Water Deficit Calculator Hypernatremia

Free Water Deficit Formula:

\[ FWD = 0.6 \times Weight_{kg} \times \left( \frac{Na}{140} - 1 \right) \]

Free Water Deficit (FWD):

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1. What is Free Water Deficit?

Free Water Deficit (FWD) is the amount of water needed to correct hypernatremia (elevated serum sodium levels). It estimates the volume of free water required to restore normal sodium concentration in the body.

2. How Does the Calculator Work?

The calculator uses the Free Water Deficit formula:

\[ FWD = 0.6 \times Weight_{kg} \times \left( \frac{Na}{140} - 1 \right) \]

Where:

\( Weight_{kg} \) — Body weight in kilograms
\( Na \) — Serum sodium concentration in mmol/L
0.6 — Estimated proportion of body weight that is water
140 — Target sodium concentration in mmol/L

Explanation: The formula calculates the water deficit based on the difference between current sodium levels and the target level of 140 mmol/L, adjusted for total body water.

3. Importance of FWD Calculation

Details: Accurate FWD calculation is crucial for safe correction of hypernatremia. Overly rapid correction can lead to cerebral edema, while insufficient correction prolongs the hypernatremic state.

4. Using the Calculator

Tips: Enter weight in kilograms and serum sodium in mmol/L. All values must be valid (weight > 0, sodium > 0). The result represents the estimated water deficit in liters.

5. Frequently Asked Questions (FAQ)

Q1: Why use 0.6 as the water proportion factor?
A: 0.6 represents the estimated fraction of body weight that is water in adult males. For females and elderly patients, this factor may be adjusted to 0.5.

Q2: What is the target sodium level of 140 mmol/L?
A: 140 mmol/L is the generally accepted normal serum sodium concentration. Some clinicians may use slightly different targets based on individual patient factors.

Q3: How quickly should hypernatremia be corrected?
A: Hypernatremia should be corrected gradually, typically over 48-72 hours, with a maximum sodium correction rate of 0.5 mmol/L per hour to avoid complications.

Q4: Are there limitations to this formula?
A: This formula provides an estimate and may need adjustment for patients with edema, dehydration, or abnormal body composition. Clinical judgment should always be used.

Q5: Should this formula be used for pediatric patients?
A: No, this formula is for adults. Pediatric water deficit calculations require different approaches based on age and body composition.