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"This is the first study to show no benefit from defibrillators if someone is at low risk," said study author Dr. Paul Chan, a fellow in cardiovascular medicine at the University of Michigan Medical School.
The report is published in the January issue of the Journal of the American College of Cardiology.
Chan worked with cardiologists at the Ohio Heart and Vascular Center in Cincinnati, studying 768 patients who were candidates for defibrillators, which deliver an electrical pulse when needed to keep the heart beating regularly. All the patients had survived heart attacks, and half of them had defibrillators implanted.
All of them had a test called microvolt T-wave alternans (MTWA), a test that has become available in the last few years. "What we were interested in was whether people identified as low risk were really low risk," Chan said.
In all, 67 percent of the patients had positive or inconclusive MTWA tests. Those who got defibrillators were 55 percent less likely to die in the three-year follow-up period than those who didn't. They were also 70 percent less likely to die of a heart rhythm disturbance.
But there was no difference in the death rate for the one third of people who had negative MTWA tests, whether or not they received implanted defibrillators.
"That was a very striking finding, because these people have a very high mortality rate," Chan said. "Over a couple of years the death rate was about 50 percent, but the defibrillators had no benefit."
Nevertheless, many cardiologists remain skeptical about use of the test, Chan said. The cost of the test, about $400, is covered by Medicare, but the machine needed to do the test costs $25,000.
It would be worth it if test results prevented implantation of defibrillators in people who would not benefit from them; the lifetime cost of an implantable defibrillator is in the neighborhood of $90,000.
In all, one life could be saved every two years for every nine implanted defibrillators in people with positive or inconclusive MTWA test results, the researchers reported. But it would take 76 implants to save one life every two years for people with negative MTWA tests.
"This is a start," said Dr. Andrea Russo, an electrophysiologist at the University of Pennsylvania, and co-author of an accompanying editorial in the journal. This study could help determine which patients should receive an implantable defibrillator, she said.
The main method used now to determine who might benefit from an implantable defibrillator is a measurement called "ejection-fraction," Russo said. Someone whose heart pumps only 25 percent to 30 percent of the normal amount of blood is regarded as a candidate, she said.
The MTWA test is not in common use because its effectiveness has not been proven, Russo said. "What we need is a large-scale, multi-center prospective study," she said.
Chan agreed. "We want to make sure that our findings are all right," he said. "That would require a controlled trial. Until that is done, the reception by the cardiology community would be tepid."
SOURCES: Paul Chan, M.D., fellow, cardiovascular medicine, University of Michigan; Andrea Russo, electrophysiologist, University of Pennsylvania, Philadelphia; January 2007, Journal of the America College of Cardiology
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