CT Coronary Angiography
Abstract & Commentary
By Michael H. Crawford, MD, Dr. Crawford is a Professor of Medicine, and Chief of Clinical Cardiology, at the University of California, San Francisco; and he is the Editor of Clinical Cardiology Alert.
Source: Mollet NR, et al. High-Resolution Spiral Computed Tomography Coronary Angiography in Patients Referred for Diagnostic Conventional Coronary Angiography. Circulation. 2005;112:2318-2323.
The newest generation spiral CT scanner with 64 slices, thin detectors and faster X-ray tube rotation provides high-resolution, near motion-free coronary artery images. In 52 patients with acute chest pain syndromes referred for invasive coronary angiography, CT scans with contrast were performed before cardiac catheterization. All were in sinus rhythm, had normal renal function, and had never had revascularization before. Patients with resting heart rates > 70 beats per minute received beta-blockers. The CT scans were compared to quantitative coronary angiography by readers blinded to the other test results. A reduction in lumen of ≥ 50% in diameter was considered a significant lesion.
Results: Mean scan time was 13 seconds. One CT scan was classified as inconclusive. Using invasive angiography as the gold standard, on a segment to segment analysis CT scanning had a sensitivity of 99%, a specificity of 95%, a positive predictive value of 76%, and a negative prediction value of 99%. CT images were judged of good quality in 90%. Poor images were most often due to motion artifacts (60%) and severe calcification (20%). Intra and interobserver variability was .73 and .79, respectively. The presence of coronary calcium tended to lead to overestimation of severity of lesions by CT. Vessel-to-vessel agreement was .85 and patient-to-patient was .95. Mollet and colleagues concluded that a 64-slice CT coronary angiogram accurately detects coronary artery disease in patients with a variety of chest pain syndromes.
Commentary
Earlier studies of coronary CT scanning looked at selective larger coronary segments and were not really comparable to conventional coronary angiography. This study examined all clinically relevant segments, as defined by the American Heart Association coronary artery disease grading system. In this study, CT angiography came out very well by analysis of coronary artery segments, vessels, or patients. Unlike noninvasive perfusion imaging, the results didn't vary by vessel. All but one patient with normal coronary arteries was correctly identified for a negative predictive value of 99%. No person with significant coronary artery disease was missed, and only one significant lesion was missed. Indeed, sensitivity was 99% and the investigators admitted they read for maximum sensitivity, as most radiologists do. Of course this caused specificity to fall a bit to 95%, but the positive predictive value was only 76%. So clearly this test is most valuable when negative.
Before we rush off and install one in every emergency room, there were some significant limitations to this study. The major problem is the selection bias of patients going to catheterization. Mollet et al claim that their study evaluated patients from a wide spectrum of clinical settings, but in fact most of the patients had a high pre-test likelihood of disease. To whit, only 12% had atypical chest pain, most had stable angina (63%), and 28% had unstable angina or nonST-elevation MI. Accordingly only 13% were found to have no coronary artery disease. The ability of any test to detect disease when the pre-test likelihood of disease is high, is enhanced by this selection bias. However to be fair, it is very difficult to define a gold standard that does not involve a selection bias. Regardless, expect CT angiography to perform less well in a truly broad patient population.
The second major problem is patient exclusions for renal dysfunction, cardiac rhythm disturbances, prior revascularization, etc. Again, expect a weaker performance when such patients are included in real world situations. Third, the more coronary calcium that was present the more CT angiography tended to overestimate the degree of stenosis. So don't expect CT angiography to solve the problem of who with coronary calcium present has significant lesions. Fourth, patients with heart rates > 70 had to have beta blockers to slow their heart rate to minimize motion artifacts. This is a logistical and clinical issue that may impede usefulness. Finally, CT angiography exposes the patient to much more radiation than cardiac catheterization. When presented with this fact, many patients decline the test. Despite these limitations, the current results with this technique are impressive, and it is being installed in my hospital's emergency room as you read this. Thus, it is time to pull our heads out of the sand and face the reality of this technique in our armamentarium.
The newest generation spiral CT scanner with 64 slices, thin detectors and faster X-ray tube rotation provides high-resolution, near motion-free coronary artery images.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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