Coronary Heart Disease Risk Factors - Part II
Coronary Artery Calcium Measurements using Electron Beam Tomography
Lets start with some explanations. Calcium is part of the fatty plaque that builds up over time in the arteries and leads to atherosclerosis. As the plaque builds up, arteries harden and become narrower, reducing blood flow. However, the most dangerous situation occurs when plaque become fragile and ruptures. This causes blood clots to form that can stop blood flow or travel elsewhere in the body. If this blocks a blood vessel that leads to the heart, it causes a heart attack. If it blocks a blood vessel to the brain, it causes a (ischemic) stroke.
Tomography is a technique uses imaging (like the images one obtains with X-rays) through the body, at various different angles. Complex mathematical algorithms (rules) enable the information from these several images to be used to construct a three-dimensional picture of the organs inside the body.
Electron beams take pictures through the body, similar to X-rays. They can be used to determine the amount of calcification in arteries, using tomographic techniques to analyze the data.
In a study (reference 10) of 5635 initially asymptomatic low to intermediate risk adults an initial measurement of coronary artery calcium (CAC) was correlated with subsequent cardiac events over a period of 37 months. They find that presence of CAC in men implies a relative risk of 10.5 (!) (3.9 to 28.4 Margin of Error). Indeed this is a very large risk ratio, compared to those associated with conventional risk factors.
However, in an editorial (reference 11) in the same issue of Circulation, a number of potential problems and possible bias factors that were not treated adequately are listed. The conclusion of the editorial is that electron beam tomography is most useful in people with uncertain risk. Those at low risk don't need it, and those with high risk should be treated aggressively regardless of the results.
The last reference, reference 12, presents a meta-study of available results. They determine an overall relative risk of 2.1 (1.6 to 2.9 Margin of Error), for a CAC score of 1 to 100. Their conclusion is that "coronary artery calcium score is an independent predictor of coronary heart disease events".
Our conclusion is that electron-beam tomography may have some valid applications, but it is not going to be a major differentiating factor between low and high risk people. Calcium deposits increase as we age, and the real question is when will they start disintegrating, causing CHD.
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