Atrial fibrillation is the most common sustained cardiac arrhythmia. More than two million Americans are believed to have intermittent or sustained atrial fibrillation. The incidence is approximately 1 percent in adults less than 45 years of age, and approximately 5 percent in adults more than 65 years of age. A review by Alexander and Flaker emphasizes new insights on etiology and treatment of atrial fibrillation, including the prevention of embolic complications.
In about 90 percent of cases, atrial fibrillation is associated with heart disease. The most commonly associated condition is hypertension, particularly if complicated by left ventricular hypertrophy. Although relatively uncommon in patients with chronic stable coronary artery disease, atrial fibrillation is an important manifestation of myocardial infarction. Atrial fibrillation develops in approximately 10 percent of patients with myocardial infarction and in up to 30 percent of patients following cardiothoracic surgery. Other cardiac conditions in which atrial fibrillation may develop include pericarditis and dilated cardiomyopathy. In alcoholic cardiomyopathy, atrial fibrillation develops in about 40 percent of patients and may be an indicator of binge drinking. Noncardiac conditions associated with atrial fibrillation include hyperthyroidism and conditions resulting in hypoxia (e.g., pulmonary embolism, pneumonia, chronic obstructive pulmonary disease). Caffeine and alcohol use may precipitate atrial fibrillation. In approximately 10 percent of patients with atrial fibrillation, no apparent cardiac association is evident. In young patients, atrial fibrillation may have a genetic etiology, be associated with bradycardia related to athletic conditioning or be a result of a medication that slows the heart rate.
In many cases, atrial fibrillation is relatively asymptomatic and is discovered incidentally. Less commonly, patients present with symptoms because of rapid ventricular response rates resulting in palpitations, dyspnea or chest discomfort. Treatment strategy depends on the degree of hemodynamic compromise and the risk of thromboembolism. Some patients require prompt control of the ventricular rate with intravenous digoxin, diltiazem or esmolol. Once the ventricular rate is controlled, electrical or pharmacologic cardioversion can be considered. Less common treatment strategies for maintaining sinus rhythm include permanent atrial-based pacing and surgery such as the Maze procedure.
Left atrial thrombus occurs in approximately 10 percent of patients with atrial fibrillation and can be identified by transesophageal echocardiography. In patients without left atrial thrombus, the risk of thromboembolism is low, and cardioversion can proceed without weeks of precardioversion anticoagulation. Cardioversion with 200 J synchronized to the R wave is successful in 75 percent of patients.
Pharmacologic cardioversion with flecainide and other medications is especially effective if initiated within 48 hours of onset of symptoms. Results from a recent study revealed that pretreatment with ibutilide enhances the rate of success of electric cardioversion in selected patients. In pharmacologic cardioversion of patients with acute atrial fibrillation, some experts recommend hospitalization with the use of antiarrhythmic medications because of the risk of inducing serious ventricular arrhythmias during the first 24 to 48 hours.
Because atrial fibrillation has a high rate of recurrence, some authorities advocate prolonged use of prophylactic antiarrhythmic medications following successful cardioversion. In clinical trials, such therapy controlled the sinus rhythm rate in 39 to 83 percent of patients but was associated with unexplained higher mortality rates.
The other major controversy in management of atrial fibrillation concerns the prevalence of thromboembolitic complications. Warfarin therapy is associated with a 64 percent reduction in stroke in patients with atrial fibrillation compared with placebo. Aspirin has been reported to have a 22 percent stroke reduction rate compared with placebo. Factors associated with increased risk of stroke include age, history of hypertension, diabetes, history of stroke and history of congestive heart failure. Current recommendations are for warfarin therapy to be offered to all patients with nonvalvular atrial fibrillation who are more than 75 years of age and to all patients less than 75 years of age who have risk factors for stroke. Studies indicate that the optimal benefit, with acceptable risks of bleeding complications, is achieved at International Normalized Ratio (INR) levels between 2 and 3. This level can be difficult to achieve and sustain. Even with support from specialized clinics and staff, only 61 percent of patients in one study maintained appropriate INR levels. Randomized trials show patient withdrawal rates from warfarin therapy to be 10 to 38 percent over a follow-up period of two to three years.