Coronary artery calcium score

Coronary artery disease

Coronary artery disease is still a leading cause of morbidity and mortality worldwide. Many acute cardiac events occur in previously asymptomatic individuals. There has been much interest in developing screening methods to identify those at risk of having a primary cardiac event. The coronary artery calcium score (CACS) is considered one such tool and is also used in some cases in the investigation of chest pain.

See also the separate articles on Stable Angina, Acute Coronary Syndrome et Acute Myocardial Infarction.

What is coronary artery calcium (CAC)?

The presence of calcium in coronary arteries is almost always indicative of atherosclerotic plaque (but bears no relationship to plaque stability or instability).¹

Cardiac risk factors and insulin resistance lead to progression of coronary artery calcification. Atherosclerotic areas, however, do not always contain calcification and the significance of coronary artery calcium remains poorly understood.²

However, first defined in 1990, the Agatston CAC score is a powerful marker of calcified coronary plaque, strongly correlates with total coronary plaque burden, and is independently associated with coronary heart disease/atherosclerotic cardiovascular disease events.³

CAC scoring is considered to be a consistent and reproducible means of assessing risk for major cardiovascular outcomes, especially useful in asymptomatic people for planning primary prevention interventions such as statins and aspirin.^{4 5}

How do we measure the coronary artery calcium score (CACS)?⁶

The coronary artery calcium score is determined by CT scanning which is non-invasive and can be of two types:

• Electron beam CT scan (EBCT).

• Multidetector CT scan (MDCT).

EBCT was used for much of the original research but has been increasingly replaced with MDCT.

The amount of calcium detected in the coronary arteries is converted to a calcium score which correlates with the severity of the atherosclerosis. The score used is the Agatston score, which is calculated from weighted density and area of the calcification identified. Scores are then used to define severity as follows:^{4 7}

The main benefit of a coronary artery calcium scan is its high negative predictive value, reported to be 95-99%.⁹ Nothing but atherosclerosis results in calcium in coronary arteries, so its presence is diagnostic.

Risk assessment using the coronary artery calcium score has been shown to motivate individuals to make lifestyle changes and adhere to medication.¹⁰ CACS predicts cardiac risk independently of established risk scoring systems such as the Framingham score.

Problems with coronary artery calcium score as a screening test include:

• Not all atherosclerotic plaques contain calcium. Obstructive coronary artery disease is possible with a CASC of 0.¹¹

• Coronary atery calcium scanning is not useful in those at very low or very high risk, or in those with known coronary artery disease.

• It has not yet been established how often it should be repeated (for example, how long the reassurance of a score of 0 lasts).

• CACS does not predict functional capacity or the degree of stenosis.

It is unclear at present as to the most appropriate use of coronary atery calcium scanning but the following are possibilities:

• Screening asymptomatic individuals with intermediate risk of coronary artery disease (ie Framingham risk score of 10-20% over 10 years). This is the recommendation in American guidelines from the American College of Cardiology Foundation/American Heart Association (ACCA/AHA), along with a lower-classed recommendation for use in those with a 6-10% risk.¹²

• Risk prediction in those with type 2 diabetes. A meta-analysis published in 2013 found a CACS of 10 or more to be predictive of all-cause mortality as well as cardiovascular events in people with type 2 diabetes.¹³

• Reclassification of risk when used together with other risk factors.¹⁴

• Screening individuals with a strong family history of premature coronary artery disease.

Individuals who have had coronary artery calcium testing with private screening companies represent a challenge to GPs presented with the results, as there are no guidelines. A non-zero score should prompt at the very least an assessment of all risk factors, and consideration of risk modification strategies (lifestyle changes, blood pressure control, statin, consideration of aspirin etc). Cardiological referral for consideration of further investigation such as stress testing may be required.

Recent cohort studies have shown a CACS 100-300 as a sign of increased cancer risk.¹⁵

A CACS above 400 is associated with worse clinical outcomes in patients with an intermediate risk of developing coronary heart disease and in those with established coronary heart disease.³

Calcium scoring can be repeated after five years to reassess cardiovascular risk, especially when there is a decision to defer statin therapy on the basis of absence of coronary calcium.¹⁶

CAC progression has been associated with higher risk for myocardial infarction and all-cause mortality.⁴