Imaging Quiz Answer
A New Cardiac Test for an Older Doctor
John D. Cantwell, MD
THE PHYSICIAN AND SPORTSMEDICINE - VOL 26 - NO. 6 - JUNE 98
The EBCT scan showed extensive calcification in the patient's left coronary system (figure 2). An additional view also showed calcium in the right coronary artery. A normal scan has a calcium score of zero; our patient's score was more than 2,500, placing him above the 90th percentile for men his age.
Because of his extensive coronary artery calcification, we questioned our patient further about symptoms. He mentioned an occasional subtle tightness in his left anterior chest early in his runs that subsided as he continued. He said he had experienced the symptoms for several years and that they occurred about a third of the time during his runs.
We performed a dual-isotope exercise stress scan to see if he had evidence of flow-limiting coronary narrowing. The scan was normal, and he did not experience chest tightness during a 13.5-minute Bruce-protocol treadmill test on which his maximum heart rate was 168 beats per minute.
We advised him to walk instead of run, add a lipid-lowering statin drug to get his LDL cholesterol below 100 mg/dL, continue on aspirin, and add antioxidant vitamins. He was advised to report any increase in the frequency or intensity of his chest tightness and to undergo periodic exercise testing.
His case illustrates the importance of thoroughly questioning patients—not just about "pain," but about any chest sensation such as heaviness, fullness, tightness, or pressure. Patients often don't equate the latter symptoms with pain.
The Role of EBCT Scanning
An EBCT scanner (figure 3) uses an electron gun and a stationary tungsten target to generate x-rays, allowing very rapid scanning times that image the heart between beats (1). The test detects calcium in the coronary arteries, which significantly increases the odds of a coronary event, as evidenced by a study (2) of 1,173 asymptomatic patients who were followed for a mean of 19 months. Those who had calcium scores higher than 100 had an odds ratio of 25.8 for an event relative to those who had scores lower than 100. Radiation exposure is equivalent to four posteroanterior and lateral chest x-rays for a man and seven for a woman. The test costs about $600, 2.5 times the cost of a treadmill test.
Calcification of a coronary artery occurs mainly along the elastic fibers of the subendothelial layer, perhaps because of the inflammatory component of atherosclerosis. Some feel it is part of an injured vessel's attempt to heal itself. The presence of calcium in an atherosclerotic plaque is thought to be associated with lipid-rich plaque, the type that can suddenly rupture, triggering thrombus formation and an acute myocardial infarction.
The validity of the EBCT scan as a measure of coronary stenosis when compared with coronary angiography has been variable (3-5). The sensitivity for a greater than 50% occlusion on angiography has ranged from 65% to 90%, and the specificity from 45% to 90%. The test isn't perfect, in other words.
One needs to remember that angiography looks at the lumen of an artery. The best way to assess the wall of the artery is by intracoronary ultrasound (ICUS) (5). As an artery becomes atheromatous, it attempts to repair itself by remodeling. In this process, the lumen diameter is maintained or even enlarged as the vessel itself enlarges. Hence, one can have atherosclerosis and calcium in the wall of the diseased artery (detectable by EBCT or ICUS), yet the lumen may appear normal on angiography (6). When one uses ICUS rather than angiography as the "gold standard," the EBCT scan has a specificity of 88%. The 12% false-positive rate may reflect speckled calcium in the adventitia of the vessel.
Patients who have false-negative EBCTs tend to be active cigarette smokers (usually in their 40s or early 50s), and they are more likely to have high LDL levels, diabetes, and single-vessel disease on angiography than patients with true-positive EBCTs (7). It is possible, in other words, to have serious atherosclerosis without a calcium buildup. In view of the test's limitations, if a patient has a normal EBCT scan but has angina pectoris, one should consider additional studies such as an exercise test or a stress scan.
In the future, the EBCT scan will probably be used to screen high-risk individuals for underlying coronary atherosclerosis and to help select those who should have exercise testing, looking for plaque buildup that significantly impairs myocardial perfusion. For symptomatic patients, however, the American Heart Association states that "presently the data are insufficient to recommend coronary artery calcium screening in lieu of stress testing for most patients with chest pain, except in those with atypical chest pain, for whom a negative study may be useful by itself or in addition to exercise testing (1)."
Dr Cantwell is a cardiologist at Cardiology of Georgia, PC, and clinical professor of medicine at Morehouse School of Medicine, both in Atlanta. He is an editorial board member of The Physician and Sportsmedicine. Address correspondence to John D. Cantwell, MD, Cardiology of Georgia, PC, 95 Collier Rd NW, Suite 2075, Atlanta, GA 30309.
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