Atrial Fibrillation (also called AF or A Fib) is an abnormal heart rhythm that is caused by electrical activity problems in the upper chambers of the heart, called the atria. In atrial fibrillation, the heart beats irregularly, and is often faster or slower than normal. When the atria fibrillate, the blood tends to pool in the atrial chambers, leading to the formation of clots. These blood clots can leave the atria, travel to the brain, and cause a stroke. Atrial fibrillation can also lead to heart failure, if there is heart damage present. People with atrial fibrillation may be at increased risk of stroke, heart failure or heart muscle disease.