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Coronary artery calcium scoring: Its practicality and clinical utility in primary care

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Release date: September 1, 2018
Expiration date: August 31, 2019
Estimated time of completion: 1 hour

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ABSTRACT

Coronary artery calcium scoring is useful as a risk-stratification tool in coronary artery disease, and it outperforms other risk-assessment methods. American College of Cardiology/American Heart Association guidelines give the test a IIB recommendation in clinical scenarios in which risk stratification is uncertain. However, if the test is not used in the appropriate clinical setting, misinterpretation of the results can lead to unnecessary cardiac testing. This review provides the primary care provider with basic knowledge about the test’s clinical utility, interpretation, risks, and limitations.

KEY POINTS

  • Coronary artery calcium testing is useful in diagnosing subclinical coronary artery disease and in predicting the risk of future cardiovascular events and death.
  • Given the high negative predictive value of the test, it can also serve to reclassify risk in patients beyond traditional risk factors.
  • Along with shared decision-making, elevated calcium scores can guide the initiation of statin or aspirin therapy.
  • A high score in an asymptomatic patient should not trigger further testing without a comprehensive discussion of the risks and benefits.


 

    References

    The United States has seen a decline in fatal myocardial infarctions, largely thanks to early detection of coronary artery disease. Current guidelines on assessment of cardiovascular risk still rely on the traditional 10-year risk model in clinical practice. However, the predictive value of this approach is only moderate, and many coronary events occur in people considered to be at low or intermediate risk.

    See related editorial

    Coronary artery calcium scoring has emerged as a means of risk stratification by direct measurement of disease. Primary care providers are either using it or are seeing it used by consulting physicians, and its relatively low cost and ease of performance have contributed to its widespread use. However, downstream costs, radiation exposure, and lack of randomized controlled trials have raised concerns.

    This article reviews the usefulness and pitfalls of coronary artery calcium scoring, providing a better understanding of the test, its limitations, and the interpretation of results.

    ATHEROSCLEROSIS AND CALCIUM

    Figure 1. Pathogenic mechanism of atherosclerotic lesions and its relationship to the coronary artery calcium score.

    Figure 1. Pathogenic mechanism of atherosclerotic lesions and its relationship to the coronary artery calcium (CAC) score. A type 1 lesion (not depicted) contains lipoproteins that initiate an inflammatory response. A type 2 lesion contains an accumulation of foam cells. The type 3 lesion contains collections of extracellular lipid droplets. Eventually, these extracellular lipid pools form a lipid core, and a type 4 lesion is created. With time, this core develops a fibrous connective-tissue thickening that can calcify and give rise to a type 5 lesion detectable by imaging. Type 6 is a complicated lesion that can include thrombus from plaque rupture.

    Atherosclerosis begins in the first few decades of life with a fatty streak in which lipoproteins are deposited in the intimal and medial layers of blood vessels (Figure 1). Inflammatory cells such as macrophages and foam cells are then recruited to the areas of deposition where they cause apoptosis, creating a necrotic core with calcium deposits.1–3

    As the calcium deposits grow, they can be detected by imaging tests such as computed tomography (CT), and quantified to assess the extent of disease.4

    CALCIFICATION AND CORONARY ARTERY DISEASE

    Coronary calcification occurs almost exclusively in atherosclerosis. Several autopsy studies5,6 and histopathologic studies7 have shown a direct relationship between the extent of calcification and atherosclerotic disease.

    Sangiorgi et al7 performed a histologic analysis of 723 coronary artery segments. The amount of calcium correlated well with the area of plaque:

    • r = 0.89, P < .0001 in the left anterior descending artery
    • r = 0.7, P < .001 in the left circumflex artery
    • r = 0.89, P < .0001 in the right coronary artery.

    Coronary artery calcium has also been associated with obstructive coronary artery disease in studies using intravascular ultrasonography and optical coherence tomography.8,9

    TECHNICAL INFORMATION ABOUT THE TEST

    First-generation CT scanners used for calcium scoring in the 1980s were electron-beam systems in which a stationary x-ray tube generated an oscillating electron beam, which was reflected around the patient table.10 A single, stationary detector ring captured the images.

    These systems have been replaced by multidetector scanners, in which the x-ray tube and multiple rows of detectors are combined in a gantry that rotates at high speed around the patient.

    Coronary calcium is measured by noncontrast CT of the heart. Thus, there is no risk of contrast-induced nephropathy or allergic reactions. Images are acquired while the patient holds his or her breath for 3 to 5 seconds. Electrocardiographic gating is used to reduce motion artifact.11,12 With modern scanners, the effective radiation dose associated with calcium testing is as low as 0.5 to 1.5 mSv,13,14 ie, about the same dose as that with mammography. The entire test takes 10 to 15 minutes.

    Table 1. Categories of coronary artery calcium scores
    Figure 2. A screenshot from a standard calcium scoring program.

    Figure 2. A screenshot from a standard calcium scoring program. The images show a small calcified plaque in the mid-left anterior descending artery (left upper panel, colored yellow), left circumflex artery (right upper panel, colored blue), and right coronary artery (right lower panel, colored red). The table in the left lower panel lists the results of the calcium score. (The Agatston score is listed as “Score”). The graph in the right lower panel shows the results of the individual patient relative to an age- and sex-matched patient population (eg, the MESA trial). This patient has a score of 62.2, indicating relatively mild calcification and falling on the 75% percentile for age, sex, and ethnicity in a control group.

    Coronary calcium on CT is most commonly quantified using the Agatston score. Calcification is defined as a hyperattenuating lesion above the threshold of 130 Hounsfield units with an area of 3 or more pixels (1 mm2). The score is calculated based on the area of calcification per coronary cross-section, multiplied by a factor that depends on the maximum amount of calcium in a cross-section (a weighted value system based on Hounsfield units of dense calcification in each major coronary artery).15 The sum of calcium in the right coronary, left anterior descending, and left circumflex arteries gives the total Agatston calcium score.

    The results fall into 4 categories, which correlate with the severity of coronary artery disease, ranging from no significant disease to severe disease (Table 1). Other scores, which are not commonly used, include the calcium volume score16 and the calcium mass score.17Figure 2 shows a screenshot from a coronary artery calcium scoring program.

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