Atrial fibrillation (also called AFib or AF) is a quivering or irregular heartbeat (arrhythmia) that can lead to blood clots, stroke, heart failure and other heart-related complications. At least 2.7 million Americans are living with AFib. Abnormalities or damage to the heart's structure are the most common cause of atrial fibrillation. Possible causes of atrial fibrillation include: High blood pressure. Heart attack. Ideally, to treat atrial fibrillation, the heart rate and rhythm are reset to normal. To correct your condition, doctors may be able to reset your heart to its regular rhythm (sinus rhythm) using a procedure called cardioversion, depending on the underlying cause of atrial fibrillation and how long you've had it The basic cause of AFib is disorganized signals that make your heart's two upper chambers (the atria) squeeze very fast and out of sync. They contract so quickly that the heart walls quiver, or fibrillate. Damage to your heart's electrical system can cause AFib.