Sodium Deficit Calculator

The sodium deficit estimates the milliequivalents of sodium needed to raise a hyponatremic serum sodium to a chosen target. Its logic is that administered sodium distributes through total body water, so raising the concentration by 1 mEq/L in a person with 42 L of body water requires roughly 42 mEq. The formula is Deficit = TBW x (Target Na - Current Na), with TBW taken as 0.60 of body weight in adult men, 0.50 in adult women and elderly men, and 0.45 in elderly women. Converting the deficit into a volume requires the sodium content of the chosen fluid: 3 percent hypertonic saline carries 513 mEq/L and 0.9 percent saline carries 154 mEq/L. Hyponatremia is defined as a serum sodium below 135 mEq/L and is the most common electrolyte disturbance among hospitalized patients. The number the formula produces matters less than the rate at which it is delivered. Correction should not exceed about 8 mEq/L in any 24 hour period, and many authorities prefer a 6 mEq/L ceiling, because brain cells that extruded organic osmolytes during chronic hyponatremia cannot re-accumulate them quickly; overshoot dehydrates oligodendrocytes and produces osmotic demyelination, with dysarthria, dysphagia, and quadriparesis appearing days after the correction. The calculation also ignores the dilutional effect of the infused volume itself and ignores potassium, which is osmotically active and raises serum sodium as effectively as sodium does. In SIADH, isotonic saline can paradoxically lower serum sodium further when urine osmolality exceeds that of the infusate.

Hyponatremia became routinely measurable with the spread of flame photometry for serum electrolytes in the late 1940s and 1950s, which made frequent sodium measurement cheap enough for ward use rather than a research procedure. The physiological basis for treating serum sodium as a function of total body water and exchangeable cations was set out by Isidore Edelman and colleagues in 1958. In 1957 William Schwartz and Frederic Bartter described the syndrome of inappropriate antidiuretic hormone secretion in patients with bronchogenic carcinoma, giving clinicians a mechanism for hyponatremia occurring in the absence of volume depletion and explaining why saline alone often fails in such patients. The safety limits came from a hard lesson. Raymond Adams, Maurice Victor, and Elliott Mancall described central pontine myelinolysis in 1959 in malnourished and alcoholic patients at Boston City Hospital, initially without an identified cause. Through the late 1970s and 1980s, clinical and experimental work, notably by Richard Sterns and colleagues, linked the lesion to the speed of sodium correction rather than to hyponatremia itself, and the term osmotic demyelination syndrome gradually replaced the older name as lesions were recognized outside the pons. That work converted the sodium deficit from a target to be reached promptly into a budget to be spent slowly, and the daily ceilings of 6 to 8 mEq/L used in current guidelines descend directly from it.