Review Question - QID 100622

All class III antiarrhythmics lengthen phase 3 (the K+ efflux phase) of the ventricular action potential, resulting in a longer action potential duration, an increased effective refractory period, and a longer QT interval.

Class III antiarrhythmics are generally considered the "antiarrhythmics of last resort" due to their potentially dangerous side effects including torsades de pointes and bradycardia. Amiodarone, a class III antiarrhythmic, has a number of unique well-known side effects that are often tested on USMLE Step 1, including pulmonary fibrosis, hepatotoxicity, hypo/hyperthyroidism, and photosensitivity.

Campbell et al. discuss the classes of antiarrhythmic drugs and proper monitoring of their use in patients. Although used less frequently than the others, class I drugs block the sodium channel and have a very high level of toxicity. Class II drugs are considered anti-sympathetic drugs since they block the beta-adrenoreceptors, which are much safer than class I. Class III are potassium channel blockers that prolong repolarization. Class IV drugs are calcium channel blockers and are usually monitored by their hemodynamic effects.

Wexler et al. review the approach to the patient with palpitations. Although palpitations in patients are mainly of cardiac etiology, psychiatric conditions such as anxiety may also produce palpitations. Even if a psychiatric etiology is considered most likely, cardiac reasons should be ruled out as cardiogenic palpitations may result in sudden death. A proper physical exam, history, and EKG can help pinpoint the origin, and cardiac imaging can rule out any structural causes. The most common causes of palpitations are ventricular and atrial premature contractions; whereas the most common cause of hospitalization due to palpitations is atrial fibrillation which may put the patient at risk for stroke.

Illustration A depicts the various phases of the cardiac action potential and shows the effect of class III antiarrhythmic drugs.

Incorrect answers:
Answer 1 & 2: Flecanide and propafenone are class IC antiarrhythmics that block sodium channels; they do not increase the length of the action potential significantly.
Answer 3: Mexiletine is a class IB antiarrhythmic that blocks sodium channels and shortens the duration of the action potential.
Answer 4: Diltiazem is a class IV antiarrhythmic and decreases conduction through the AV node; it would have the above mentioned effects on the action potential.