A 60-year-old man with ischemic cardiomyopathy and palpitations is started on amiodarone. Which phase of the cardiac action potential is its primary site of action?
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Class III antiarrhythmics like amiodarone act on phase 3 by blocking potassium channels.
Step 1: Understanding the Question:
The question requires matching the anti-arrhythmic drug Amiodarone to the specific phase of the cardiac action potential it primarily influences. Step 2: Detailed Explanation:
Vaughan-Williams Classification: Amiodarone is classified as a Class III anti-arrhythmic agent. While it has some properties of all four classes (alpha/beta blockade, Na channel block, and Ca channel block), its primary mechanism is the blockade of potassium (\(K^+\)) channels.
Phase 3 - Repolarization: In the cardiac action potential, Phase 3 is characterized by the rapid efflux of potassium ions, which brings the cell back to its resting membrane potential.
Mechanism: By blocking these potassium channels, Amiodarone prolongs the duration of the action potential and the effective refractory period (ERP). This makes it highly effective against both supraventricular and ventricular arrhythmias.
Impact on ECG: On an electrocardiogram, this prolongation of the action potential manifests as an increase in the QT interval.
Clinical context: It is widely used in patients with structural heart disease (like ischemic cardiomyopathy) because it carries a lower risk of pro-arrhythmia compared to other agents, though it has significant non-cardiac side effects (thyroid, pulmonary, and hepatic).
Step 3: Final Answer:
Amiodarone primarily acts on Phase 3 (Repolarization) by blocking potassium channels and prolonging the refractory period.
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