Atrial fibrillation is the rapid and uncontrolled beating of the atria, the upper chambers of the heart, and is the most common cardiac arrhythmia. It can result from congestive heart failure, myocardial infarction, valvular disease, hypertension, infection, or, in 20 to 35% of cases, without any evidence of other cause. Patients with AFib experience palpitations, chest discomfort, weakness, fainting and breathlessness. Their quality of life can be so severely reduced that in one study the average quality of life was equivalent to that of patients crippled from rheumatoid arthritis and below that for patients with congestive heart failure or recent myocardial infarction.Patients with AFib have a severely elevated risk of stroke (5% risk of stroke per year or 5 to 6 times the risk of the general population). In the U.S. approximately 100,000 strokes per year are due to AFib. Because of the risk of stroke, patients require lifelong treatment with anticoagulants. This involves two office visits per year and monthly blood tests to measure clotting rates. Even so, such therapy only reduces the stroke risk by two-thirds, (to double the risk of those without AFib), and carries a 2.4% yearly risk of major hemorrhage.