Normal heart rhythm versus atrial fibrillation
In a normal heart rhythm, an electrical signal travels from the heart's upper chambers (atria) to the heart's lower chambers (ventricles). In atrial fibrillation, electrical signals fire from multiple locations in the upper chambers — most often from the pulmonary veins. The atrioventricular (AV) node doesn't stop the extra signals from entering the ventricles. The result is a rapid and chaotic heartbeat.