The heart is made up of four chambers. The rhythm of your heart is normally controlled by a natural pacemaker (the sinus node) located in the right atrium. The sinus node produces electrical impulses that normally start each heartbeat. The electrical impulses then arrive at a cluster of cells called the AV node. The AV node sends a signal to the ventricles (bottom chambers of the heart). The electrical impulses cause the ventricle to contract and pump blood to the rest of the body.