What is a sample intravenous amiodarone dosing calculation for a 70‑kg adult with a ventricular arrhythmia?

IV Amiodarone Dosing Calculation for a 70-kg Adult with Ventricular Arrhythmia

For a 70-kg adult with life-threatening ventricular arrhythmia, administer 150 mg IV over 10 minutes (diluted in 100 mL D5W), followed by 1 mg/min for 6 hours (360 mg total), then 0.5 mg/min for 18 hours (540 mg total), with a maximum 24-hour dose of 2.2 g. 1, 2, 3

Step-by-Step Dosing Protocol

Initial Loading Dose

• Preparation: Dilute 150 mg amiodarone in 100 mL of 5% dextrose in water (D5W) to achieve a concentration of 1.5 mg/mL 2, 3
• Administration rate: Infuse over 10 minutes for hemodynamically unstable ventricular tachycardia or ventricular fibrillation 1, 2
• Weight consideration: For this 70-kg patient, 150 mg equals approximately 2.1 mg/kg, which is within the therapeutic range 3
• Critical requirement: Use only D5W as the diluent; normal saline causes drug precipitation 2, 3

Early Maintenance Infusion (Hours 1-6)

• Rate: 1 mg/min continuously for 6 hours 1, 2, 3
• Total delivered: Approximately 360 mg during this phase 2, 3
• Concentration limit: If using peripheral IV, keep concentration ≤2 mg/mL; concentrations >2 mg/mL require central venous access to prevent severe phlebitis 1, 2

Late Maintenance Infusion (Hours 7-24)

• Rate: 0.5 mg/min for the remaining 18 hours 1, 2, 3
• Total delivered: Approximately 540 mg during this phase 2, 3
• Cumulative 24-hour dose: Loading (150 mg) + early maintenance (360 mg) + late maintenance (540 mg) = 1,050 mg total 2, 3

Maximum Dose Ceiling

• Absolute limit: Do not exceed 2.2 g of amiodarone in any 24-hour period 1, 2, 3
• Supplemental boluses: Up to 6-8 additional 150 mg boluses (each over 10 minutes) may be given for breakthrough arrhythmias within 24 hours, but total exposure must remain ≤2.2 g 2

Practical Calculation Example for 70-kg Patient

Hour 0-0.17 (first 10 minutes):

• 150 mg IV bolus = 150 mg

Hours 0.17-6:

• 1 mg/min × 60 min/hr × 5.83 hours = 350 mg (approximately 360 mg)

Hours 6-24:

• 0.5 mg/min × 60 min/hr × 18 hours = 540 mg

Total first 24 hours: 150 + 360 + 540 = 1,050 mg

Essential Administration Requirements

Equipment and Setup

• Infusion device: Use a volumetric infusion pump; drop-counter devices can underdose by up to 30% 2
• In-line filter: Mandatory for all amiodarone infusions 2, 3
• IV access: Peripheral access acceptable if concentration ≤2 mg/mL; central line required for higher concentrations 1, 2, 3

Monitoring Parameters (Mandatory)

• Continuous ECG monitoring: Watch for bradycardia (occurs in 4.9% of patients), AV block, QT prolongation, and QRS changes 1, 2, 3
• Blood pressure: Hypotension develops in 16-26% of patients and is rate-dependent 1, 2, 3
• Heart rate threshold: If HR falls below 50 bpm with symptoms, reduce infusion rate by 50% or temporarily discontinue 3

Timing of Therapeutic Effect

• Onset of action: Antiarrhythmic effects typically appear within 20-30 minutes after administration 2, 3
• Peak conversion time: Most conversions to sinus rhythm occur after 6-8 hours and usually require a cumulative dose ≥1 g 2, 4, 5
• Clinical implication: The delayed onset makes amiodarone less appropriate as first-line therapy unless the patient is clinically stable 2

Absolute Contraindications (Must Exclude Before Starting)

• Second- or third-degree AV block without a functioning pacemaker 1, 2, 3
• Severe sinus node dysfunction without a pacemaker 2, 3
• Cardiogenic shock 3
• Marked sinus bradycardia 3

Relative Contraindications (Use Only if Life-Threatening)

• Baseline heart rate <60 bpm 2, 6
• Systolic blood pressure <100 mmHg 2
• Moderate-to-severe left ventricular dysfunction 2, 7
• Decompensated heart failure 3

Critical Drug Interactions

• Digoxin: Reduce dose by 50% when starting amiodarone; levels will double 1, 2, 3, 8
• Warfarin: Reduce dose by 30-50% and monitor INR weekly for 6 weeks; interaction peaks at 7 weeks 1, 2, 3
• Beta-blockers or calcium-channel blockers: Additive bradycardia risk; consider dose reduction 6

Management of Adverse Effects

Hypotension (16% incidence)

• Action: Slow or temporarily discontinue the infusion; hypotension is directly related to infusion rate and usually resolves with rate reduction 2, 3, 5
• Pitfall: Most hypotensive episodes occur during the rapid bolus, not the maintenance infusion 2

Bradycardia (4.9% incidence)

• Action: If HR <50 bpm with symptoms, reduce infusion rate by 50% and hold other rate-slowing drugs 6, 3
• Obtain ECG: Exclude second- or third-degree heart block 6, 3

Heart Block

• Action: Discontinue immediately; this is an absolute contraindication without pacemaker support 2, 3

Special Considerations for Cardiac Arrest

• VF/pulseless VT protocol: Give 300 mg IV/IO bolus over 10 minutes after defibrillation and epinephrine 2
• Second dose: 150 mg IV/IO if rhythm persists 2
• Weight-based dosing: 5 mg/kg rapid bolus (350 mg for 70-kg patient), may repeat up to total 15 mg/kg/day 2, 3

Common Pitfalls to Avoid

1. Do not use rapid bolus in stable patients: Rapid administration offers no therapeutic advantage and markedly increases hypotension risk 3
2. Never use normal saline: Only D5W is compatible; saline causes precipitation 2, 3
3. Do not exceed 2 mg/mL peripherally: Higher concentrations require central access to prevent phlebitis 1, 2, 3
4. Do not discontinue abruptly: In patients on chronic amiodarone or beta-blockers, abrupt cessation risks rebound ischemia and arrhythmias 3
5. Electrical cardioversion takes priority: If the patient becomes hemodynamically unstable during pharmacologic therapy, immediately perform synchronized cardioversion 2, 3