Major
Amiodarone × Metoprolol
Class III antiarrhythmics (Vaughan Williams)×Selective β1-adrenoceptor blockers
Mechanism
Dual mechanism. Amiodarone non-competitively blocks β-adrenoceptors and depresses atrioventricular (AV) conduction; metoprolol is a competitive β-blocker. Effects add up: lowered heart rate and AV conduction. Amiodarone also inhibits CYP2D6 and metoprolol plasma levels rise.
Symptoms
Bradycardia (heart rate below 50/min), dizziness, syncope. ECG: first- to second-degree AV block, prolonged PR. In heart failure: worsening dyspnoea and oedema.
Management
The combination is appropriate in cardiology (atrial fibrillation, post-infarction care). Halve the metoprolol dose when starting amiodarone. Check pulse and ECG at 1 and 4 weeks. If pulse drops below 50 or second-degree AV block appears, adjust metoprolol further.
Sources
- Lexicomp: Lexicomp Drug Interactions (2024)– Wolters Kluwer Clinical Drug Information, Inc. Lexi-Interact Online, 2024
- ESC: 2024 ESC Guidelines for the management of atrial fibrillation developed in collaboration with the EACTS (2024)– Eur Heart J 2024;45(36):3314–3414