Hypernatremia Correction Calculation Example
For this 10-kg intubated boy with serum sodium of 160 mmol/L, calculate the free water deficit using the formula: Free Water Deficit (L) = 0.6 × body weight (kg) × [(current Na ÷ desired Na) - 1], then administer hypotonic fluid or D5W at a rate that lowers sodium by 0.5 mmol/L per hour (maximum 10-12 mmol/L per 24 hours).
Step-by-Step Calculation
1. Calculate Free Water Deficit
Using the water deficit equation 1:
- Body weight: 10 kg
- Current Na: 160 mmol/L
- Target Na: 140 mmol/L
Free Water Deficit = 0.6 × 10 kg × [(160 ÷ 140) - 1]
- = 0.6 × 10 × [1.143 - 1]
- = 6 × 0.143
- = 0.86 liters (860 mL)
2. Determine Safe Correction Rate
Maximum correction rate: 0.5 mmol/L per hour 2, 3
- Total sodium decrease needed: 160 - 140 = 20 mmol/L
- Time required at 0.5 mmol/L/hr: 20 ÷ 0.5 = 40 hours minimum
- Maximum correction in first 24 hours: 10-12 mmol/L
3. Calculate Infusion Rate for First 24 Hours
Target sodium at 24 hours: 160 - 10 = 150 mmol/L
For the first 24 hours:
- Administer approximately 500-600 mL of free water (as D5W or enteral free water)
- Infusion rate: 21-25 mL/hour
4. Fluid Selection
Use D5W (dextrose 5% in water) parenterally 4:
- D5W provides approximately 2.25 mmol/L decrease in sodium per liter administered
- More effective than enteral free water in critically ill patients
- Allows precise titration in intubated patients
Critical Considerations
Monitoring Requirements
- Check serum sodium every 4-6 hours during active correction 5
- Adjust infusion rate based on measured sodium changes
- Monitor for signs of cerebral edema (though rare with slow correction)
Important Caveats
Formulas have significant limitations 6, 1:
- The water deficit equation underestimates total body water losses by 40-50%
- Individual variations can be extreme (±4-5 mmol/L from predicted)
- Serial sodium measurements are essential—do not rely solely on calculations
Faster correction may be safe in specific contexts 3:
- Recent meta-analysis shows faster correction (>0.5 mmol/L/hr) may reduce mortality in severe admission-related hypernatremia
- However, no major neurological complications reported only when correction <1 mmol/L/hr
- For this critically ill child on vasopressors, maintain conservative 0.5 mmol/L/hr rate
Ongoing Fluid Management
Account for all fluid sources 5:
- Include dopamine and epinephrine infusion volumes
- Add maintenance fluids (use isotonic solutions per AAP guidelines 7)
- Consider insensible losses and urine output
- Adjust free water administration accordingly
Maintenance Fluids Alongside Correction
For this 10-kg child:
- Maintenance requirement: ~1000 mL/24hr (100 mL/kg/day for first 10 kg)
- Use isotonic fluid (0.9% NaCl or PlasmaLyte) with appropriate KCl and dextrose 7
- Restrict to 65-80% of calculated maintenance (650-800 mL/24hr) given critical illness and vasopressor use 5
Practical Implementation
Hour 0-24:
- D5W at 21-25 mL/hr (500-600 mL total)
- Isotonic maintenance fluid at 27-33 mL/hr (650-800 mL total)
- Vasopressor infusions (volumes included in total fluid calculation)
- Check sodium at hours 4,8,12,18, and 24
Adjust rate if:
- Sodium drops >0.5 mmol/L/hr → decrease D5W rate
- Sodium drops <0.3 mmol/L/hr → increase D5W rate slightly
- Sodium drops >12 mmol/L in 24 hours → stop D5W, consider hypertonic saline if overcorrection
The key principle: formulas provide starting estimates only; frequent sodium monitoring and rate adjustments based on actual response are mandatory for safe correction 6, 1.