Purdue News

September 21, 1981 Implantable Device Restarts Heart West Lafayette, Ind. – An implantable device that automatically delivers electric shocks if the heart stops may mean life instead of death for certain high-risk heart patients. Purdue University researchers are making technological refinements to such a device. "The device is about the size of a pack of cigarettes and is implanted like a pacemaker," says Dr. W. A. Tacker, a research physician with Purdue's Biomedical Engineering Center. If a heart stops, as it often does during a heart attack or after a drug overdose or perhaps in an accident that involves a blow to the chest, "the defibrillator applies an electric shock to start it back up again," Tacker explains. At Purdue, defibrillation tests are done in experimental animals. Clinical testing of the implantable defibrillator on humans has been going on for about a year and a half elsewhere, most notably at Johns Hopkins University, Tacker says. "Two kinds of obstacles have slowed use of the device on humans," says the Purdue researcher. First, engineering components such as miniaturization and signal processing must be perfected. Then, medical decisions about who should receive an implantable defibrillator have to be made. Also, Tacker adds, there must be some guarantee that the device doesn't do more harm than good. "If an automatic defibrillator applies an electric shock to a beating heart, it could kill the patient," he says. So the system used to monitor the heart must be as near fail-safe as possible. "Emphasis at Purdue has been on the safety and reliability of the device," Tacker notes. "The cardiac defibrillation unit is implanted under the skin on the chest or in the abdominal cavity," Tacker says. Wires run from the unit to electrodes in the heart, and electric impulses from the heart tell the defibrillator how that organ is doing--the device actually takes a continual electrocardiogram. "The Johns Hopkins defibrillator evaluates the electrocardiogram and decides whether to give a shock based on the electrocardiogram alone," Tacker says. "But what if the electrocardiogram were wrong for some reason?" The Purdue unit monitors another indicator of heart activity in addition to the EKG. "We put two electrodes in the heart and pass a very low-current electrical signal between them. Resistance changes with each heartbeat--it is high when the heart is empty of blood and it is low when the heart is full of blood." So, by measuring the resistance, the defibrillator confirms the diagnosis, Tacker explains. This measurement of the heart's mechanical activity happens only after the electrocardiogram indicates something is wrong. If the measurement confirms the EKG, then the defibrillator is activated "But both indicators have to show the heart is stopped before the unit is used." A person who needs an implanted cardiac defibrillator might already have a pacemaker, Tacker says. Abnormally slow heart rate requires treatment with a weak shock. A pacemaker does that. Fibrillation requires a much larger shock, which the implantable defibrillator would deliver. Eventually, Tacker says, research probably will lead to "a combination unit that would either pace or defibrillate as needed." Currently, externally used defibrillation equipment is standard to hospitals, clinics, doctors' offices and emergency medical vehicles. "But if the heart stops while a person is asleep or alone, the person would die," says Tacker. Purdue News Service: (765) 494-2096; purduenews@purdue.edu