Severity of Coronary Artery Calcification by Electron Beam Computed Tomography Predicts Silent Myocardial Ischemia  Zuo-Xiang He, MD; Thomas D. Hedrick,MD; Craig M. Pratt, MD; Mario S. Verani, MD; Vincent Aquino, MD; Robert Roberts, MD; John J. Mahmarian, MD Cardiovascular Institute and Fu Wai Hospital, Beijing, and Baylor College of Medicine, Houston.  Circulation January 2000;101:244-251

Background – Detection of subclinical coronary artery disease (CAD) before the development of life-threatening cardiac complications has great potential clinical relevance.  Electron beam computed tomography (EBCT) is currently the only noninvasive test that can detect CAD in all stages of its development and thus has the potential to be an excellent screening technique for identifying asymptomatic subjects with underlying myocardial ischemia

Methods and Results – Over 2.5 years, we prospectively studied 3,895 generally asymptomatic subjects with EBCT, 411 of whom had stress myocardial perfusion tomography (SPECT) within a close (median, 17 days) time period.  SPECT and exercise treadmill results were compared with the coronary artery calcium score (CACS) as assessed by EBCT.

The total CACS identified a population at high risk for having myocardial ischemia by SPECT.

No subject with CACS <10 had an abnormal SPECT compared with 2.6% of those with scores from 11 to 100, 11.3% of those with scores form 101 to 399, and 46% of those with scores ³400 (P>0.0001).  CACS predicted an abnormal SPECT regardless of subject age or sex. 10% of the total population of 3,895 subject had a score of > 400.

The mean CAS was significantly lower in subjects with a normal compared with abnormal SPECT regardless of exercise ECG findings.

Conclusion  - CACS identifies a high-risk group of asymptomatic subjects who have clinically important silent myocardial ischemia.  Our results support the role of EBCT as the initial screening tool for identifying individuals at various stages of CAD development for whom therapeutic decision making may differ considerably.

Editor’s comment:  This study is the first to describe the relationship between the severity of coronary artery calcification as detected by EBCT Coronary Artery Scan and the presence of stress-induced ischemia in a large cohort of predominately asymptomatic subjects. The presence and extent of stress induced myocardial ischemia detected by SPECT is known to predict subsequent outcome.  The findings emphasize that EBCT is more sensitive than either ETT (Exercise Treadmill Test) or SPECT for detecting subclinical CAD . The Coronary Artery Scan can further identify the 10% of asymptomatic individuals among an otherwise heterogeneous low risk population with cardiac risk factors, who have severe coronary artery calcification and are likely to have silent myocardial ischemia. This is important since patients with silent myocardial ischemia are known to be at higher risk for subsequent cardiac events. The results of this study support the role of EBCT as the initial screening tool through which coronary atherosclerosis can be detected and potentially treated before the development of critical coronary artery stenosis and/or stress–induced myocardial ischemia.

 

Identification of Patients at Increased Risk of First Unheralded Acute Myocardial Infarction by Electron-Beam Computed Tomography  Paolo Raggi, MD; Tracy Q. Callister, MD; Bruce Cooil, PhD; Zuo-Xiang He, MD; Nicholas J. Lippolis, MD; Donald J. Russo, MD; Alan Zelinger, MD; John J. Mahmarian, MD EBCT Research Foundation and Vanderbilt University (B.C.), Nashville, Tenn.; Bailer College of Medicine, Houston, Tex; and Christ Hospital and Medical Center and Rush Medical College, Oak Lawn, Ill (AZ).   Circulation, February, 2000;101:850-855

Background - There is a clear relationship between absolute calcium scores (CS) and severity of coronary artery disease. However, hard coronary events have been shown to occur across all ranges of CS.

 Methods and Results - We conducted 2 analyses:

in group A, 172 patients underwent electron-beam CT (EBCT) imaging within 60 days of suffering an unheralded myocardial infarction. In group B, 632 patients screened by EBCT were followed up for a mean of 32±7 months for the development of acute myocardial infarction or cardiac death. The mean patient age and prevalence of coronary calcification were similar in the 2 groups (53±8 versus 52±9 years and 96% each).

 In group B, the annualized event rate was 0.11% for subjects with CS of 0, 2.1% for CS 1 to 99, 4.1% for CS 100 to 400, and 4.8% for CS >400, and 7% of the patients had CS >400.

Conclusions - Coronary calcium is present in most patients who suffer acute coronary events. Although the event rate is greater for patients with high absolute CSs, few patients have this degree of calcification on a screening EBCT. Conversely, the majority of events occur in individuals with high CS percentiles. Hence, CS percentiles constitute a more effective screening method to stratify individuals at risk.  

Editor’s Comment:  In the first group, imaging showed calcification in 96% of patients. In the second group, 96% of 27 patients who had a cardiac event also showed calcification by EBCT, which was statistically significant, compared with the patients who did not have an event. In the second group, it is noted that although 22% of events occurred in patients with calcium scores over 400, this constituted 7% of the entire group, and the majority of cardiac events occurred in patients with mild to moderate scores. In both groups, mild, moderate, and extensive absolute calcium scores were distributed similarly.. However, by dividing calcium scores into quartiles, the researchers found that 70% of the patients in each group who had a cardiac event had calcium scores greater than the 75th percentile. High age and sex-adjusted calcium score percentiles appear to be closely related to the occurrence of subsequent hard events, and we suggest that this measurement should be used in the assessment of the risk of a hard event in asymptomatic individuals undergoing EBCT screening.