What is the mechanism of action of Amiodarone?

• A. Prolong the action potential by blocking Sodium channels
• B. Shorten the action potential by blocking Sodium channels
• C. Prolong the action potential by blocking Potassium channels
• D. Shorten the action potential by blocking Potassium channels

Answer: (C)

Amiodarone is a class III antiarrhythmic, and its main effect is to prolong repolarization by blocking potassium channels in cardiac cells, especially the rapid delayed rectifier current (IKr). This slows phase 3 of the action potential, extending the overall action potential duration and the QT interval, which increases the refractory period and reduces the chance of re-entry tachyarrhythmias. While it also has other actions on sodium channels, calcium channels, and beta receptors, the key mechanism behind prolonging the action potential is potassium-channel blockade.