Water Deficit Calculation in Hypernatremia
Standard Formula
The water deficit in hypernatremia is calculated using the formula: Water deficit (L) = 0.5 × ideal body weight (kg) × [(current serum Na/desired serum Na) - 1], where the factor 0.5 represents the proportion of total body weight that is water. 1
Key Components of the Formula
Multiplication factor (0.5): Represents total body water as a fraction of body weight 1
- Use 0.6 for men and children
- Use 0.5 for women and elderly patients
- This accounts for differences in body composition 2
Ideal body weight: Use the patient's premorbid or ideal body weight, not current dehydrated weight 2
Desired serum sodium: Typically target 140 mEq/L, though initial correction should aim for 145 mEq/L in severe hypernatremia 1
Critical Correction Rate Guidelines
The induced change in serum osmolality must not exceed 3 mOsm/kg/h, and total sodium correction should not exceed 10-15 mEq/L per 24 hours for chronic hypernatremia to prevent cerebral edema. 1, 3
Correction Rate Based on Chronicity
- Acute hypernatremia (<48 hours): Can correct more rapidly, up to 1 mmol/L/hour if severely symptomatic 3
- Chronic hypernatremia (>48 hours): Maximum 10-15 mmol/L per 24 hours 1, 3
- Corrections faster than 48-72 hours: Associated with increased risk of pontine myelinolysis 3
Fluid Selection for Correction
The primary treatment involves administration of hypotonic fluids, with 5% dextrose in water (D5W) being the preferred solution as it delivers no renal osmotic load. 1, 3
Hypotonic Fluid Options
- D5W (5% dextrose in water): Preferred as it provides pure free water replacement 3
- 0.45% NaCl (half-normal saline): Contains 77 mEq/L sodium, appropriate for moderate hypernatremia 3
- 0.18% NaCl (quarter-normal saline): Contains ~31 mEq/L sodium, for more aggressive free water replacement 3
Critical Contraindication
Avoid isotonic saline (0.9% NaCl) in hypernatremia as it worsens the condition by delivering excessive osmotic load—requiring 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid. 1, 3
Initial Fluid Administration Rates
Adults
Pediatric Patients
Important Limitations of the Formula
The water deficit equation (WD) systematically underestimates both total body water (TBW) and free water (FW) losses by 1.5-2.5 L for TBW and 0.5-1.0 L for FW. 2
Why the Formula Underestimates
- The equation assumes all water loss is free water, but dehydration involves both electrolyte and water losses 2
- Actual plasma sodium and osmolality values differ from assumed constants 2
- Body composition varies significantly between individuals 2
Improved Alternative Formula
For more accurate free water estimation, use: Water deficit = 0.6 × body mass × [1 - (290 ÷ plasma osmolality)], which estimates free water loss to within 0.06-0.16 L. 2
Monitoring During Correction
Check serum sodium every 2-4 hours initially during active correction, then every 6-12 hours once stable. 3
Essential Monitoring Parameters
- Daily weights and fluid balance 3
- Urine output, specific gravity, and osmolality 3
- Serum electrolytes including potassium, chloride, bicarbonate 3
- Neurological status for signs of cerebral edema 3
Special Clinical Scenarios
Nephrogenic Diabetes Insipidus
Patients with renal concentrating defects require ongoing hypotonic fluid administration to match excessive free water losses, and isotonic fluids will worsen hypernatremia. 3
Patients with Impaired Renal Function
Adjust fluid rates and monitor more frequently to ensure safe correction, as impaired renal function affects sodium and water handling. 1
Common Pitfalls to Avoid
- Correcting chronic hypernatremia too rapidly: Leads to cerebral edema, seizures, and permanent neurological injury 3
- Using isotonic saline as initial therapy: Worsens hypernatremia by providing excessive osmotic load 1, 3
- Inadequate monitoring during correction: Results in overcorrection or undercorrection 3
- Relying solely on the formula: The calculated deficit is only an estimate; serial sodium measurements must guide therapy 4