Atrial fibrillation (A-Fib) is an irregular and often very rapid heart rhythm. An irregular heart rhythm is called an arrhythmia. A-Fib can lead to blood clots in the heart. The condition also increases the risk of stroke, heart failure and other heart-related complications. During atrial fibrillation, the heart’s upper chambers — called the atria — beat chaotically and irregularly. They beat …