By examining both the nervous system and the heart’s electrical system, cardiologists could better identify which patients were at highest risk of cardiac arrest or death, even years after a heart attack, according to research presented in the Dec.11 issue of the Journal of the American College of Cardiology (JACC).
“This is important because past studies, focusing on a single test, failed to identify most people at risk,” said Derek V. Exner, MD, MH, an associate professor at the University of Calgary’s Libin Cardiovascular Institute of Alberta, in Canada. “We developed a simple method of identifying approximately twice as many people at risk. That means we can potentially save more lives.”
The Risk Estimation Following Infarction, Noninvasive Evaluation (REFINE) study assessed two factors critical to the development of cardiac arrhythmias. An electrical system that has been damaged by a heart attack sets the stage for disturbances to the heart’s rhythm. At the same time, a nervous system on high alert, even without a patient realizing it, makes it more likely that a serious arrhythmia will take place, and progress to cardiac arrest or death.
Exner and his colleagues enrolled 322 patients who had suffered a heart attack and had at least a mild abnormality in the heart’s pumping ability. Within two to four weeks of the heart attack, and again at 10 to 14 weeks, they performed a variety of tests to measure the status of both the nervous system and the heart’s electrical system. They tracked patients for an average of four years.
The participants wore a heart monitor for 18 to 24 hours a day as they went about their daily activities. Using the electrocardiogram, researchers analyzed the heart’s electrical system by looking for T-wave alternans (TWA), a subtle abnormality in the heart’s rhythm that must be detected by a computer. Researchers also analyzed the electrocardiogram for evidence that the nervous system was on high alert by looking for abnormalities in heart rate turbulence (HRT), a measure of the heart’s ability to adapt to change.
Early after a heart attack, TWA and impaired HRT were not accurate warning signs of future risk. However, at the 10- to 14-week mark, TWA and impaired HRT identified patients at increased risk, and they were particularly powerful when used in combination.
The 20 percent of patients who had both TWA and impaired HRT on the all-day heart monitor and had a persistent abnormality in the heart’s pumping ability, faced more than six times the risk of cardiac arrest or death during follow-up when compared to other patients in the study.
The researchers said that using an exercise test to detect TWA was effective, but not as simple and efficient as the all-day heart monitor to look for impaired HRT and TWA at the same time.
Exner said that studies expected to begin in 2008 will evaluate whether an implantable cardioverter-defibrillator can save the lives of patients with abnormalities in both the nervous system and the heart’s electrical system. Until then, those at highest risk should receive intensive follow-up, he added.
Alberta Heritage Foundation for Medical Research, the Canadian Institutes of Health Research, and the Heart and Stroke Foundation of Alberta funded the REFINE study. GE Healthcare and Cambridge Heart provided unrestricted donations of equipment and testing materials for the study.