The University Record, April 16, 2001

High-tech scan for blocked arteries matches conventional tests in finding root of symptoms, according to U-M study

By Kara Gavin
Health System Public Relations

Sanjay Saint (left), assistant professor of internal medicine, and Brahmajee Nallamothu, cardiology fellow, examine charts in University Hospital. Saint and Nallamothu contributed to the U-M study on electron beam computed tomography and its effectiveness in diagnosing blocked arteries in patients with symptoms of coronary artery disease. Photo by Bill Wood, U-M Photo Services
Patients experiencing chest pain and other symptoms may be able to find out equally well whether blocked arteries are the cause by lying still for a quick heart scan or by running on a treadmill for a stress test, according to a new U-M study.

The analysis of electron beam computed tomography (EBCT) shows the new test can diagnose blocked arteries just as reliably as stress tests in patients with symptoms of coronary artery disease. The paper, which compared data from nine studies of EBCT using a process called meta-analysis, appears in the March 26 issue of the Archives of Internal Medicine.

EBCT, which takes ultrafast pictures that reveal the presence of blockages by “seeing” the calcium that concentrates in them, is gaining popularity. The test is seen as faster, more consistent and easier to perform than stress tests. But the equipment usually used to do the scan is expensive, and some studies have called the test’s accuracy into question. The U-M study’s results, based on research involving 1,662 patients, found the test to be accurate.

That confirmation may help hospitals and imaging centers decide whether or not to invest in EBCT machines in order to diagnose obstructed coronary arteries in patients who come to the emergency room or a checkup with chest pain, shortness of breath or arm pain. The study also may help insurers decide whether to pay for the scans.

The study did not address the use of EBCT to screen the arteries of patients without symptoms, an increasingly common practice for which patients themselves often pay. Nor did it compare the cost of the scanning equipment or an EBCT scan with the cost of various kinds of stress tests.

“This analysis should help settle at least part of the debate over EBCT’s value compared with other methods of detecting arterial blockages in symptomatic patients,” says lead author Brahmajee Nallamothu, a cardiology fellow. “Frankly, we were surprised that the test showed this level of reliability in diagnosing coronary artery disease in these patients.”

Adds co-author Mark Fendrick, “Now that we have an indication that this test is worthwhile for symptomatic patients, we hope our result will lead others to study its cost effectiveness and its accuracy in diagnosing coronary artery disease among asymptomatic patients.” Fendrick co-directs the Consortium for Health Outcomes, Innovation and Cost-Effectiveness Studies and is associate professor of internal medicine and of health management and policy.

Co-author Melvyn Rubenfire, director of the Health System Preventive Cardiology program and professor of internal medicine, says, “Additionally, the data regarding increasing risk of coronary events with increasing calcium scores is convincing enough to use EBCT as an adjunct to standard risk factor assessment when deciding who would benefit from cholesterol-lowering drug therapy.”

EBCT, which uses an electron beam and a CT scanner to make split-second images from various angles around the patient, is now used at more than 150 medical and imaging centers around the world for various imaging purposes. Since the heart and nearby blood vessels are always in motion, EBCT has been seen as an especially good tool for capturing cardiovascular images because of its ability to scan quickly and to be linked to a patient’s heartbeat.

But though the technique has been studied by many researchers and judged accurate by the American Heart Association, its use has not yet been approved by insurers. The machine’s cost has been prohibitive for many diagnostic centers. And its inability to spot plaques that aren’t calcified or to distinguish easily between plaques and the normal calcium that builds up in the arteries of older people also are drawbacks.

The U-M analysis looked at studies that evaluated EBCT’s use in diagnosing patients with symptoms, specifically in finding plaques that blocked more than half of any given artery—enough to alter blood flow dangerously. They compared how often the test found real plaques and how often it gave a false positive.

Overall, they found that at the optimal setting for reliability, EBCT could find large plaques 75 percent of the time in people who have them (a measure called sensitivity), while still giving 75 percent of those without plaques an accurate diagnosis (a measure called specificity).

If the test is adjusted to be more sensitive to the presence of calcium, its sensitivity can be as high as 92 percent of the time, preventing doctors from missing plaques but also raising the risk of false positives to nearly 50 percent. Other settings can minimize the risk of false positives but also reduce the chance that the scan will pick up every blockage bigger than 50 percent.

These levels of reliability are similar to what can be achieved by a trained stress-test operator using monitors and/or radioactive tracers to assess a patient’s blood flow while he or she is exercising on a treadmill.

Similar levels of accuracy, though based on far fewer patients, are being seen with an EBCT-like scan that can be performed with state-of-the-art CT scanners equipped with special software. This approach, which analyzes calcium density in arteries but doesn’t require the purchase of a special EBCT machine, is what the Health System is exploring for its Cardiovascular Center.