Amiodarone prolongs the cardiac action potential by blocking which ion channels?

• A. Sodium channels
• B. Potassium channels
• C. Calcium channels
• D. Chloride channels

Answer: (B)

Amiodarone prolongs the cardiac action potential mainly by blocking potassium channels, especially the delayed rectifier potassium currents responsible for phase 3 repolarization. When these channels are inhibited, repolarization slows, the action potential duration increases, and the effective refractory period lengthens. This is the hallmark of class III antiarrhythmic action.

While amiodarone also affects sodium and calcium channels and has beta-blocking effects, these play secondary roles in its overall antiarrhythmic action. Blocking sodium channels would mainly slow the upstroke and conduction, not extend the duration of the action potential, and calcium channel effects alter the plateau rather than extend the repolarization phase. Chloride channels are not the primary players in ventricular action potential duration.