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The free water deficit is used to assess the amount of water replacement needed to correct hypernatremia. As with many medical equations, the result is an estimate that may not be valid for a given patient. Repeated measurements of sodium may be needed to ensure safe treatment. The calculation is based on the formula:
water deficit = (total body water)*(1-(140/Na))
total body water = correction factor * weight
The correction factor is 0.6 for men, 0.5 for women and elderly men, and 0.45 for elderly women.
Also offered is a calculated flow rate and duration for IV D5W. This calculation is based on a conservative correction rate of 0.5 meq/L/hr to avoid cerebral edema. It does not take into account ongoing water losses.
hours to correct = |(serum sodium-140)|/0.5
infusion rate = water deficit/hours to correct
- Adrogué HJ and Madias NE. Hypernatremia. New Engl J Med 342(20): 1493-9, 2000.