Amiodarone's classification as a class III antiarrhythmic is based on potassium channel blockade, but it also possesses actions of which other Vaughan-Williams classes?

• A Class I only (sodium channel blockade)
• B Classes I, II, and IV in addition to class III ✓
• C Class IV only (calcium channel blockade) in addition to class III
• D Classes II and IV only

Explanation

Amiodarone is a multi-channel blocker with properties spanning all four Vaughan-Williams classes: Class I (sodium channel blockade), Class II (non-competitive beta-adrenoceptor antagonism), Class III (potassium channel blockade prolonging the action potential), and Class IV (calcium channel blockade). This broad mechanism explains its efficacy across many arrhythmia types. It also has complex pharmacokinetics with an extremely long half-life (40–55 days), significant thyroid effects, and pulmonary toxicity with chronic use.

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

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