What is the recommended intravenous amiodarone dosing regimen for rapid conversion of atrial or ventricular arrhythmias in an adult without contraindications, and what monitoring and contraindications should be considered?

Intravenous Amiodarone Dosing and Administration

For life-threatening ventricular arrhythmias or hemodynamically unstable atrial fibrillation with rapid ventricular response, administer IV amiodarone as a 150 mg bolus over 10 minutes, followed by 1 mg/min for 6 hours, then 0.5 mg/min for the remaining 18 hours, delivering approximately 1,050 mg in the first 24 hours. 1, 2

Standard Three-Phase Infusion Protocol

Rapid Loading Phase

• Dilute 150 mg of amiodarone in 100 mL of 5% dextrose in water (D5W) and infuse over 10 minutes as the initial bolus. 1, 2
• Never use normal saline or lactated Ringer's solution as the diluent, as these cause drug precipitation and loss of efficacy. 3

Early Maintenance Phase

• Continue at 1 mg/min for the next 6 hours, delivering approximately 360 mg during this phase. 1, 2

Late Maintenance Phase

• Reduce the infusion rate to 0.5 mg/min for the remaining 18 hours, delivering approximately 540 mg. 1, 2
• The total 24-hour dose should not exceed 2.2 g. 3

Cardiac Arrest Modification

• For pulseless ventricular tachycardia or ventricular fibrillation unresponsive to defibrillation and epinephrine, administer 300 mg IV/IO bolus over 10 minutes. 1
• If the arrhythmia persists, give a second 150 mg bolus. 1

Breakthrough Arrhythmias

• For recurrent ventricular fibrillation or hemodynamically unstable ventricular tachycardia during the maintenance infusion, repeat the 150 mg loading bolus over 10 minutes. 3, 2
• This supplemental bolus may be given 10–30 minutes after the initial dose if breakthrough arrhythmias persist. 3

Administration Requirements

Vascular Access and Concentration

• Administer through a central venous catheter whenever possible, as peripheral vein phlebitis occurs frequently with concentrations >2 mg/mL. 3
• If using a peripheral line, ensure the concentration does not exceed 2 mg/mL. 3
• Use an in-line filter during administration. 3

Expected Onset of Action

• The antiarrhythmic effect typically becomes apparent 20–30 minutes after administration. 3
• Most conversions to sinus rhythm occur after 6–8 hours of therapy and usually require a cumulative dose of at least 1 g. 3, 4
• This delayed onset makes amiodarone less appropriate as a first-line agent for immediate conversion unless the patient is clinically stable. 3

Absolute Contraindications

• Second- or third-degree AV block unless a functioning pacemaker is in place. 1, 2
• Marked sinus bradycardia or sick sinus syndrome without a pacemaker. 1, 2
• Cardiogenic shock. 2
• Known hypersensitivity to amiodarone or iodine. 2

Relative Contraindications Requiring Extreme Caution

• Baseline heart rate <60 beats/min: Use only if the arrhythmia is immediately life-threatening and no safer alternatives exist. 3
• Systolic blood pressure <100 mm Hg: Higher risk for further hemodynamic compromise during infusion. 3
• Moderate or severe left ventricular dysfunction: Can exacerbate hypotension and bradycardia. 3

Critical Monitoring Parameters

Cardiovascular Monitoring

• Continuous ECG monitoring is mandatory throughout the infusion, assessing heart rate, AV conduction, QT interval, PR interval, and QRS duration. 3, 1
• Hypotension occurs in 16–26% of patients receiving IV amiodarone; slow the infusion rate if blood pressure drops, and consider vasopressors, positive inotropic agents, or volume expansion as needed. 3, 1, 2
• Bradycardia and AV block occur in 4.9% of patients; slow or discontinue the infusion if these develop. 3, 1, 2
• QT prolongation is common but rarely causes torsades de pointes. 1

Specific Monitoring in Patients with Pre-existing Bradycardia

• If the patient has a heart rate of 57 bpm, IV amiodarone should be used with extreme caution and is relatively contraindicated unless a pacemaker is in place or the clinical situation is immediately life-threatening. 3
• Monitor heart rate continuously; if it decreases by 10 beats per minute, reduce the infusion rate. 3
• Watch for second- or third-degree heart block, which represents an absolute contraindication to continued therapy without pacemaker support. 3

Drug Interactions Requiring Immediate Action

Digoxin

• Reduce digoxin dose by 50% immediately when starting amiodarone, as amiodarone doubles digoxin levels. 1, 3

Warfarin

• Reduce warfarin dose by 30–50% and check INR within 3–5 days, as amiodarone significantly increases anticoagulation effects. 1, 3
• Monitor INR at least weekly during the first 6 weeks, as interaction effects don't peak until 7 weeks. 3

Other Rate-Control Agents

• Concomitant beta-blockers, calcium channel blockers, or digoxin create additive effects and increase the risk of bradycardia. 3
• Consider reducing doses of these agents approximately 6 weeks after starting amiodarone as its rate-control effect develops. 5

Transition to Oral Therapy

Timing and Dosing

• Begin oral amiodarone while the IV infusion is still running at 0.5 mg/min, due to amiodarone's extremely long half-life (15–100 days, average 58 days). 5
• After <1 week of IV therapy, start 800–1,600 mg oral daily in divided doses. 5, 1
• After 1–3 weeks of IV therapy, start 600–800 mg oral daily in divided doses. 5, 1
• Continue oral loading until a cumulative total of approximately 10 g is reached (typically 1–2 weeks), then reduce to maintenance dosing of 200–400 mg daily. 5, 1

Overlap Strategy

• Continue the IV infusion at 0.5 mg/min for 24 hours after the first oral dose to ensure adequate tissue saturation while oral absorption catches up. 5
• Providing IV and oral overlap for a median of 4 hours does not decrease the rate of early tachyarrhythmia recurrence, so extended overlap beyond 24 hours is not necessary. 6

Common Pitfalls and Caveats

• Do not use amiodarone as a first-line agent for rapid conversion of atrial fibrillation in the emergency department, as most conversions occur after 6–8 hours. 3, 4
• Avoid grapefruit and grapefruit juice during amiodarone therapy, as they inhibit CYP3A-mediated metabolism and increase plasma levels. 3
• Monitor liver and thyroid function every 6 months during maintenance therapy. 3
• Screen for pulmonary toxicity, as amiodarone may cause potentially fatal pulmonary fibrosis. 3
• Amiodarone-associated bradycardia requiring permanent pacemaker is more common in women than in men. 3