PACEMAKER

electronic cardiac-support device that produces rhythmic electrical impulses that take over the regulation of the heartbeat in patients with certain types of heart disease. A healthy human heart contains its own electrical conducting system capable of controlling both the rate and the order of cardiac contractions. Electrical impulses are generated at the sinoatrial node in the right atrium, one of the chambers of the heart. They then pass through the muscles of both atria to trigger the contraction of those two chambers, which forces blood into the ventricles. The wave of atrial electrical activity activates a second patch of conductive tissue, the atrioventricular node, initiating a second discharge along an assembly of conductive fibres called the bundle of His, which induces the contraction of the ventricles. When electrical conduction through this system is interrupted, as is the case in a number of diseases including congestive heart failure and as an aftereffect of heart surgery, the condition is called heart block. An artificial pacemaker may be employed temporarily until normal conduction returns, or permanently to overcome the block. In temporary pacing, a miniature electrode attached to fine wires is introduced into the heart through a vein, usually in the arm. The pacing device, an electric generator, remains outside the body and produces regular pulses of electric charge to maintain the heartbeat. In permanent pacing, the electrode may again be passed into the heart through a vein or it may be surgically implanted on the heart's surface; in either case the electrode is generally located in the right ventricle. The electric generator is placed just beneath the skin, usually in a surgically created pocket below the collarbone. The first pacemakers were of a type called asynchronous, or fixed, and they generated regular discharges that overrode the natural pacemaker. The rate of asynchronous pacemakers may be set at the factory or may be altered by the physician, but once set they will continue to generate an electric pulse at regular intervals. Most are set at 70 to 75 beats per minute. More recent devices are synchronous, or demand, pacemakers that trigger heart contractions only when the normal beat is interrupted. Most pacemakers of this type are designed to generate a pulse when the natural heart rate falls below 68 to 72 beats per minute. These instruments have a sensing electrode that detects the atrial impulse. Once in place, the electrode and wires of the pacemaker usually require almost no further attention. The power source of the implanted pulse generator, however, requires replacement at regular intervals, generally every four to five years. Most current pacemakers use batteries as a power source, but there has been some exploration of generators energized by radioactive isotopes such as plutonium-238.