NIH Study Shows MRI Provides Faster, More Accurate Way To Diagnose Heart Attacks

Researchers evaluated the ability of high-resolution MRI to detect acute coronary syndrome (heart attack or unstable angina) in emergency department patients with chest pain. MRI results were compared with three standard diagnostic tests: an electrocardiogram (ECG or EKG), blood enzyme test, and the TIMI risk score, which assesses the risk of complications or death in patients with chest pain based on a combination of several clinical characteristics. MRI detected all of the patients' heart attacks, including three in patients who had normal EKGs. In addition, MRI detected more patients with unstable angina than the other tests.

Of the more than 5 million patients who visit emergency departments with chest pain each year, only about 40 percent can be immediately diagnosed with heart attack using standard tests. The majority of patients must undergo a number of tests and further hospitalization for a conclusive diagnosis to be reached.

"This study lays the groundwork for what could mean a dramatic change in how heart attacks are diagnosed - and how rapidly patients receive treatment once they arrive at the hospital," said NHLBI Director Dr. Claude Lenfant. "Using MRI to detect heart problems in the emergency department will ultimately save lives. Because patients will be diagnosed and treated more quickly, cardiac MRIs might save costs, as well."

The study is part of a collaborative program among the NHLBI, the Warren Grant Magnuson Clinical Center - both components of the National Institutes of Health (NIH), the Federal Government's primary agency for biomedical and behavioral research - and Suburban Hospital in Bethesda, Maryland. Another area of the program, supported by NIH's National Institute of Neurological Disorders and Stroke, is studying the use of MRI to diagnose and treat stroke. NIH is part of the U.S. Department of Health and Human Services.

In the chest pain study, 161 patients whose initial EKG did not indicate a heart attack were evaluated with traditional diagnostic tests and a cardiac MRI after their condition was stabilized. The MRI lasted around 38 minutes. All patients were followed up 6 to 8 weeks after the initial visit to the emergency department.

Researchers studied the ability of each test to detect acute coronary syndrome (sensitivity) as well as how often an abnormal test result correctly identified a patient with acute coronary syndrome (specificity). MRI accurately diagnosed 21 of the 25 patients (84 percent) determined to have acute coronary syndrome - a significantly higher level of sensitivity than EKG criteria for ischemia (restricted blood flow), blood enzyme levels, and TIMI risk score. MRI was also more specific than abnormal EKG.

"MRI was the strongest predictor of acute coronary syndrome - for both heart attacks and unstable angina," said Dr. Andrew Arai, principal investigator of the study, who leads the cardiovascular imaging section of the NHLBI Laboratory of Cardiac Energetics. "MRI allows us to look at how well the heart is pumping, how good the supply of blood to the heart is in specific areas, and whether there is evidence of permanent damage to the heart."

MRI is a type of body scan that uses magnets and computers to provide high-quality images based on varying characteristics of the body's tissues. The technology allows physicians to study the heart's overall structure and functioning continuously in three dimensions. MRI addresses another critical issue in assessing patients with acute coronary syndrome: time. Patients can be scanned in under 40 minutes; if severe blockages are found, they could receive vital treatment to restore blood flow, such as clot-busting drugs, angioplasty, or coronary artery bypass surgery. Current recommendations are for such therapies to begin within one hour from the start of a heart attack for optimal effectiveness.

EKG records the electrical activity of the heart to detect abnormal heart rhythms, some areas of damage, inadequate blood flow, and heart enlargement. Like MRI, EKG is noninvasive. Because of its low degree of sensitivity, however, EKG immediately diagnoses only about 10 percent of patients with acute coronary syndrome. It is not uncommon for an EKG to appear normal during a heart attack or an episode of unstable angina.

Patients suspected of having a heart attack that is not confirmed by an EKG typically have their blood tested for enzymes or other substances ("markers") that indicate permanent damage to the heart tissue. Because the markers are not evident in the blood until several hours after a heart attack, patients whose EKG appears normal may need to stay in the hospital for 12 to 24 hours to ensure the blood test is accurate. Furthermore, because the blood test detects only permanently damaged tissue, it does not detect unstable angina.

Unstable angina can be considered an "impending heart attack." Heart attacks are caused by a blood clot from a tear or break in fatty deposits (plaque) that have built up inside a coronary artery; when the blood clot suddenly cuts off most or all blood supply to the heart, heart tissue is permanently damaged. In unstable angina, the coronary artery has many or all of the same characteristics as a heart attack, except that the problems are not quite severe enough to cause permanent heart damage. Because no heart cells die in unstable angina, the condition is harder to detect with standard tests.

Approximately two percent of patients with a heart attack are discharged home from the emergency department without the heart attack being detected and treated. In addition, many patients with unstable angina are sent home without diagnosis; they may progress to a heart attack after discharge or require urgent medical therapy soon thereafter. Patients with undetected acute coronary syndrome are twice as likely to die as those whose condition is detected and treated. In the study, three of the patients were not characterized as having acute coronary syndrome until their follow-up visit.

"MRI provides us with additional and more precise information than is currently accessible from other imaging methods, such as echocardiography, coronary angiography, and positron emission tomography," added Dr. Robert S. Balaban, scientific director of the NHLBI Laboratory Research Program, and a co-author of the paper. "MRI technology could help us get another 20 percent of patients with acute coronary syndrome to life-saving treatment more quickly, and reduce the number of patients spending hours in the hospital for long-term EKG and enzyme monitoring."

Balaban estimates that MRI to detect acute coronary syndrome in the emergency department could be used in hospitals nationwide within a few years. Many U.S. hospitals currently have equipment that could be upgraded for this use.

"This study represents a shift of moving technology directly from the basic science laboratory to the clinical setting," added Balaban. "Working with physicists and engineers, we hope to further develop MRI to provide even more precise imaging and quantifiable ways to measure coronary blockages.

"MRI is a noninvasive imaging tool that we can now interact with in 'real time' allowing us to see soft tissues, such as the wall of a diseased artery or the heart muscle itself while measuring physiological functions such as contraction, blood flow, and viability," Balaban noted. "The future of MRI is to go from diagnostic uses, as described in this study, to therapeutic applications. MRI can be used to guide minimally invasive procedures, including cardiac catheterization to open blocked arteries, direct the injection of therapeutic agents such as gene vectors or stem cells into damaged areas of the heart, replace heart valves, or remove cells that are causing arrhythmias."