Purdue News

November 22, 1982 Purdue Engineers Develop Two-Speed Cardiac Pacemaker West Lafayette, Ind. – A cardiac pacemaker that automatically increases the heart rate in response to exercise has been developed by Purdue University's Biomedical Engineering Center. To a person currently limited by a fixed-rate pacemaker, the exercise-responsive device could mean a more active, normal life, says Neal Fearnot, research engineer at Purdue who developed the new pacemaker. "Ideally, a person using a pacemaker is able to carry on the same activities done before the pacemaker was implanted," says Fearnot. "But today, a person using a pacemaker that causes the heart to beat at a constant rate may have a limited ability to do exercise. With this pacemaker, the person could carry on a routine exercise program." Though the new pacemaker has not yet been tested in humans, laboratory research with animals has been very successful, says Fearnot. The device is currently being scaled down to a size for implantation in humans. Using a thermal sensor to measure the temperature of blood circulating through the heart, the new pacemaker can increase the heart rate to accommodate physical activity, and resume a normal heart rate shortly after the activity slows down or stops, says Fearnot. "When a person exercises, the muscles are working and generating heat. Since muscles are only 15 to 20 percent efficient, 80 percent of the energy is given off in heat, which is then carried away by the blood to the heart," he explains. It is at this point that a micro-computer in the new pacemaker can detect the level of activity, based on the temperature readings, and choose between the "exercise" and "resting" rates, says Fearnot. "The pacemaker we have developed is now set to work at two heart rates, which would be set by a physician after the pacemaker is implanted," says Fearnot. "For example, a patient with the pacemaker set at a heart rate of 72 beats per minute could have the pacemaker choose a rate of 98 to accommodate a moderate amount of exercise." The exercise-responsive pacemaker has been very successful in dogs with abnormal heart rates, says Fearnot. The dogs are able to exercise and run regularly. Currently, some 300,000 Americans are living with some type of implanted pacemaker, Fearnot says. "The type of pacemaker used and the rate at which it operates will depend upon the problem being treated." Pacemakers are generally used by patients whose hearts either cannot properly initiate or cannot respond to the electrical signals needed for contraction. Depending upon the particular problem, the pacemaker might be used continuously to stimulate the heart, or serve as a back-up to the natural pacemaker, triggered when there is a long pause or abnormal heart beat. The pacemaker developed at Purdue could be used by patients who do not send any electrical signals at all, or in cases where the signals are unreliable, says Fearnot. Today, these patients are limited by constant-rate pacemakers. The limits associated with constant-rate pacemakers are due to the continuous "resting" pace used by the devices now available, Fearnot explains. "The heart naturally responds to exercise by beating faster, allowing more oxygen to flow to muscles where it is needed. With a constant-rate pacemaker, the heart must compensate for the slower rate by pumping more blood through the heart with each contraction." This response is more strenuous for the heart than the natural "speeding-up process," says Fearnot. "A patient doing exercise might become dizzy or nauseated after a short time, or might even pass out if the heart is forced to work too hard." Another group of pacemaker patients, those who send electrical signals but whose hearts do not respond correctly, may enjoy the benefits of a similar exercise-responsive pacemaker in the near future. The DDD (a universal code used by physicians to describe the various functions of a pacemaker) is currently being used in clinical tests around the country for patients with "good atrial electrical signals," says Fearnot. "If a patient's natural pacemaker is functioning and sending strong signals, the DDD pacemaker can be rate-responsive," he says. "But if the patient is not sending any electrical signals, or if the natural pacemaker is not reliable, a pacemaker such as ours could be used." Researchers at Purdue are currently investigating the possibility of a pacemaker that would operate at more than two rates. "At this point, we don't know how many speeds are needed, but the technology is there to develop pacemakers that will respond to the needs of the body," says Fearnot. The exercise-responsive pacemaker is not presently available, he adds, but could be available to patients in the next few years. The research conducted at Purdue is sponsored by Cook Pacemaker Co. in Leechburg, Pa. Purdue News Service: (765) 494-2096; purduenews@purdue.edu