Amiodarone is classified as a Vaughan-Williams class III antiarrhythmic but also exhibits properties of classes I, II, and IV. Its predominant electrophysiological effect is:
- A Rapid sodium channel blockade causing QRS prolongation at therapeutic concentrations
- B Blockade of repolarising potassium channels (IKr, IKs), markedly prolonging the action potential duration and QTc interval ✓
- C Selective L-type calcium channel blockade reducing SA and AV node automaticity
- D Non-selective beta-adrenoceptor agonism increasing refractory periods
Correct answer: B. Blockade of repolarising potassium channels (IKr, IKs), markedly prolonging the action potential duration and QTc interval
Explanation
The dominant mechanism of amiodarone is class III activity — blockade of multiple repolarising potassium currents (IKr, IKs, IK1) — which prolongs action potential duration and effective refractory period in all cardiac tissues, manifesting as QTc prolongation on ECG. This explains its broad-spectrum efficacy across supraventricular and ventricular arrhythmias. It also has class I (sodium channel), class II (non-competitive beta-block), and class IV (calcium channel) properties but these are secondary to its potassium channel blockade.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.