Calculating Free Water Deficit and Correction Strategy for Hypernatremia
For a serum sodium of 154 mEq/L, calculate the free water deficit using the formula: Free water deficit (L) = 0.5 × body weight (kg) × [(current Na ÷ 140) – 1], then correct with hypotonic fluids (0.45% NaCl or D5W) at a maximum rate of 10–15 mmol/L per 24 hours, monitoring serum sodium every 2–4 hours initially. 1, 2
Free Water Deficit Calculation
Use the standard formula: Free water deficit (L) = 0.5 × body weight (kg) × [(154 ÷ 140) – 1] 2, 3
This calculation provides only an initial estimate—you must add ongoing free water losses (insensible losses, urine output) to the calculated deficit 2, 3
Important caveat: All published formulae systematically underestimate actual total body water losses by 1.5–2.5 liters, though they reasonably estimate free water requirements when using osmolality-based calculations 4, 5
Fluid Selection for Correction
Primary hypotonic fluid options:
Half-normal saline (0.45% NaCl) contains 77 mEq/L sodium with osmolarity ~154 mOsm/L—this is the preferred initial choice for moderate hypernatremia 1
D5W (5% dextrose in water) provides pure free water replacement with no sodium load and is especially valuable when renal concentrating defects are present 1
Quarter-normal saline (0.18% NaCl) contains ~31 mEq/L sodium and provides more aggressive free water replacement for severe cases 1
Never use isotonic saline (0.9% NaCl) as initial therapy—it will worsen hypernatremia because the kidneys must excrete approximately 3 liters of urine to eliminate the osmotic load from just 1 liter infused 1, 3
Correction Rate Guidelines
Maximum safe correction rates:
For chronic hypernatremia (>48 hours duration): limit correction to 10–15 mmol/L per 24 hours or ≤0.5 mmol/L per hour 1, 2
Faster correction risks cerebral edema, seizures, and permanent neurological injury as brain cells rapidly gain water after losing intracellular osmolytes during chronic adaptation 1, 2
The induced change in serum osmolality should not exceed 3 mOsm/kg H₂O per hour 2
Combining Free Water and Half-Normal Saline
Practical administration strategy:
Start with 0.45% NaCl at 25–30 mL/kg per 24 hours in adults, which provides both free water and some sodium replacement 1
- For a 70 kg patient: 1750–2100 mL per 24 hours (~73–88 mL/hour)
If correction is too slow with 0.45% NaCl alone, add enteral free water via nasogastric tube (after confirming proper placement to prevent aspiration) 1, 2
If correction is too rapid (>0.5 mmol/L per hour), slow the hypotonic fluid rate or temporarily add small volumes of isotonic fluid 2
Calculate the expected sodium change from your chosen fluid using: Change in Na = (fluid Na – patient Na) ÷ (total body water + 1) 3, 6
- For 0.45% NaCl (77 mEq/L) in a patient with Na 154 mEq/L and TBW 35 L: (77 – 154) ÷ 36 = –2.1 mEq/L per liter infused
Monitoring Protocol
Essential serial measurements:
Check serum sodium every 2–4 hours during active correction, then every 6–8 hours once stable 1, 2
Monitor urine output, urine specific gravity, and urine electrolytes to assess ongoing losses and renal concentrating ability 1, 3
Track daily weight, vital signs, and neurological status 1
Assess for volume status changes—hypovolemia requires more aggressive initial fluid replacement 1, 3
Critical Pitfalls to Avoid
Do not correct chronic hypernatremia faster than 10–15 mmol/L per 24 hours—this is the single most important safety limit 1, 2
Do not use isotonic saline in patients with renal concentrating defects (nephrogenic diabetes insipidus, osmotic diuresis)—it will exacerbate hypernatremia 1, 2
Do not rely solely on the calculated free water deficit—ongoing losses often exceed the initial calculation, requiring continuous reassessment 2, 3
Do not assume all published correction formulae are accurate—they systematically underestimate total body water losses, so frequent sodium monitoring is mandatory 4, 5