Amiodarone is used for both atrial fibrillation and ventricular arrhythmias because it acts on multiple ion channels. Amiodarone is classified as which Vaughan-Williams class, and why is this classification considered an oversimplification?

• A Class I (Na+ channel blocker), but it also inhibits repolarization by blocking K+ channels
• B Class II (beta-blocker) with additional Class IV (Ca2+ channel blocking) properties
• C Class III (K+ channel blocker), but it also blocks Na+ channels, Ca2+ channels, and has non-competitive beta-blockade ✓
• D Class IV (Ca2+ channel blocker) with Na+ channel blocking activity at toxic doses only

Explanation

Amiodarone is primarily classified as a Class III antiarrhythmic because it prolongs the action potential duration and refractory period by blocking outward potassium channels (IKr, IKs, Ito). However, it also potently blocks inactivated fast sodium channels (Class I effect), blocks L-type calcium channels (Class IV), and exerts non-competitive beta-adrenergic blockade (Class II). This multi-channel mechanism is responsible for both its broad antiarrhythmic efficacy and its diverse toxicity profile (pulmonary, thyroid, hepatic, corneal). Amiodarone contains iodine and can cause both hypothyroidism and hyperthyroidism.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.