The rhythmic, regular beating of the heart is controlled by a natural pacemaker--a small patch of cells at the top of the right atrium called the sinus node which sends rhythmic electric impulses along specific conducting fibers tothe heart muscles, stimulating them to contract and relax in a regular sequence. When the heart muscles fail to receive the pacemaker's signals, the heart ceases pumping blood ceases. Within a few minutes, the patient faints, andwithin a few more minutes, dies--unless the heart muscles can be stimulated to resume contracting. An artificial pacemaker is designed to help a damaged heart beat normally, programmed to send an electrical impulse to stimulate theheart muscle if it does not sense a normal heart beat within a specific amount of time. Dual-chamber pacemakers, the most common type, sense and pace activity in both the atrium (upper chamber) and ventricle (lower chamber). Single-pass pacemakers use only one lead and sense only one chamber, usually the ventricle. Faster and more easily implanted than the dual lead type, this pacemaker is indicated only in certain instances.

The English surgeon W. H. Walshe first suggested using electric impulses to restart the heart in 1862. Nearly a century later, Harvard-educated American cardiologist, Paul Zoll, believed he could use the heart's responsiveness to electrical stimulation to treat cases of "heart block." His first attempt, passing an electrode down the esophagus, failed, but in 1952 he developed an external pacemaker, passing an electric shock to the heart through electrodes placed on a patient's chest. In October 1952, Zoll's pacemaker was used to maintain a heartbeat in a man suffering from congestive heart failure; after twodays, the patient's own heart took over again.

Zoll's machine, while effective, had inherent limitations: The shocks were painful to the chest muscles, and the machine--and thus the movement of the patient--were restricted to the nearest electrical outlet. Researchers envisioned an implantable pacemaker, and American inventor Wilson Greatbatch had dreamed of building one since he first heard of heart block in 1951. The restrictive size of vacuum tubes and storage batteries made it impossible, however. Astransistors became widely available in the late 1950s, Greatbatch mentionedhis idea to Dr. William Chardack of the Buffalo, New York, Veterans Administration Hospital and, with Chardack's encouragement, put together an implantable pacemaker in three weeks, working in the barn behind his home. After two years of animal testing, in 1960, Chardack and his associates implanted the first pacemakers in the chest wall of a human patient. Also in 1960, Ake Senningand his colleague, Elmqvist, also designed an implantable pacemaker with anexternal coil and internal receiver. Modern pacemakers are much improved overthese designs. Lightweight and relatively easy to install, and their lithiumbatteries last up to ten years as opposed to the mercury-zinc battery's lifeof twenty months. The first generation of pacemakers sent signals at a preset rate; researchers Ken Anderson and Dennis Brumwell of Medtronic, Inc., in Minneapolis, Minnesota, advanced pacemaker technology immensely when they invented activity-responsive pacing in 1981 by using piezoelectric crystals thatreacted to differing levels of body exertion. Medtronic's "Activitrax," introduced in 1985, was the first pacemaker to adjust pacing rate to exercise level. Several even more advanced pacemakers became available in the late 1980s,among them Medtronic's "Legend." These devices can be reprogrammed while they're still implanted, using radio-frequency signals to reset the pacemaker's microprocessor. They also store information about cardiac events, and some models can even transmit that information over the telephone, directly from thepatient's chest to the doctor's office.

Two concerns surrounding the pacemaker are interference by digital wireless telephones and the "Year 2000" effect. Research indicates that some digital phones may interfere with some pacemakers but pose no more risk than metal detectors at airport security checkpoints or X-rays, and that interference is unlikely if the phone is held six in (15.24 cm) or more from the pacemaker. Research is underway to determine how this interference can be prevented. Medtronic, the world's largest supplier of implantable medical devices, issued a statement in March 1998 that the anticipated "Year 2000" computer problems willin no way affect their computer-aided pacemakers because therapy is based oninternal counters which are unrelated to any specific calendar date.

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