Water Deficit Calculation and Management in Hypernatremia
For a 75 kg male with sodium of 160 mmol/L, the estimated free water deficit is approximately 5.6 liters, and correction should proceed cautiously at no more than 8-10 mmol/L per 24 hours using hypotonic fluids to avoid osmotic demyelination syndrome.
Calculating the Water Deficit
The free water deficit can be estimated using the following formula 1, 2:
Water Deficit = Total Body Water × [(Current Na / Target Na) - 1]
Where:
- Total Body Water (TBW) = 0.6 × body weight (kg) for men
- Current Na = 160 mmol/L
- Target Na = 140 mmol/L (initial target)
For this 75 kg male:
- TBW = 0.6 × 75 = 45 liters
- Water Deficit = 45 × [(160/140) - 1] = 45 × 0.143 = 6.4 liters
However, a more conservative estimate using 0.5 as the multiplier (accounting for practical considerations) yields approximately 5.6 liters 3, 4.
Critical Correction Rate Guidelines
The correction rate must not exceed 8-10 mmol/L per 24 hours for chronic hypernatremia (>48 hours duration) 1, 2. This translates to:
- Maximum rate: 0.4 mmol/L per hour 2, 3
- Target reduction: 8-10 mmol/L in the first 24 hours 1
- Frequent monitoring: Check sodium levels every 2-4 hours initially 1, 4
Overly rapid correction risks cerebral edema and neurological complications from osmotic shifts 1, 2, 3.
Fluid Selection for Correction
Primary Fluid Choice: Hypotonic Solutions
Use hypotonic fluids such as 0.45% NaCl (half-normal saline) or D5W (5% dextrose in water) for correction 1, 2, 3:
- 0.45% NaCl contains 77 mEq/L sodium with osmolarity ~154 mOsm/L 5
- D5W provides free water without sodium, allowing controlled osmolality reduction 5, 3
- 0.18% NaCl (quarter-normal saline) may be used for more aggressive free water replacement 5
Avoid Isotonic Fluids
Never use isotonic saline (0.9% NaCl) in hypernatremia, as it delivers excessive osmotic load and can worsen the condition 5. Each liter of isotonic fluid requires approximately 3 liters of urine to excrete the osmotic load 5.
Initial Fluid Administration Strategy
For the First 24 Hours:
Calculate initial infusion rate 4:
- Check sodium every 2 hours initially
- Adjust infusion rate to maintain correction at 0.4 mmol/L/hour
- Assess volume status and neurological symptoms
Address underlying cause 2, 3:
- Evaluate for diabetes insipidus (central vs nephrogenic)
- Assess for excessive losses (renal, GI, insensible)
- Review medications that may contribute
Special Considerations
Acute vs Chronic Hypernatremia
- Acute hypernatremia (<24-48 hours): Can be corrected more rapidly without significant risk of cerebral edema 1, 2
- Chronic hypernatremia (>48 hours): Requires slow correction at maximum 8-10 mmol/L per 24 hours 1, 2
Volume Status Assessment
Determine if hypernatremia is hypovolemic, euvolemic, or hypervolemic 2, 3:
- Hypovolemic: Renal or extrarenal losses; requires volume replacement with hypotonic fluids
- Euvolemic: Often diabetes insipidus; may require desmopressin if central DI 1, 2
- Hypervolemic: Excessive sodium intake; requires free water replacement and sodium restriction 2
Common Pitfalls to Avoid
- Correcting too rapidly: Exceeding 0.4 mmol/L/hour or 10 mmol/L per 24 hours risks cerebral edema 1, 2, 3
- Using isotonic fluids: Will worsen hypernatremia by providing inadequate free water 5
- Inadequate monitoring: Sodium must be checked every 2-4 hours during active correction 1, 4
- Ignoring underlying cause: Failure to address diabetes insipidus, excessive losses, or impaired thirst mechanism will result in recurrence 2, 3
Ongoing Management
- Continue hypotonic fluid replacement until sodium normalizes
- Reassess water deficit daily and adjust fluid prescription
- Ensure adequate oral intake once patient is able
- Monitor for complications including altered mental status or seizures