pacemaker, artificial, device used to stimulate a rhythmic heartbeat by means of electrical impulses. Implanted in the body when the heart's own electrical conduction system (natural pacemaker) does not function normally, the battery-powered device emits impulses that trigger heart-muscle contraction at a rate that is preset or is determined by demand. The device today may be as small as one inch (2.5 cm) in diameter and weigh as little as 0.5 oz. (14 gm). It is implanted, using local anesthetic, under a flap of skin in the chest or abdomen. One or more electrodes are threaded through a vein from the device to the right side of the heart. First developed in the 1960s, pacemakers originally sent one steady beat to the heart. Modern versions can monitor the heart and activate only when necessary; they are also less sensitive to outside sources of electromagnetic radiation than earlier versions. Most pacemakers run on lithium batteries, which need to be replaced about every 10 years. See also arrhythmia.

Source of rhythmic electrical impulses that trigger heart contractions. In the heart's electrical system, impulses generated at a natural pacemaker are conducted to the atria and ventricles. Heart surgery or certain diseases can interrupt conduction (heart block), requiring use of a temporary or permanent artificial pacemaker. A small electrode attached to an electric generator outside the body is threaded through a vein into the heart. The generator, inserted beneath the skin, produces regular pulses of electric charge to maintain the heartbeat. Permanent pacemakers can also be implanted on the heart's surface.

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