Electron beam computed tomography in chest pain centers
James K. McCord, M.D.

Dr. McCord is cardiology director, chest pain unit, Henry Ford Hospital and Medical Center, Detroit, Michigan.

Electron beam computed tomography (EBCT) is highly sensitive in detecting calcium in coronary arteries. The development of coronary artery calcium is associated with atherosclerosis. Coronary artery calcium is absent in the normal vessel, and occurs almost exclusively in an atherosclerotic artery. EBCT may have a role in risk stratifying patients who do not have a history of coronary artery disease.

Technologic considerations. EBCT uses an electron gun rather than a standard x-ray tube to generate x-rays, which allows for very rapid screening times. Transmural images are obtained in 100 msec with 3-mm to 6-mm cuts during one to two breath holds. Scanning is triggered by electrocardiogram (ECG) at 80% of the R-R interval (near end-diastole before atrial contraction). No intravenous contrast is required, and an entire scan can be completed in 10 to 15 minutes (few seconds of scanning time). A scan costs approximately $500.

Histological studies have shown that tissue densities >130 Hounsfield units are associated with calcified plaque. A calcium score, which is the product of the area of calcification and a factor based on the maximal calcium density, is calculated. A composite score of the entire coronary artery system is typically used: 0, normal; 1 to 99, mild; 100 to 400, moderate; and >400, severe calcification. Coronary artery scores must be age- and sex-adjusted because coronary calcification is more extensive in older and male patients.

Risk stratification. Several studies have demonstrated that the degree of coronary artery calcium is a predictor of the level of risk for future cardiac events.1-3 In one study of 1,173 patients with an average of 19 months of follow-up, there was a significant association between coronary calcium and major coronary events (unadjusted odds ratio [OR] 20.0 to 35.4).2

However, when adjusting for risk factors, the incremental benefit of EBCT to predict future risk is less impressive. In one study, when controlling for sex, age, diabetes mellitus, ECG left-ventricular hypertrophy, smoking, hypertension, family history, and cholesterol levels, the coronary artery calcium score was not a significant estimator of cardiac death or myocardial infarction (MI).3 The recently published American College of Cardiology/American Heart Association consensus document on EBCT for the prognosis of coronary artery disease states, "Importantly, the incremental value of EBCT over traditional multi-variate risk assessment models has yet to be established."4

EBCT in chest pain units. To date two trials have been published using EBCT in the emergency department (ED) setting to evaluate patients with possible acute coronary syndrome (ACS). A study by Laudon5 evaluated 105 patients who presented to the ED with chest pain of uncertain etiology. Patients were included if they had a nondiagnostic ECG and normal cardiac biomarkers. Female patients between the ages of 40 and 65 and male patients between the ages of 30 and 55 were studied. Patients with a history of coronary artery disease were excluded. All patients had an EBCT (blinded to clinicians) and a coronary artery calcium score >0 was considered positive.

The responsible clinicians determined cardiac testing, which included stress testing in 58, stress nuclear in 19, stress echocardiogram in 11, and heart catheterization in 25. The sensitivity and specificity for a positive test was calculated for EBCT. The sensitivity, negative predictive value, and specificity for EBCT was 100%, 100%, and 63%, respectively. The EBCT negative group had no cardiac events at 4-month follow-up. The conclusion of the authors was, "... a negative EBCT scan allows the patient to be safely discharged from the emergency department without further testing."5

Similarly, McLaughlin6 studied 134 patients who presented to the ED with chest pain of uncertain etiology. The mean age was 53, and patients with a diagnostic ECG or positive cardiac biomarkers were excluded. A coronary calcium score >1 was considered positive. Events were recorded at 30 days: death, acute MI, and need for revascularization.

In this study, 48 of the 134 patients (36%) had negative EBCT. There were seven events in the positive EBCT group (4 AMI, 2 CABG, and 1 PTCA). Of the 48 EBCT-negative patients there was one event (one AMI in a patient who was using cocaine). Likewise, the authors suggest that a negative EBCT scan may allow for an earlier, safe discharge from the ED.

Discussion. Each year, over 6 million patients in the United States present to EDs for evaluation of possible ACS. However, most of these patients (~80%) ultimately do not have an ACS. In patients with a nondiagnostic ECG and negative cardiac biomarkers, stress testing has proven to be an effective strategy for risk stratification. The experience with EBCT in this patient population is limited; therefore, in this setting EBCT is not ready to supplant strategies using stress testing.

The two studies utilizing EBCT in the ED involved a very small number of patients. In addition, EBCT has potential problems with positive predictive value and negative predictive value. In the study by McLaughlin, 63% of patients had a positive EBCT, which would lead to further testing.6 Thus, the need for "double testing" in many patients may make EBCT economically unattractive.

On the other hand, although coronary artery calcium is associated with atherosclerosis, EBCT does not detect vulnerable plaques, which are most likely to lead to ACS. In fact, the most vulnerable plaques may have no calcium at all. In a study of 118 patients with ACS, 10% had a negative EBCT.7 Young female smokers may be more prone to have ACS without coronary artery calcium. Two of the seven patients in the study by McLaughlin were female smokers with relatively low coronary calcium scores (30 and 31).

Conclusion

EBCT is an easy, fast, and relatively inexpensive test when compared with stress testing with imaging. EBCT may have a role in the risk stratifying of some patient populations who present to the ED with chest pain of uncertain etiology. However, at present, insufficient information is known that would warrant the replacement of stress testing with EBCT in this setting. The low specificity of EBCT may require that a significant number of patients undergo further testing and there may be a subpopulation of patients with ACS that have a negative EBCT scan. Larger prospective trials are required to determine which patients with chest pain of uncertain etiology may be effectively risk stratified with EBCT in the ED.

References

1. Detrano RC, Wong ND, Doherty TM, et al. Coronary calcium does not accurately predict near-term future coronary events in high-risk adults. Circulation 1999;99:2633-2638.
2. Arad Y, Spadaro LA, Goodman K, et al. Predictive value of electron beam computed tomography of the coronary arteries. 19-month follow-up of 1173 asymptomatic subjects. Circulation 1996;93:1951-1953.
3. Secci A, Wong N, Tang W, et al. Electron beam computed tomographic coronary calcium as a predictor of coronary events: comparison of two protocols. Circulation 1997;96:1122-1129.
4. OÕRourke RA, Brundage BH, Froelicher VF, et al. American College of Cardiology/American Heart Association expert consensus document on electron beam computed tomography for the diagnosis and prognosis of coronary artery disease. Circulation 2000;102:126-140.
5. Laudon DA, Vukov LF, Breen JF, et al. Use of electron-beam computed tomography in the evaluation of chest pain patients in the emergency department. Ann Emerg Med 1999;33:15-21.
6. McLaughlin VV, Balogh T, Rich S. Utility of electron beam computed tomography to stratify patients presenting to the emergency room with chest pain. Am J Cardiol 1999;84;327-328.
7. Schmermund A, Baumgart D, Gunter G, et al. Coronary artery calcium in acute coronary syndromes: a comparative study of electron-beam computed tomography, coronary angiography, and intracoronary ultrasound in survivors of acute myocardial infarction and unstable angina. Circulation 1997;96:1461-1469.