Atrial fibrillation (say "A-tree-uhl fih-bruh-LAY-shun") is a type of irregular heartbeat (arrhythmia) in which the heart's upper chambers quiver, or fibrillate. This increases the risk of blood clots, which can cause a stroke or other problems. The lower chambers beat without a regular rhythm and may beat too fast. This can cause symptoms like lightheadedness or chest pain.
You may need medicine, electrical shock (cardioversion), or other treatments to stop the arrhythmia. If your heart doesn't return to a normal rhythm, you may need long-term treatment with medicine to keep your heart from beating too fast and blood-thinning medicine to help prevent a stroke.