A normal heartbeat begins with a group of specialized pacemaker cells called the sinoatrial node in the heart's upper, right chamber, or atrium. Regular electrical impulses from these cells cause the atria to contract or squeeze, pushing blood into the lower chambers, or ventricles. Traveling down to a junction called the atrioventricular, or AV node, and on to the ventricles, these impulses cause the ventricles to contract. This pumps blood out to the lungs and the rest of the body. A ventricular premature beat, or VPB, is an extra heartbeat caused by electrically unstable muscles in the ventricles. Also called premature ventricular contractions, or ventricular premature contractions, VPB's occur in almost everyone, sometimes rarely and sometimes often.
