Hyponatremia Correction Formula
The sodium deficit formula is: Desired increase in Na (mEq/L) × (0.5 × ideal body weight in kg), which estimates the total sodium needed to achieve the target correction. 1
Calculating Sodium Deficit
- Use the formula: Sodium deficit = Desired increase in sodium (mEq/L) × (0.5 × ideal body weight in kg) to determine the appropriate amount of sodium supplementation needed 1
- The factor 0.5 represents the estimated total body water as a fraction of body weight in adults 1
- This calculation provides the total sodium deficit in mEq that needs to be replaced 1
Determining Hypertonic Saline Infusion Rate
- For 3% hypertonic saline, the initial infusion rate (mL/kg per hour) can be estimated by: body weight (kg) × desired rate of increase in sodium (mmol/L per hour) 2
- Administer 100 mL of 3% hypertonic saline IV over 10 minutes as boluses for severe symptomatic hyponatremia, which can be repeated up to three times at 10-minute intervals until symptoms improve 3
- Each 100 mL bolus of 3% saline typically raises serum sodium by approximately 1-2 mEq/L 3
Critical Correction Rate Limits
- Target an initial sodium increase of 4-6 mEq/L in the first hour for severely symptomatic patients (seizures, coma), but never exceed 8 mmol/L total correction in 24 hours 1, 3
- For severe symptoms, correct by 6 mmol/L over 6 hours or until symptoms resolve 1, 3
- If 6 mmol/L is corrected in the first 6 hours, limit additional correction to only 2 mmol/L in the following 18 hours 3
- The maximum safe correction is 8 mmol/L in 24 hours, 12-14 mmol/L in 48 hours, and 14-16 mmol/L in 72 hours 4
High-Risk Patient Adjustments
- Patients with advanced liver disease, alcoholism, malnutrition, or prior encephalopathy require even more cautious correction at 4-6 mmol/L per day, not exceeding 8 mmol/L in 24 hours 1
- Chronic hyponatremia (>48 hours duration) should not be corrected faster than 1 mmol/L per hour 1
Monitoring Requirements
- Check serum sodium every 2 hours during initial correction for severe symptoms 1, 3
- Monitor every 4 hours after resolution of severe symptoms 1
- Track strict intake and output, daily weights, and watch for signs of osmotic demyelination syndrome (dysarthria, dysphagia, oculomotor dysfunction) typically occurring 2-7 days after rapid correction 1, 3
Common Pitfalls
- Overcorrection exceeding 8 mmol/L in 24 hours risks osmotic demyelination syndrome, a potentially fatal neurological complication 1, 3, 5
- Inadvertent overcorrection can occur from unexpected water diuresis—frequent monitoring and desmopressin administration can prevent or reverse this 4, 6
- The formula provides an estimate only; actual correction depends on ongoing losses, fluid intake, and underlying pathophysiology 2