Atrial fibrillation is the most common type of arrhythmia, which is a problem with the rate or rhythm of the heartbeat during which the heart can beat too fast, too slow or with an irregular rhythm. Atrial fibrillation, or AF, occurs when rapid, disorganized electrical signals cause the atria, which consists of the heart's two upper chambers, to contract very fast and irregularly. According to the National Heart, Lung and Blood Institute, when a person is suffering from AF, blood begins to pool in his or her atria. While that blood pools in the atria, it is not being pumped completely into the heart's two lower chambers, and this prevents the lower and upper chambers from working together effectively. Though AF does not always produce noticeable symptoms, some people with AF experience chest pain or heart failure, especially when the heart rhythm is rapid. AF can increase a person's risk of stroke, and it may surface periodically or become an ongoing problem that lasts for years.