Atrial fibrillation is an abnormal heart rhythm that causes the upper chamber of the heart to beat in a disorganized manner. This causes the pulse to be irregular and usually rapid.

During atrial fibrillation, the electrical impulse in the upper chambers (atria) of the heart is disorganized. This causes the atria to contract abnormally, pumping less blood into the ventricles. During atrial fibrillation, the heart is unable to pump blood as efficiently because the ventricles are not optimally filled with blood.