A normal heartbeat is called sinus rhythm. That is a steady regular pulse of about 60 beats per minute. An abnormal heartbeat is called an arrhythmia. The commonest arrhythmia is atrial fibrillation. This is an irregular chaotic heartbeat that has no rhythm to it. It is not a good thing to have because atrial fibrillation allows blood to pool in the heart giving rise to the possibility of clot formation and stroke. 15% of all strokes are due the a fib.