In a normal heart rhythm, a tiny cluster of cells at the sinus node sends out an electrical signal. The signal then travels through the atria to the atrioventricular (AV) node and then passes into the ventricles, causing them to contract and pump out blood. In atrial fibrillation, electrical signals fire from multiple locations in the atria (typically pulmonary veins), causing them to beat chaotically. The AV node — your heart's natural pacemaker — is unable to prevent all of these chaotic signals from entering the ventricles. Your ventricles respond to these extra, chaotic signals by beating faster than normal.