Atrial fibrillation is a type of abnormal heart rhythm (arrhythmia) of your heart. Atrial fibrillation starts in the upper chambers of your heart (the atria) and causes them to quiver (fibrillate), instead of beating normally. This can mean your heart doesn’t pump blood around your body as efficiently as it should.Some people have a single episode of atrial fibrillation. For others it can come and go (sporadic or paroxysmal), or be permanent (persistent).It’s important to diagnose and treat atrial fibrillation because it can cause blood clots that can block blood supply to your vital organs and lead to a stroke. It may also be a sign that you have heart disease.