Abstract
Coronary heart disease (CHD) is the leading cause of death in the United States (US). Approximately half of deaths from CHD occur out of hospital, most being sudden. The majority of sudden cardiac deaths occur in asymptomatic subjects. Recent lipid-lowering trials in asymptomatic subjects have demonstrated the potential for risk reduction for CHD events by primary prevention. It is, however, generally acknowledged that risk will be underestimated in asymptomatic subjects who fall into the category of intermediate risk by the traditional risk factors. Non-invasive measurements of subclinical atherosclerosis, which is the end result of risk-factor exposure, have the possibility of improving the risk stratification of asymptomatic subjects in intermediate-risk.
Electron-beam computed tomography (EBCT) is a non-invasive and highly sensitive means to detect calcification within coronary arterial wall. Coronary artery calcification (CAC) is a recognized marker of atherosclerosis. Atherosclerotic burden of coronary arteries correlates strongly with the amount of CAC measured by EBCT. Studies examining the predictive value of CAC among asymptomatic subjects consistently reported that CAC is a risk for CHD. Two studies reported that unadjusted odds ratios of CAC for CHD were over 20. Incremental value of EBCT over the traditional risk assessment models, however, has not been established.
Although CHD mortality in Japan remains uniquely low in industrialized countries, among men aged 30-49, risk factor profiles for CHD are similar between men in the US and Japan, except higher prevalence of cigarette smoking in Japan and higher prevalence of obesity in the US. It is reported that the declining trend in CHD mortality in Japan has recently slowed down in metropolitan areas, especially in men aged 30-49, and that the incidence has increased in middle-aged workers in a metropolitan area. A mortality validation study reported that the differences in CHD mortality between the US and Japan were not as large as suggested by vital statistics. It is, therefore, important to evaluate subclinical atherosclerosis measured by EBCT in men in recent birth cohorts in Japan and compare it to that in men in the US in order to predict future trend in CHD in Japan.
