Comparison of prevalence and severity of coronary calcium determined by electron beam tomography among various ethnic groups

Abstract

Electron beam tomography (EBT) can accurately and noninvasively quantitate the amount of coronary artery calcium (CAC), thus, providing an index of atherosclerotic burden. We sought to evaluate whether ethnic differences exist in the prevalence and severity of coronary calcium in asymptomatic, highrisk patients.  We evaluated 1,426 minority participants (828 men, 598 women) who underwent EBT scanning. These patients were were age- and gender-matched 3:1 with Caucasians (2,484 men, 1,794 women). We utilized the mean age of the minority participants and percentages of men and women to match to our physician-referred Caucasian population. All participants (minorities and matched Caucasians) were asymptomatic and referred by their primary physician to evaluate the presence and amount of CAC. Physicians referred patients to assess their cardiovascular risk. Data on risk factors for coronary artery disease (CAD) were collected at interview before the EBT procedure. The presence and number of risk factors for a participant were calculated based on the National Cholesterol Education Program guidelines. 1 Risk factors included: men aged 45 years, women aged 55 years, current cigarette smokers, history of premature coronary disease in a first-degree relative (men 55 years, women 65 years), diabetes, hypertension, and hypercholesterolemia. Cigarette smoking was defined as smoking 10 cigarettes/day. Participants currently using insulin or oral hypoglycemic agents, or those with dietcontrolled diabetes were classified as diabetic. Systemic hypertension was defined by current use of antihypertensive medication or known and untreated hypertension; hypercholesterolemia was similarly defined by use of cholesterol-lowering medication or known but untreated serum total cholesterol 240 mg/dl. Participants were excluded from the study if they had documented CAD before entry into the study. The study protocol was approved by the institutional review board of the Harbor-UCLA Research and Education Institute. One thousand four hundred twenty-six minority and 4,278 Caucasian subjects at Harbor-UCLA Medical Center underwent EBT using an Imatron C-150XL Ultrafast computed tomographic scanner (Imatron, Inc., South San Francisco, California). Coronary artery visualization was obtained without contrast medium injection, and 30 consecutive images were obtained at 3-mm intervals beginning 1 cm below the carina and progressing caudally to include the entire coronary tree. Exposure time was 100 ms/image slice, and total skin irradiation dose was 600 mrads/ scan. Electrocardiographic triggering was used and adjusted such that image acquisition occurred at the same point in the cardiac cycle starting in late systole.2,3 A computed tomographic threshold of 2 pixels and 130 Hounsfield units was utilized for identification of a calcific lesion. Each focus exceeding the minimum criteria was scored using the algorithm developed by Agatston et al, 4 calculated by multiplying the lesion area by a density factor derived from the maximal Hounsfield unit within this area. A total coronary calcium score was determined by summing individual lesion scores from each of 4 anatomic sites (left main, left anterior descending, circumflex, and right coronary arteries). EBT scoring was performed by a cardiologist who was blinded to the clinical, electrocardiographic, and angiographic information. Categorical data are presented as number (percentages), and continuous data as mean value SD.