How do you calculate the volume of hypertonic saline (3% sodium chloride) to administer to a patient with severe hyponatremia?

Calculating Hypertonic Saline Volume for Hyponatremia

Primary Calculation Formula

Use the sodium deficit formula to calculate the required volume of 3% hypertonic saline: Sodium deficit (mEq) = Desired increase in Na (mEq/L) × (0.5 × body weight in kg). 1, 2

Since 3% NaCl contains 513 mEq/L of sodium, divide the calculated sodium deficit by 513 to determine the volume in liters needed. 3

Step-by-Step Calculation Approach

Step 1: Determine Target Sodium Increase

• For severe symptomatic hyponatremia (seizures, altered mental status, coma): Target 6 mEq/L increase over 6 hours or until symptoms resolve 1, 2
• Maximum correction limit: Never exceed 8 mEq/L in 24 hours to prevent osmotic demyelination syndrome 1, 2, 4
• For high-risk patients (cirrhosis, alcoholism, malnutrition): Limit to 4-6 mEq/L per day 1, 2

Step 2: Calculate Sodium Deficit

Formula: Desired Na increase (mEq/L) × 0.5 × body weight (kg) = Total sodium deficit (mEq) 1, 2

• The factor 0.5 represents total body water as a fraction of body weight 1
• For women and elderly patients, some sources suggest using 0.45 instead of 0.5 3

Step 3: Convert to Volume of 3% NaCl

3% hypertonic saline contains 513 mEq/L of sodium 3

Volume (L) = Sodium deficit (mEq) ÷ 513 mEq/L 3

Step 4: Determine Infusion Rate

For bolus administration (preferred for symptomatic hyponatremia): Give 100-150 mL of 3% NaCl over 10 minutes, can repeat up to 3 times at 10-minute intervals 1, 5

For continuous infusion: Initial rate (mL/kg/hour) = body weight (kg) × desired rate of Na increase (mEq/L/hour) 3

• Target 1-2 mEq/L per hour until symptoms resolve 3
• Then slow to achieve no more than 8 mEq/L total in 24 hours 1, 4

Practical Example

For a 70 kg patient with severe symptomatic hyponatremia requiring 6 mEq/L increase:

1. Sodium deficit = 6 mEq/L × (0.5 × 70 kg) = 210 mEq
2. Volume needed = 210 mEq ÷ 513 mEq/L = 0.41 L = 410 mL of 3% NaCl
3. Administer as 100-150 mL boluses over 10 minutes, repeated as needed 5

Critical Monitoring Requirements

• Check serum sodium every 2 hours during initial correction for severe symptoms 1, 2
• Check every 4 hours after symptom resolution 1
• Recalculate and adjust infusion rate based on actual sodium response 1, 4

Essential Safety Considerations

Never exceed 8 mEq/L correction in 24 hours for chronic hyponatremia (>48 hours duration) to prevent osmotic demyelination syndrome 1, 2, 4, 6

If overcorrection occurs: Immediately stop hypertonic saline, administer D5W (5% dextrose in water), and consider desmopressin to relower sodium 1

High-risk populations requiring slower correction (4-6 mEq/L per day): advanced liver disease, alcoholism, malnutrition, prior encephalopathy, severe hyponatremia <120 mEq/L 1, 2

Alternative Oral Approach for Non-Severe Cases

For mild-moderate symptomatic hyponatremia without ICU access, oral sodium chloride tablets can deliver equivalent correction: Calculate hourly dose to match 0.5 mL/kg/hour of 3% NaCl 7

This requires 1 gram NaCl tablet (17 mEq sodium) given hourly with careful monitoring 2, 7