Pacemakers: A small battery operated device that helps the heart beat in a regular rhythm. A pacemaker uses batteries to send electrical impulses to the heart to help it pump properly. An electrode is placed next to the heart wall and small electrical charges travel through the wire to the heart. Most pacemakers are demand pacemakers, which have a sensing device that turns the signal off when the heartbeat is above a certain level and turns it back on when the heartbeat is too slow.