Review Question - QID 218529

This patient who presents with palpitations likely has paroxysmal atrial fibrillation. Amiodarone may be used to achieve rhythm control in patients who have atrial fibrillation.

Amiodarone is a class III anti-arrhythmic drug; however, it is highly lipophilic and also has class I, II, and IV effects. It works by blocking potassium channels, delaying the repolarization of myocytes. This results in a longer action potential duration. Amiodarone is primarily used to achieve rhythm control in atrial fibrillation and atrial flutter, and may also be used in wide complex tachycardia such as ventricular tachycardia. Because amiodarone increases the duration of the action potential, it also lengthens the QT interval. However, use of amiodarone typically does not have a pro-arrhythmic effect and the risk of inducing torsade de pointes is not significantly increased. Amiodarone has significant toxicities including pulmonary fibrosis, hepatotoxicity, optic neuropathy, and hypothyroidism. Therefore, prior to initiating amiodarone, a thyroid function test, liver function test, baseline ophthalmologic exam, and pulmonary function test should be performed. Of note, rhythm control is not the preferred modality of treating chronic atrial fibrillation given the side-effects associated with this medication.

Shenthar et al. examined the incidence of drug-induced torsade de pointes with amiodarone. They found that the incidence of drug-induced torsade de pointes with amiodarone was about 1.5% and that risk factors for developing torsade de pointes included female sex, left ventricular dysfunction, electrolyte abnormalities, baseline prolonged QT interval, concomitant beta-blocker, and digoxin therapy. They recommended treating amiodarone-induced torsade de pointes promptly with magnesium or defibrillation to achieve good prognosis.

Incorrect Answers:
Answer 2: Digoxin is a cardiac glycoside that directly inhibits Na/K ATPase which leads to inhibition of the Na/Ca exchanger, leading to an increase in intracellular calcium and positive inotropy in cardiac myocytes. Digoxin works to achieve rate control in atrial fibrillation by decreasing conduction of the atrioventricular node and is not used for rhythm control.

Answer 3: Metoprolol is a class II anti-arrhythmic (beta blocker). This medication can be used in atrial fibrillation to reduce the ventricular rate, but does not control rhythm.

Answer 4: Procainamide is a class IA anti-arrhythmic drug that blocks Na+ channels, slowing depolarization. It is typically used for re-entrant supraventricular tachycardia and is not used for rhythm control of atrial fibrillation.

Answer 5: Verapamil is a class IV anti-arrhythmic (non-dihydropyridine calcium channel blocker). This drug functions by decreasing conduction velocity at the atrioventricular node and therefore helps to achieve rate control in atrial fibrillation.

Bullet Summary:
Amiodarone is used to achieve rhythm control in patients with atrial fibrillation and increases the duration of the action potential as well as the QT interval.