Cardiovascular System MCAT Practice Question
A 45-year-old man with a history of hypertension presents to the emergency department with palpitations and dyspnea. Cardiac monitoring reveals atrial fibrillation with a ventricular rate of 145 bpm. An ECG shows a prolonged QT interval. He is started on IV amiodarone, a Class III antiarrhythmic that prolongs the action potential duration by blocking potassium channels. Within 2 hours, his heart rate decreases to 88 bpm and his rhythm converts to normal sinus rhythm. Which of the following best explains the mechanism by which amiodarone's prolongation of the action potential duration terminates the atrial fibrillation?
Answer choices
- AIncreased action potential duration enhances sodium-potassium pump activity, restoring normal resting membrane potential more rapidly between consecutive beats
- BExtended refractoriness period prevents reentry by making tissue unable to conduct premature impulses that would otherwise perpetuate the arrhythmiaCorrect answer
- CProlonged repolarization increases intracellular calcium accumulation, which strengthens atrial contractility and mechanically terminates fibrillatory activity
- DDelayed repolarization decreases the slope of phase 4 diastolic depolarization, reducing spontaneous ectopic activity from latent pacemaker cells in the atria
- EExtended action potential duration increases the maximum diastolic potential, making atrial myocytes hyperpolarized and resistant to any incoming signals
- FProlonged action potential duration slows conduction velocity throughout the atrium, allowing AV nodal tissue to recover and block all atrial impulses
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