What is the calculated infusion rate of 3% NaCl for a patient with severe hyponatremia?

Calculation of 3% NaCl Infusion Rate for Severe Hyponatremia (Na 115 mg/dL)

For a patient with severe hyponatremia (Na 115 mg/dL), the 3% NaCl infusion rate should be calculated using the sodium deficit formula: Desired increase in Na (mEq) × (0.5 × ideal body weight in kg).

Understanding the Clinical Context

Severe hyponatremia (Na <125 mEq/L) requires prompt intervention to prevent serious neurological complications including cerebral edema, seizures, and death 1. With a sodium level of 115 mg/dL, this patient falls into the severe category requiring careful management.

Key Treatment Principles:

• Maximum correction rate: 8 mmol/L in 24 hours 2, 1
• Initial target: 6 mmol/L correction in first 6 hours for severe symptoms 2
• After initial correction, slow down to prevent exceeding 8 mmol/L in 24 hours 1

Calculation Method

To calculate the appropriate 3% NaCl infusion rate:

1. Determine sodium deficit:

   • Formula: Sodium deficit = Desired increase in Na (mEq) × (0.5 × ideal body weight in kg) 2
   • For severe symptomatic hyponatremia, aim for 6 mEq/L increase in first 6 hours

2. Calculate infusion rate:

   • 3% NaCl contains 513 mEq/L of sodium
   • Infusion rate (mL/hr) = Sodium deficit (mEq) ÷ 513 mEq/L × 1000 mL/L ÷ hours of infusion

Example Calculation

For a 70 kg patient with Na 115 mg/dL:

1. Sodium deficit for 6 mEq/L increase:

   • Deficit = 6 mEq/L × (0.5 × 70 kg) = 210 mEq

2. Infusion rate for 6-hour correction:

   • Rate = 210 mEq ÷ 513 mEq/L × 1000 mL/L ÷ 6 hours = 68.2 mL/hr

Monitoring and Adjustment

• Check serum sodium every 2 hours for severe symptoms 1
• Monitor urine output and specific gravity every 4 hours 1
• Adjust infusion rate based on sodium levels
• Stop 3% NaCl when severe symptoms resolve or target correction is achieved 2
• Switch to mild symptom protocol when appropriate 2

Important Cautions

• Do not exceed correction of 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome 2, 1
• If sodium increases too rapidly, consider administering D5W to slow correction 3
• Patients with severe symptoms are at higher risk of overcorrection (38% vs 6%) 1
• Be vigilant for water diuresis which can accelerate sodium correction 1

Alternative Approaches

For patients without IV access or where ICU transfer is not possible:

• Hourly oral NaCl tablets can be used as an alternative to IV 3% NaCl 3
• Calculate equivalent oral dose to deliver sodium comparable to 0.5 mL/kg/hr of 3% NaCl

This calculation method provides a systematic approach to treating severe hyponatremia while minimizing the risk of complications from overly rapid correction.