Hypernatremia is defined as a serum sodium concentration greater than 145 mEq/L. It is almost always a result of a deficit in free water relative to total body sodium. Calculating the free water deficit helps clinicians estimate the volume of water required to normalize the patient's serum sodium level.
Total Body Water (TBW)
To estimate the free water deficit, you first need to estimate the patient's Total Body Water (TBW). TBW varies based on age, sex, and body composition. It is expressed as a fraction of total body weight in kilograms.
- Children: 0.6 L/kg
- Adult Males: 0.6 L/kg
- Adult Females: 0.5 L/kg
- Elderly Males: 0.5 L/kg
- Elderly Females: 0.45 L/kg
The Calculation Formula
Once TBW is estimated, the free water deficit can be calculated using the following formula:
Where Desired Na⁺ is typically set at 140 mEq/L.
Clinical Example
A 70 kg adult male presents with a serum sodium of 154 mEq/L.
1. Estimate TBW:
TBW = 70 kg × 0.6 = 42 Liters
2. Calculate Deficit:
Deficit = 42 × [ (154 / 140) - 1 ]
Deficit = 42 × [ 1.1 - 1 ] = 42 × 0.1 = 4.2 Liters
Correction Rate Guidelines
Replacing the free water deficit requires careful attention to the rate of correction. Correcting hypernatremia too rapidly can lead to cerebral edema and permanent neurological damage, especially if the hypernatremia is chronic.
- Acute Hypernatremia (developing <48 hours): Can often be corrected relatively quickly (e.g., lowering sodium by 1-2 mEq/L per hour).
- Chronic Hypernatremia (developing >48 hours, or unknown duration): Correction must be slow. The goal is typically to lower the serum sodium by no more than 10-12 mEq/L in a 24-hour period.
Practical Fluid Management
When prescribing fluids, remember:
- Ongoing Losses: The calculated free water deficit only accounts for existing missing water. You must also add the patient's ongoing daily sensible and insensible losses (e.g., urine, sweat, gastrointestinal) to the fluid prescription.
- Fluid Choice: Free water can be administered enterally (via mouth or feeding tube) or intravenously (e.g., D5W, or 0.45% NS if some sodium is also needed, though 0.45% NS is only half free water).
- Frequent Monitoring: Because calculations are estimates and renal water handling may change, frequent monitoring of serum sodium (e.g., every 4-6 hours) is essential to ensure the correction rate is appropriate.
References
- Adrogué, H. J., & Madias, N. E. (2000). Hypernatremia. New England Journal of Medicine, 342(20), 1493-1499.
- Sterns, R. H. (2015). Disorders of plasma sodium—causes, consequences, and correction. New England Journal of Medicine, 372(1), 55-65.