Atrial fibrillation (AF) is the most common arrhythmia. An arrhythmia is a problem with the speed or rhythm of the heartbeat. A disorder in the heart’s electrical system causes AF and other types of arrhythmia. AF occurs when rapid, disorganized electrical signals in the heart’s two upper chambers, called the atria, cause them to contract very fast and irregularly (this is called fibrillation). As a result, blood pools in the atria and isn’t pumped completely into the heart’s two lower chambers, called the ventricles. When this happens, the heart’s upper and lower chambers don’t work together as they should. Often, people who have AF may not even feel symptoms. However, even when not noticed, AF can lead to an increased risk of stroke. In many patients, particularly when the rhythm is extremely rapid, AF can cause chest pain, heart attack, or heart failure. AF may occur rarely or every now and then, or it may become a persistent or permanent heart rhythm lasting for years.