Hypernatremia, characterized by elevated serum sodium levels (>145 mEq/L), is a life-threatening electrolyte imbalance requiring precise correction. Central to its management is the concept of Free Water Deficit (FWD), a calculation that determines the volume of water needed to restore sodium balance safely. This article explains FWD, its clinical applications, and practical considerations for healthcare providers.
What is Free Water Deficit?
Free Water Deficit (FWD) estimates the amount of pure water required to lower serum sodium to a target level in patients with hypernatremia. It accounts for:
- Current and desired sodium concentrations
- Patient weight and age
- Total Body Water (TBW) variations based on sex and body composition
Why is FWD Important?
Rapid sodium correction can cause cerebral edema, while delayed treatment risks neurological damage. FWD guides clinicians in:
- Avoiding over- or under-correction of sodium
- Tailoring fluid therapy to individual patient factors
- Preventing complications of hypernatremia (e.g., seizures, coma)
How is Free Water Deficit Calculated?
The formula for FWD is:
*Example: TBW = 27 L, Serum Na = 150 → FWD = 27 × [(150/140)-1] ≈ 1.9 L
Key Variables:
- Total Body Water (TBW) = Factor x Weight (kg)
- Serum Sodium: Measured serum sodium (mEq/L)
- Desired Na: Typically 140 mEq/L
- Adult Male: 0.6
- Adult Female: 0.5
- Elderly Male: 0.5
- Elderly Female: 0.45
- Child: 0.6
🧮 Free Water Deficit (FWD) Calculator
🧪 Overview:
This calculator estimates the free water deficit in patients with elevated serum sodium (hypernatremia). It helps guide fluid replacement to restore normal serum osmolality.
📐 Formula:
FWD (L) = TBW × [(Serum Na / 140) - 1]
TBW (Total Body Water) = Weight × Factor
Factors based on sex/age group:
- Adult Male (Age 18–64): 0.6
- Adult Female (Age 18–64): 0.5
- Elderly Male (Age ≥65): 0.5
- Elderly Female (Age ≥65): 0.45
- Child (Age <18): 0.6
📊 Serum Sodium Normal Range:
135–145 mEq/L
🖊️ Enter Patient Details:
Clinical Considerations
- Rate of Correction: Limit sodium reduction to 0.5 mEq/L per hour to avoid osmotic demyelination syndrome.
- Adjustments: Modify TBW estimates for obesity, edema, or dehydration.
- Route of Administration: Oral or intravenous (5% dextrose) based on patient status.
Case Example
A 65-year-old woman (60 kg) presents with a serum sodium of 150 mEq/L:
- TBW = 60 kg × 0.45 = 27.0 L
- FWD = 27.0 × (150/140 − 1) = 1.9 liters
Correction would involve administering 1.9 liters of water over 24–48 hours, with frequent sodium monitoring.
Limitations of FWD
- Assumes no ongoing sodium/water losses.
- Does not account for insensible fluid loss (e.g., fever).
- Requires clinical judgment for patients with renal or cardiac disease.
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