Atrial fibrillation (AFib) is the most common type of irregular heartbeat. The abnormal firing of electrical impulses causes the atria (the top chambers in the heart) to quiver (or fibrillate). View an animation of atrial fibrillation. Additional common symptoms of atrial fibrillation. Sometimes people with AFib have no symptoms and their condition is only detectable upon physical examination. Still, others may experience one or more of the following symptoms: