Atrial fibrillation (AF or A-fib) is an abnormal heart rhythm characterized by rapid and irregular beating. Often it starts as brief periods of abnormal beating which become longer and possibly constant over time.Most episodes have no symptoms. Occasionally there may be heart palpitations, fainting, shortness of breath, or chest pain. The disease is associated with an increased risk of heart failure, dementia, and stroke. It is a type of supraventricular tachycardia.