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
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