ICDs, or implantable cardioverter defibrillators, are designed to shock damaged hearts back into rhythm and save patients from sudden cardiac death, which kills 300,000 Americans each year.
But a study finds that while many of these patients will benefit from their ICDs, a large number won't, and a simple heart-rhythm test can tell who's who.
In fact, the study suggests that if the test were used on the majority of ICD candidates, as many as one-third could be spared the operation to implant a device, without raising their risk of sudden death.
The study, published in the Journal of the American College of Cardiology ( JACC ), is based on data from 768 patients who were candidates for ICDs at Christ Hospital and the Ohio Heart and Vascular Center in Cincinnati. All of the patients had survived myocardial infarctions but had permanent damage to their heart muscle caused by lack of blood flow, a condition called ischemic cardiomyopathy.
Each patient received a test called microvolt T-wave alternans or MTWA, along with a battery of other tests, during their evaluation. Half of the patients went on to receive ICDs, although the MTWA test results weren't used in the decision-making process. The patients' health and the causes of any deaths were tracked for up to three years.
After that time, the data were analyzed by researchers from the University of Michigan Cardiovascular Center and VA Ann Arbor Healthcare System, in cooperation with the Ohio team.
In all, 67 percent of patients had positive or inconclusive MTWA test results. Of them, the patients who went on to receive an ICD were 55 percent less likely to die in the follow-up period than those who hadn't gotten an ICD. They were also 70 percent less likely to die suddenly due to a heart-rhythm disruption. But at the same time, the one-third of patients who had negative MTWA tests and then received ICDs were no less likely to die than those with similar test results who didn't receive ICDs.
" This is the first study to demonstrate that a subset of patients who meet current criteria for defibrillator placement may not benefit at all from ICDs," says Paul Chan, at the U-M Medical School. " Use of the MTWA test could truly help us tell which ICD candidates will benefit most."
In all, the authors calculate, one life could be saved every two years for every nine ICDs implanted in people with positive or inconclusive MTWA results. But it would take 76 ICD implantations in people with negative MTWA tests to save one life every two years.
The reductions in death risk were present even after the authors corrected for many other variables and differences between the two MTWA-positive and MTWA-negative groups.
Chan and his Ohio colleagues, led by first author Theodore Chow, of the Lindner Clinical Trial Center at Christ Hospital and the OHVC, have studied the use of MTWA in predicting patients' risk for several years.
They published results from the same group of ischemic cardiomyopathy patients, showing that the MTWA test was able to predict the risk of death from any cause, even after they adjusted the data for other heart-rhythm test results and medical issues.
In recent years, ICDs have been shown to reduce the overall risk of sudden cardiac death well enough to be given approval by the FDA ( Food and Drug Administration ) and to receive coverage by Medicare and other insurers.
In 2004 Medicare expanded the group of patients who were eligible for ICD therapy, leading to estimates that 50,000 new patients each year can qualify for the devices based on criteria relating to their heart rhythm and pumping capability.
Source: University of Michigan Health System, 2007
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