A 69-year-old retired teacher presents with a 1-month history of palpitations, intermittent shortness of breath, and fatigue. She has a history of hypertension. An electrocardiogram (ECG) shows atrial fibrillation with a ventricular response of 122 beats/min (bpm) and signs of left ventricular hypertrophy. She is anticoagulated with warfarin and started on sustained-release metoprolol, 50 mg/d. After 7 days, her rhythm reverts to normal sinus rhythm spontaneously. However, over the ensuing month, she continues to have intermittent palpitations and fatigue. Continuous ECG recording over a 48-hour period documents paroxysms of atrial fibrillation with heart rates of 88–114 bpm. An echocardiogram shows a left ventricular ejection fraction of 38% (normal ≥ 60%) with no localized wall motion abnormality. At this stage, would you initiate treatment with an antiarrhythmic drug to maintain normal sinus rhythm, and if so, what drug would you choose?
Cardiac arrhythmias are a common problem in clinical practice, occurring in up to 25% of patients treated with digitalis, 50% of anesthetized patients, and over 80% of patients with acute myocardial infarction*. Arrhythmias may require treatment because rhythms that are too rapid, too slow, or asynchronous can reduce cardiac output. Some arrhythmias can precipitate more serious or even lethal rhythm disturbances; for example, early premature ventricular depolarizations can precipitate ventricular fibrillation. In such patients, antiarrhythmic drugs may be lifesaving. On the other hand, the hazards of antiarrhythmic drugs—and in particular the fact that they can precipitate lethal arrhythmias in some patients—have led to a reevaluation of their relative risks and benefits. In general, treatment of asymptomatic or minimally symptomatic arrhythmias should be avoided for this reason.
Arrhythmias can be treated with the drugs discussed in this chapter and with nonpharmacologic therapies such as pacemakers, cardioversion, catheter ablation, and surgery. This chapter describes the pharmacology of drugs that suppress arrhythmias by a direct action on the cardiac cell membrane. Other modes of therapy are discussed briefly (see Box: The Nonpharmacologic Therapy of Cardiac Arrhythmias, later in the chapter).
The Nonpharmacologic Therapy of Cardiac Arrhythmias
It was recognized over 100 years ago that reentry in simple in vitro models (eg, rings of conducting tissues) was permanently interrupted by transecting the reentry circuit. This concept is now applied in cardiac arrhythmias with defined anatomic pathways—eg, atrioventricular reentry using accessory pathways, atrioventricular node reentry, atrial flutter, and some forms of ventricular tachycardia—by treatment with radiofrequency catheter ablation or extreme cold, cryoablation. Mapping of reentrant pathways and ablation can be carried out by means of catheters threaded into the heart from peripheral arteries and veins. Studies have also shown that paroxysmal and persistent atrial fibrillation may arise from one or more of the pulmonary veins. Both forms of atrial fibrillation can be cured by electrically isolating the pulmonary veins by radiofrequency or cryotherapy catheter ablation or during concomitant cardiac surgery. Because catheter ablation therapy can often permanently cure atrial fibrillation, ...