Atrial fibrillation (AF) is the most common heart-rhythm disturbance in the United States, with 5 percent of Americans over age 65 being affected. Over 2 million Americans will seek medical attention for this condition. (Atrial fibrillation should not be confused with ventricular fibrillation, a more acute condition that sometimes occurs during heart attacks.)

AF is a chaotic rhythm disturbance involving both of the upper chambers of the heart, called the atria. During AF, the atria are being electrically activated at a rate much faster than normal.

During normal heart rhythm (usually 60 to 100 beats per minute), each atrial beat conducts in a one-to-one fashion with the main pumping chambers, or ventricles. In contrast, during AF, the rapid atrial rhythm (up to 500 beats per minute) corresponds to an irregular ventricular rhythm of about 110-140 beats per minute.

Aside from causing a rapid and irregular pulse, AF can result in blood pooling in the left atrium, which can lead to the formation of a blood clot that could potentially break free and pass on to the brain, causing a stroke.

In some cases, AF appears in patients without other diseases (lone AF), and it may even run in families. Generally, AF is associated with advancing age and cardiovascular conditions such as hypertension, coronary artery disease, valvular disease, cardiomyopathy (heart enlargement and weakening), and congestive heart failure. AF may also be caused by elevated thyroid hormone and by lung disease.

Episodes of AF may at times be exacerbated by drinking alcohol or caffeine, or by traveling to high altitudes. After a patient has developed AF, the chance of stroke is increased by the following risk factors: age; high blood pressure; diabetes I and II; congestive heart failure; and a prior history of either stroke, mini-stroke (known as a “transient ischemic attack”), or other blood clot originating in the heart (embolus). The risk of stroke is especially high in patients with mitral valve disease (disorders of the valve between the left atrium and left ventricle).

A physician can diagnose atrial fibrillation by monitoring the heartbeat with an electrocardiogram (ECG) machine. If AF is intermittent, an outpatient rhythm monitor may be necessary to record the arrhythmia. After establishing a diagnosis, a complete history and physical exam should be taken, along with blood tests (including thyroid-function tests) and an echocardiogram (to establish whether cardiac abnormalities are present).

The most important complication of AF is stroke, which becomes more likely with the risk factors described above. In cases where AF results in a high heart rate over a period of weeks or months, heart failure can result due to weakening of the heart.

The diagnosis of AF requires prompt medical attention, including rate control and anticoagulation (anti-clot drugs) in most cases. The patient can reduce (but not eliminate) their risk by controlling hypertension and diabetes with diet and exercise.