Coronary Artery Calcium Score

Synonym: CACS

Coronary artery disease is unfortunately still a leading cause of morbidity and mortality in Europe. There are a number of ways to predict a patient's risk of a cardiac event, the most well known being the Framingham Risk Score. Unfortunately such scores do not identify all "at risk" - and the definitive investigation, coronary angiography, is associated with significant morbidity and mortality. The provision of an additional non-invasive method of determining coronary disease at an early stage may help us combat some of these problems.¹

What is coronary artery calcium?

Coronary artery disease results from the development of atherosclerotic plaques. These plaques consist of abnormal macrophages that accumulate lipids and abnormalities of the endothelium. These plaques can become calcified - which is a common finding. Cardiac risk factors and insulin resistance lead to progression of coronary artery calcification - this may help in deciding who should be investigated with this methodology.

What is the coronary artery calcium score (CACS)?

The CACS is a measure of the calcium in the the coronary arteries. The calcium is detected by computerised tomography scanning. The presence of calcium in the coronary tree is diagnostic of the presence of coronary atherosclerosis.²

How do we measure the CACS?

The CACS is measured using CT scanning. This is non-invasive and can be of two types: Electron Beam CT scan (EBCT) and Multidetector CT scan (MDCT). EBCT is the commonest technology currently used to measure CACS.

The amount of calcium detected in the coronary arteries is converted to a calcium score which correlates with the severity of the blockage. The results are reported on a numerical scale. The numerical scale is continuous.

The question that arises from the studies investigating the usefulness of EBCT scans in coronary artery disease is whether it could become a screening test, or be used rather than the Exercise Tolerance Test (ETT) or myocardial perfusion scans. The following table looks at some of the advantages and disadvantages of the EBCT over the ETT. The main disadvantage is that CACS does not tell us about the patient's functional capacity which could be inferred from an ETT.

At present there are no guidelines available however, with more and more studies being performed on its usefulness the following groups are probably most likely to benefit:

• Asymptomatic middle aged men and women (men from 45 years of age and women from 55 years of age).
• Presence of just one risk factor e.g. mild hypertension.
• Presence of a strong family history of premature coronary artery disease.

Increased CACS predicts subsequent development of heart disease in patients followed up for at least 3.5 years. This is based on a meta-analysis which looked at four studies.⁵

A later study on asymptomatic men aged 40-50 years also confirmed a strong relationship between CACS and coronary heart disease.⁶ The Saint Francis Heart Study took these observations further and observed that CACS predicts events independent of standard risk factors.⁷ In this study CACS was concluded to be more accurate than the predictive use of risk factors and predicted both non-fatal cardiac events and fatal coronary events. This trial also looked at older patients up to the age of 70. This finding has also been repeated in other trials.^8,9,10

A prospective observational study with a median follow up of 7 years found the use of CACS with the Framingham Risk Score to be better at determining a patient's risk especially those in the intermediate risk group.¹¹ However, an earlier trial in 1999, the South Bay Heart Watch did not find that CACS added much to the Framingham Risk Score and thus further work is needed here.¹² A more recent study suggests that some patients identified as low risk with traditional scoring methods would fall in to higher risk categories with CAC scoring.¹³

At present it is difficult to know what the role is of CAC scans in comparison to myocardial perfusion scans - some evidence suggests disparity between the two.¹⁴ Further evidence and guidance is required to determine how to effectively use CAC scans and where they are to be placed with our current cardiac tests.