Amiodarone prolongs the action potential duration primarily by blocking which current, and what is the consequence for refractory period?

• A IKr (rapid delayed rectifier K+ current); prolongs effective refractory period ✓
• B INa (fast sodium current); shortens effective refractory period
• C ICaL (L-type calcium current); shortens effective refractory period
• D IKs (slow delayed rectifier K+ current); has no net effect on refractory period

Explanation

Amiodarone is a Class III anti-arrhythmic that predominantly blocks IKr (rapid delayed rectifier potassium current), preventing potassium efflux during phase 3 repolarisation. This prolongs the cardiac action potential duration and the effective refractory period in all cardiac tissues — the basis for its broad-spectrum anti-arrhythmic activity. Prolonged ERP reduces re-entry circuit sustainability. Its multiple channel-blocking properties (Na, Ca, K + beta-blockade) account for its efficacy across multiple arrhythmia types.

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

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