Since heart attacks are usually caused by plaques breaking off from the inner linings of arteries leading to the heart, doctors use a test called Coronary Artery Calcium Score (CAC) to predict which people have the largest plaques and therefore are at high risk for suffering heart attacks that cause one out of four deaths in North America. The CAC test uses x-ray computed tomography (CT scan) to measure the extent of plaques in the arteries leading to the heart. This test categorizes patients into risk for suffering a heart attack by Agatson Units (named after the man who invented the test):
0 = no plaques
1-99 = mild plaques formation
100-399 = moderate plaque formation
>400 = severe plaque formation

CAC scores have been found to predict chances of dying of a heart attack in asymptomatic people (Ann Intern Med, Jul 7, 2015;163(1):14-21), and to correlate with heart attack risk factors such as high LDL cholesterol, low HDL cholesterol, high blood pressure, smoking, and high triglycerides (J Am College of Cardiology, July 5, 2005;46(1):158-165). However, not everyone with a high calcium score is at high risk for a heart attack, because a CAC can miss the plaques without calcium that are most likely to cause heart attacks.  One study showed that people who have no heart disease whatever have a 25 percent chance of having extensive plaques in their arteries, compared to 47 percent in people who have died from heart attacks (Atherosclerosis, Jul 2013;229(1):124-9).  Among the people who died of heart attacks, more than half of the lesions leading to sudden death had no calcium. Since the CAC measures calcium, these people who died of heart attacks would have had a CAC score showing no plaques at all. Healthy people with low risk factors for heart attacks and a CAC score of zero, followed for 10 years, still have a 3.1 percent chance of suffering a heart attack (Eur Heart J, July 1, 2018;39(25):2401–2408).

CAC Images Can Predict Which Plaques Are Likely to Break Off
A radiologist can look at CAC images and predict which plaques are likely to break off to cause heart attacks. Most of the calcium is in the inner lining of the plaque that holds the plaque in place inside an artery. In effect, calcium is the glue that helps to keep the plaque in place and prevent it from breaking off. The more fat in the plaque, the weaker it is and the more likely it is to break off. If the radiologist looks at a plaque on a CAC and it has lots of calcium, particularly on its inner lining, and very little fat inside the plaque, it is a stable plaque that is not likely to break off to cause a heart attack. Exercise helps to prevent heart attacks primarily by stabilizing plaques, and athletes who eat an anti-inflammatory diet can be at extremely low risk for heart attacks, even though they may have a relatively high calcium score.

Statins Can Increase CAC Scores Yet Help to Prevent Heart Attacks
Statins are known to help to prevent heart attacks, but they can increase CAC scores (Nature Partner Journals, Aging and Mechanisms of Disease, June 22, 2018;4(7)). Statins increase CAC scores because they stabilize plaques by depositing extra calcium in existing plaques, which lowers heart attack risk. This study showed that statins raise CACs by 29.7 percent each year, but adding another drug called a PCSK9 inhibitor to the statin drug slowed the rate that CAC scores rose to only 14.3 percent each year. A 10-year-follow up showed that patients with CAC scores greater than zero benefit from statins, while those with CAC scores of zero had no reduction in heart attack rates whatever (Arteriosclerosis, Thrombosis, and Vascular Biology, Aug 29, 2019;39(10). Statins help to prevent heart attacks in other ways besides lowering LDL cholesterol, such as lowering inflammation, widening narrowed blood vessels and reducing oxidation.

My Recommendations
High CAC scores are associated with increased risk for suffering a heart attack. However, some people, particularly athletes and people taking statins, may have high CAC scores even though they are at low risk for heart attacks, because both vigorous exercise and statins raise CAC scores in a way that helps to prevent heart attacks. In these people, a high calcium content in plaques can indicate stable plaques, with a reduced chance of breaking off to cause a heart attack.

A high CAC score also may not show increased risk for a heart attack because it can include calcifications in valves that do not increase risk for a heart attack. If you have a high calcium score, talk with your doctor and share these references with him or her, to see if any of these factors apply to you.