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Water Deficit Formula:
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Free Water Deficit (FWD) is a calculation used to estimate the amount of free water needed to correct hypernatremia (elevated serum sodium levels). It represents the volume of water required to return serum sodium to normal levels (140 mmol/L).
The calculator uses the Free Water Deficit formula:
Where:
Explanation: The equation calculates how much water is needed to dilute the excess sodium in the body back to the normal level of 140 mmol/L.
Details: Accurate FWD estimation is crucial for safely correcting hypernatremia, which if untreated can lead to neurological complications. Proper calculation helps prevent overly rapid correction which can cause cerebral edema.
Tips: Enter total body water in liters and current serum sodium in mmol/L. All values must be valid (TBW > 0, Na > 0). For estimating TBW, a common approximation is 0.6 × body weight (kg) for men and 0.5 × body weight (kg) for women.
Q1: What is hypernatremia?
A: Hypernatremia is a condition characterized by elevated serum sodium levels (>145 mmol/L), typically caused by water loss exceeding sodium loss.
Q2: How quickly should hypernatremia be corrected?
A: Correction should be gradual, typically not exceeding 0.5 mmol/L per hour or 10-12 mmol/L per day to avoid cerebral edema.
Q3: What factors affect TBW estimation?
A: Age, sex, body composition, and certain medical conditions can affect TBW. Elderly and obese individuals typically have lower TBW percentages.
Q4: Are there limitations to this calculation?
A: This formula provides an estimate and doesn't account for ongoing losses. Clinical judgment and monitoring are essential during correction.
Q5: When should this calculation not be used?
A: This formula may not be appropriate for patients with severe renal impairment, those on dialysis, or with complex fluid and electrolyte disorders.