How do I calculate the norepinephrine (noradrenaline) infusion rate for a 75‑kg adult patient?

Norepinephrine Infusion Calculation for a 75-kg Patient

For a 75-kg adult patient, start norepinephrine at 0.1–0.5 mcg/kg/min (7.5–37.5 mcg/min), which translates to 2–10 mL/hr using standard concentration, then titrate to blood pressure response.

Standard Preparation and Concentration

The FDA-approved preparation is straightforward 1:

• Add 4 mg (one 4-mL vial) of norepinephrine to 1,000 mL of 5% dextrose
• This creates a concentration of 4 mcg/mL
• Must use dextrose-containing solutions (D5W or D5NS) to prevent oxidation
• Never use saline alone

Dose Calculation for Your 75-kg Patient

Starting Dose Range

The American Heart Association guidelines specify 2:

• Weight-based dosing: 0.1–0.5 mcg/kg/min
• For a 70-kg adult: 7–35 mcg/min
• For your 75-kg patient: 7.5–37.5 mcg/min

Converting to Infusion Rate (mL/hr)

Using the standard 4 mcg/mL concentration:

Starting dose (0.1 mcg/kg/min):

• 75 kg × 0.1 mcg/kg/min = 7.5 mcg/min
• 7.5 mcg/min ÷ 4 mcg/mL = 1.875 mL/min
• 1.875 mL/min × 60 = 112.5 mL/hr (round to 113 mL/hr)

Alternatively, start at 2–3 mL/min as FDA label suggests:

• 2 mL/min = 120 mL/hr (8 mcg/min = 0.107 mcg/kg/min)
• 3 mL/min = 180 mL/hr (12 mcg/min = 0.16 mcg/kg/min)

Maintenance range (0.5–1 mL/min per FDA):

• 0.5 mL/min = 30 mL/hr (2 mcg/min = 0.027 mcg/kg/min)
• 1 mL/min = 60 mL/hr (4 mcg/min = 0.053 mcg/kg/min)

Practical Dosing Algorithm

1. Initial infusion: Start at 2–3 mL/min (120–180 mL/hr) 1

   • This delivers 8–12 mcg/min for your 75-kg patient
   • Equals approximately 0.1–0.16 mcg/kg/min

2. Target blood pressure:

   • Maintain systolic BP 80–100 mmHg 1
   • For previously hypertensive patients: no higher than 40 mmHg below baseline
   • For septic shock: target MAP ≥65 mmHg 3

3. Titration:

   • Adjust rate based on BP response every few minutes
   • Average maintenance: 0.5–1 mL/min (30–60 mL/hr) 1
   • Maximum can go much higher if needed (up to 68 mg/day reported) 1

Critical Safety Considerations

Administration route:

• Strongly prefer central venous access 2
• Extravasation causes tissue necrosis
• If extravasation occurs: infiltrate 5–10 mg phentolamine diluted in 10–15 mL saline immediately 2

Monitoring requirements:

• Continuous BP monitoring (arterial line preferred for high doses)
• Titrate to clinical response, not fixed dose
• Correct hypovolemia first—norepinephrine is relatively contraindicated in volume depletion 2

Important contraindications and cautions:

• Do not mix with sodium bicarbonate or alkaline solutions 2
• Use cautiously in ischemic heart disease (increases myocardial oxygen demand) 2
• Monitor for occult blood volume depletion if requiring very high doses 1

Weight-Based Dosing Considerations

Recent research shows that obese patients require lower weight-based doses but similar absolute doses compared to non-obese patients 4. For a 75-kg patient of normal body habitus, standard weight-based dosing is appropriate. However, if your patient is obese (BMI ≥30), consider using ideal body weight or starting with lower weight-based doses while monitoring absolute dose requirements.

The dose-response relationship is approximately linear, with general anesthesia roughly doubling the slope compared to awake patients 5, though this is less relevant for shock states where the primary concern is maintaining adequate perfusion pressure.