Atrial fibrillation (A-tre-al fi-bri-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm. AF occurs when rapid, disorganized electrical signals cause the atria (AY-tree-uh), the two upper chambers of the heart, to fibrillate. The term "fibrillate" means to contract very fast and irregularly. In AF, blood pools in the atria and isn't pumped completely into the ventricles (VEN-trih-kuls), the heart's two lower chambers. As a result, the heart's upper and lower chambers don't work together as they should.Often, people who have AF may not feel symptoms. However, even when not noticed, AF can increase the risk of stroke. In some people, AF can cause chest pain or heart failure, particularly when the heart rhythm is very rapid. AF may occur rarely or every now and then, or it may become a persistent or permanent heart rhythm lasting for years.