Converting Infusion Rates to mL/hr
To convert any infusion rate to mL/hr, use the formula: Infusion rate (mL/hr) = [Weight (kg) × Dose (mcg/kg/min) × 60 min/hr] ÷ Concentration (mcg/mL). 1
Standard Conversion Formula
The universal formula for converting weight-based infusion rates to mL/hr is:
Infusion rate (mL/hr) = [Weight (kg) × Dose (mcg/kg/min) × 60] ÷ Concentration (mcg/mL) 1
- This formula applies to all continuous infusions including vasopressors, inotropes, and sedatives 1
- The factor of 60 converts minutes to hours 1
The "Rule of 6" Method (Pediatric Simplified Approach)
For pediatric patients, the "Rule of 6" provides a simplified preparation method:
- Multiply 6 × body weight (kg) = number of mg of drug to add to 100 mL total volume 1
- With this preparation, 1 mL/hr delivers 1 mcg/kg/min 1
- For dopamine specifically: 0.6 × body weight (kg) = mg diluted to 100 mL; then 1 mL/hr = 0.1 mcg/kg/min 2
This method eliminates complex calculations at the bedside but requires specific drug concentrations 1, 2.
Standard Drug Concentrations
Noradrenaline
- Standard concentration: 4 mg in 250 mL D5W = 16 mcg/mL 3
- For a 70 kg adult at 0.02 mcg/kg/min: (70 × 0.02 × 60) ÷ 16 = 5.25 mL/hr 3
Dopamine
- Standard concentration: 400 mg in 500 mL D5W = 800 mcg/mL 1, 4
- Alternative concentrations: 1600 mcg/mL or 3200 mcg/mL for fluid-restricted patients 4
- For a 70 kg adult at 5 mcg/kg/min: (70 × 5 × 60) ÷ 800 = 26.25 mL/hr 1
Epinephrine
- Using "Rule of 6": 0.6 × body weight (kg) = mg diluted to 100 mL; then 1 mL/hr = 0.1 mcg/kg/min 1
- For standard concentrations, apply the universal formula 1
Critical Safety Considerations
Always use an infusion pump, preferably a volumetric pump, for all vasoactive medications 4. Manual calculations carry significant error risk:
- Only 15% of anesthesiologists correctly calculated pediatric drug concentrations in one study 5
- Calculation errors ranged from 50 times too low to 56 times too high 5
- Electronic calculators reduce infusion errors from 32.6% to <1% 6
Avoid inadvertent bolus administration by carefully controlling infusion rates 4. The dead volume in IV tubing can contain substantial drug mass that may be accidentally delivered 7.
Practical Examples
Example 1: Dopamine for 20 kg child at 10 mcg/kg/min
Example 2: Noradrenaline for 80 kg adult at 0.1 mcg/kg/min
- Standard concentration: 16 mcg/mL
- Calculation: (80 × 0.1 × 60) ÷ 16 = 30 mL/hr 3
Example 3: Epinephrine for 15 kg child at 0.3 mcg/kg/min
- Using Rule of 6: 0.6 × 15 = 9 mg in 100 mL (concentration = 90 mcg/mL)
- Calculation: (15 × 0.3 × 60) ÷ 90 = 3 mL/hr 1
Monitoring Requirements
- Continuous hemodynamic monitoring is mandatory during all vasoactive infusions 2, 4
- Use arterial line monitoring when available, especially at higher doses 2
- Monitor for extravasation, which can cause severe tissue injury; treat with phentolamine 0.1-0.2 mg/kg (up to 10 mg) diluted in 10 mL saline injected intradermally at the site 2
Common Pitfalls
- Never flush IV lines containing vasoactive drugs - this can deliver a dangerous bolus 2
- When carrier fluids are stopped, drug delivery can be profoundly reduced due to dead volume effects 7
- After changing infusion rates, allow time to reach steady state before assessing clinical effect 7
- Cover dopamine infusions with foil to prevent light degradation 2