1. What is Free Water Deficit?

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 concentration to normal levels.

2. How Does the Calculator Work?

The calculator uses the Free Water Deficit formula:

Where:

• \( FWD \) — Free Water Deficit (L)
• \( TBW \) — Total Body Water (L)
• \( Na \) — Serum Sodium (mmol/L)

Explanation: The equation calculates the water deficit based on the current sodium level and total body water volume, with 140 mmol/L representing the target sodium concentration.

3. Importance of FWD Calculation

Details: Accurate FWD estimation is crucial for managing hypernatremia, guiding fluid replacement therapy, and preventing complications from overly rapid sodium correction.

4. Using the Calculator

Tips: Enter total body water in liters and serum sodium in mmol/L. All values must be valid (TBW > 0, Na > 0).

5. Frequently Asked Questions (FAQ)

Q1: How is Total Body Water (TBW) estimated?
A: TBW is typically estimated as 60% of body weight in kilograms for men and 50% for women, though this varies with age and body composition.

Q2: What is the target sodium level in this calculation?
A: The formula uses 140 mmol/L as the target sodium concentration, which is the upper limit of normal range.

Q3: How quickly should the water deficit be corrected?
A: Correction should generally occur over 48-72 hours to avoid cerebral edema, with sodium decreasing no faster than 0.5 mmol/L per hour.

Q4: Are there limitations to this calculation?
A: This formula doesn't account for ongoing water losses and may need adjustment based on clinical context and patient-specific factors.

Q5: When should this calculation be used with caution?
A: Use caution in patients with heart failure, renal impairment, or those at risk for fluid overload. Always consider clinical context.