A patient with recurrent ventricular tachycardia is treated with amiodarone. Amiodarone's predominant antiarrhythmic mechanism involves which of the following, and which class of action does this classify it under?
- A Blockade of fast sodium channels during phase 0 — Class IA
- B Selective beta-1 adrenergic blockade — Class II
- C L-type calcium channel blockade in AV node — Class IV
- D Blockade of potassium channels prolonging action potential duration — Class III, but also has I, II, and IV properties ✓
Explanation
Amiodarone is classified as a Class III antiarrhythmic because it predominantly blocks cardiac delayed rectifier potassium channels (IKr/IKs), prolonging the action potential duration and refractory period. However, it also blocks sodium channels (Class I), has non-competitive beta-adrenergic blockade (Class II), and blocks L-type calcium channels (Class IV) — making it the broadest-spectrum antiarrhythmic available. It also has significant effects on thyroid metabolism (structural similarity to thyroxine). Despite its adverse effect profile, it is the most effective antiarrhythmic for both supraventricular and ventricular arrhythmias.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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