Using magnetic resonance imaging (MRI) scans of the heart wall, researchers at Johns Hopkins have found that people whose muscle wall thickness contained more than 25 percent scar tissue were approximately nine times more likely to test positive for a fast and dangerous heart rhythm known as ventricular arrhythmia.
Patients at risk of such arrhythmias often have a heart defibrillator implanted, a small device that delivers an electrical shock to restore their cardiac rhythm in case the heart beats too rapidly to pump enough blood to the rest of their body. Statistics from the United States Centers for Disease Control and Prevention estimate that each year more than 400,000 Americans suffer a sudden cardiac death, at least 30 percent of which are due to arrhythmia.
"If further tests confirm that MRI measurements of scar tissue accurately predict the risk of arrhythmia-related sudden death, these could become the gold standard for screening who really needs or does not need a defibrillator," says the study's senior author, electrophysiologist Henry Halperin, M.D., a professor of medicine, radiology and biomedical engineering at The Johns Hopkins University School of Medicine and its Heart Institute. "While tests are widely available to screen patients with coronary artery disease for risk of sudden cardiac death, tests are not so effective for identifying the many who will die suddenly from arrhythmias."
Indeed, while the U.S. National Center for Health Statistics estimates that more than 1 million Americans currently have a defibrillator, national studies published early this year have shown that only 5 percent of these devices ever fire to correct a heartbeat.
The latest Hopkins findings, which appear in today's edition of the journal Circulation, are believed to be the first to search in the heart's architecture - rather than its pumping function and electrical signaling - and so far the only study to analyze this architecture for clues about arrhythmias in patients with poor heart function but no arterial disease.
According to the researchers, defibrillators are prescribed when tests show abnormalities in the heart's ejection fraction (ability to squeeze blood to the rest of the body) and/or its resistance to electrical impulses that try to stimulate an arrhythmia.
"Our MRI technique has significant advantages over existing methods because it avoids the risks of infection that come with surgery, it is noninvasive, there are no catheters, and it is relatively easy to perform, taking only 45 minutes," says study co-author and cardiologist João Lima, M.D., an associate professor of medicine and radiology at Hopkins.
Lima notes that a patient with an ejection fraction of 60 percent has normal pumping ability, but anything less than 30 percent for a period of nine months or longer is considered low and an immediate risk factor for arrhythmia. He adds that if a patient has an ejection fraction that is slightly above 30 percent, then an electrophysiology test is used to determine if a patient requires a defibrillator. In this test, a thin catheter is inserted into the heart to try to induce an arrhythmia, something that will fail if the heart is healthy and not at risk. However, if it happens once, it is known to be two to four times more likely to happen again, he says.
Twenty-six patients from the Baltimore area participated in the study, which took place from July 2003 to February 2005. Participants were men and women, with an average age of 53, referred by community physicians to Hopkins for cardiac assessment. None had previous signs of coronary artery disease, another leading cause of sudden cardiac death, yet were experiencing other symptoms of heart disease, such as shortness of breath, instant fatigue and the inability to walk up stairs.
As part of a baseline MRI, the researchers used a technique developed at Hopkins to map and gauge the precise amount and distribution of scar tissue in the heart's muscle wall. The amount of scar tissue was measured as a percentage of the thickness of the muscle wall, which is on average about 1 centimeter. Composed of dense, fibrous tissue, with little to no blood supply, scar tissue was clearly visible on the image, the researchers say. After MRI, each patient underwent a standard electrophysiological assessment with a catheter.
Statistical analysis showed that the five patients who tested positive had the characteristic scar pattern, ranging from 26 percent to 75 percent scar tissue, with MRI. While MRI did not explain why the scar tissue forms, such scar patterns have been previously noted on autopsy studies of patients with heart disease. The researchers believe that previous inflammation, injury or excess stress on the heart wall may lead to this fibrosis and scar formation.
"Our study is yet another example of the potential applications of cardiac MRI in the prevention and treatment of cardiovascular disease," says the study's lead author, Saman Nazarian, M.D., a cardiac electrophysiology, clinical and research fellow at Hopkins. "Cardiac MRI is already useful for assessing the structure and function of the heart and the extent of structural changes due to coronary artery disease. MRI can also help identify patients in need of aggressive medical therapy and can help in the planning of invasive heart surgery or identification of the best candidates for bypass surgery."
Nazarian points out that these results also offer promise that cardiac MRI might prove useful in screening people at moderate risk of sudden cardiac death from arrhythmias - those without significant coronary artery disease and ejection fractions between 30 percent and 50 percent.
Another therapeutic implication, he says, is that identifying the telltale scar pattern could potentially improve existing procedures to ablate, or burn off, regions of the heart muscle that trigger arrhythmia.