Water Deficit Calculation and D5W Infusion Rate for Severe Hypernatremia
For this 44-year-old woman weighing 45 kg with serum sodium of 169 mmol/L, the estimated free water deficit is approximately 4.1 liters, and D5W should be infused at an initial rate of approximately 85 mL/hour to safely correct the hypernatremia over 48 hours.
Water Deficit Calculation
The free water deficit is calculated using the formula: Free water deficit (L) = 0.5 × body weight (kg) × [(current Na/140) - 1] 1
For this patient:
- Free water deficit = 0.5 × 45 kg × [(169/140) - 1]
- Free water deficit = 22.5 × (1.207 - 1)
- Free water deficit = 22.5 × 0.207
- Free water deficit ≈ 4.66 liters
However, this formula uses 0.5 as the total body water fraction for women 1. The actual deficit also depends on ongoing losses, which must be added to the calculated deficit 1.
Safe Correction Rate Guidelines
The maximum correction rate for chronic hypernatremia (>48 hours duration) must not exceed 8-10 mmol/L per 24 hours, with a target rate of 0.5 mmol/L per hour or less 1, 2. This slower correction prevents cerebral edema, seizures, and neurological injury from osmotic water shift into brain cells 1.
For this patient with sodium of 169 mmol/L, the correction should aim for:
- Maximum decrease: 8-10 mmol/L per 24 hours 1, 2
- Target rate: ≤0.5 mmol/L per hour 1
- Correction period: 48-72 hours minimum for chronic hypernatremia 2
D5W Infusion Rate Calculation
D5W (5% dextrose in water) is the preferred fluid for hypernatremia correction because it delivers no renal osmotic load and allows slow, controlled decrease in plasma osmolality 3.
Initial Rate Calculation:
To correct 8-10 mmol/L over 24 hours:
- If targeting 8 mmol/L correction in first 24 hours
- Estimated fluid needed: approximately 2-2.5 liters over 24 hours (roughly half the total deficit)
- Initial D5W rate: 85-105 mL/hour (2,040-2,520 mL per 24 hours)
Alternative Calculation Approach:
For maintenance plus deficit replacement over 48 hours:
- Total deficit: ~4.66 liters
- Maintenance fluid: ~30 mL/kg/24h = 1,350 mL/day 1
- Total over 48 hours: 4,660 + 2,700 = 7,360 mL
- Rate: approximately 75-80 mL/hour over 48 hours
Critical Management Principles
Hypotonic fluids such as D5W or 0.45% NaCl are the appropriate choices for hypernatremia correction 1, 4. Isotonic saline (0.9% NaCl) must be avoided in patients with renal concentrating defects or nephrogenic diabetes insipidus, as it delivers excessive osmotic load requiring 3 liters of urine to excrete the osmotic load from just 1 liter of isotonic fluid, which risks worsening hypernatremia 3, 1.
Serial measurements of serum sodium are required every 2-4 hours initially, then every 6-8 hours once stable 1. Therapy should be adjusted based on the rate of sodium correction, with slowing of free water administration if sodium is correcting too rapidly (>0.5 mmol/L/h) 1.
Monitoring Protocol
- Check serum sodium every 2-4 hours during initial correction phase 1
- Monitor urine output and urine electrolytes to assess ongoing losses 1
- Adjust infusion rate if correction exceeds 0.5 mmol/L per hour 1
- Target serum sodium reduction of 8-10 mmol/L per 24 hours maximum 1, 2
Common Pitfalls to Avoid
Do not correct chronic hypernatremia faster than 8-10 mmol/L per 24 hours, as rapid correction can cause cerebral edema and seizures 1, 2. Avoid isotonic fluids in patients with impaired renal concentrating ability 1. Do not use the calculated deficit as the sole guide—ongoing losses must be continuously assessed and replaced 1.
For this specific patient: Start D5W at 85 mL/hour, check sodium every 2-4 hours, and adjust the rate to maintain correction at ≤8-10 mmol/L per 24 hours 1, 2.