What fluid and rate should be used to correct hypernatremia at 0.5 mmol/L/hour in a 68kg male with a sodium level of 148 meq/L?

Management of Hypernatremia in a 68 kg Male with Sodium Level of 148 mEq/L

For a 68 kg male with hypernatremia (Na 148 mEq/L), administer hypotonic 0.45% NaCl at a rate of 100-120 mL/hour to achieve a correction rate of 0.5 mmol/L/hour.

Assessment of Hypernatremia Severity

Hypernatremia is defined as a serum sodium concentration >145 mEq/L. At 148 mEq/L, this represents mild hypernatremia that requires careful correction to prevent complications:

• Mild hypernatremia: 146-150 mEq/L
• Moderate hypernatremia: 151-160 mEq/L
• Severe hypernatremia: >160 mEq/L

Fluid Selection

The choice of fluid depends on the severity of hypernatremia and the patient's volume status:

• Hypotonic 0.45% NaCl (half-normal saline) is the most appropriate fluid for mild hypernatremia correction when the corrected serum sodium is elevated 1.
• 5% Dextrose in water (D5W) could be considered for more severe hypernatremia but may cause too rapid correction in mild cases.

Calculation of Fluid Rate

To correct hypernatremia at a rate of 0.5 mmol/L/hour:

1. Calculate free water deficit:

   • Formula: Free water deficit = 0.5 × body weight (kg) × [(current Na/140) - 1]
   • For this patient: 0.5 × 68 kg × [(148/140) - 1] = 1.94 liters

2. Calculate infusion rate:

   • For 0.45% NaCl (which is approximately 50% free water):
   • To provide 1.94 L of free water over approximately 16 hours (to achieve 0.5 mmol/L/hour correction)
   • Required rate: 100-120 mL/hour of 0.45% NaCl

Monitoring Protocol

Careful monitoring is essential to ensure appropriate correction:

• Check serum sodium every 4 hours initially
• Adjust fluid rate based on measured correction rate
• Monitor for signs of cerebral edema (headache, altered mental status, seizures)
• Track fluid input/output and daily weights

Important Considerations

1. Maximum correction rate: The total correction should not exceed 8-10 mmol/L in 24 hours to prevent osmotic demyelination syndrome 1, 2.

2. Duration of hypernatremia: If hypernatremia has been present for >48 hours (chronic), correction should be even more cautious, not exceeding 8 mmol/L/day 2.

3. Volume status assessment: Ensure the patient is not hypovolemic before starting hypotonic fluids. If signs of hypovolemia exist, begin with isotonic saline before transitioning to hypotonic fluids 1.

4. Underlying cause: While correcting the sodium level, identify and address the underlying cause of hypernatremia (impaired thirst, diabetes insipidus, excessive water losses, etc.) 3.

Adjustment Algorithm

If the correction rate is:

• Too slow (<0.3 mmol/L/hour): Increase infusion rate by 20%
• Too fast (>0.7 mmol/L/hour): Decrease infusion rate by 20% or switch to isotonic saline temporarily

Pitfalls to Avoid

• Overly rapid correction: Can cause cerebral edema and neurological complications
• Inadequate monitoring: Failure to check sodium levels frequently can lead to under or overcorrection
• Ignoring underlying causes: Treating only the sodium level without addressing the cause will lead to recurrence
• Inappropriate fluid selection: Using isotonic fluids may worsen hypernatremia

The most recent evidence suggests that while rapid correction has traditionally been considered dangerous, the risk may be lower than previously thought, particularly in ICU settings 4. However, in the absence of life-threatening symptoms, the safer approach is controlled correction at 0.5 mmol/L/hour.