effective volume depletion, the syndrome of inappropriate ADH secretion (SIADH), thiazide diuretics, adrenal insufficiency, or hypothyroidism. (See "Causes of hyponatremia in adults".)

Patients with true volume depletion and some with SIADH require saline administration to raise the serum sodium. Isotonic saline is typically sufficient in true volume depletion but is ineffective in SIADH where, if saline is given, a hypertonic solution is typically required. The mechanisms responsible for these conclusions are described elsewhere. (See "Overview of the treatment of hyponatremia in adults".)

When treating patients with moderate to severe hyponatremia with saline, formulas have been used to estimate the sodium deficit as a guide to initial therapy and to estimate the effect of a given volume of saline. The utility and limitations of these formulas will be reviewed here. An overview of the treatment of hyponatremia is presented separately. (See "Overview of the treatment of hyponatremia in adults".)

SODIUM DEFICIT FORMULA

When saline is given to treat hyponatremia, the quantity of sodium required to achieve the desired elevation in the serum sodium concentration can be estimated from the product of the serum sodium deficit per liter and the total body water (TBW):

   Sodium deficit  =  TBW  x  (desired serum Na  -  actual serum Na)

     

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Literature review current through: Oct 2014. | This topic last updated: Jun 23, 2014.
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