Cardiovascular disease is the leading cause of death worldwide, responsible for an estimated 30% of global deaths in 2008. Most of these deaths result from coronary artery disease and stroke. Coronary artery disease refers to the narrowing of the coronary arteries (the arteries providing blood supply to the heart) by a substance called atheromatous plaque. These plaques are formed from lipid-containing white blood cells and fibrous tissue, and tend to calcify and become stiff over time, sometimes referred to as ‘hardening of the arteries’. As these plaques become larger over time, the arteries supplying the heart become narrow, resulting in less blood flow. Eventually, the flow can slow down or stop to the point where the heart can no longer function, resulting in a myocardial infarction or MI (also known as a ‘heart attack’).
Understanding your risk for coronary heart disease
Doctors can reliably diagnose heart attacks and angina (chest pain resulting from coronary artery disease). The problem is that approximately 70% of heart attacks and 50% of sudden deaths from coronary artery disease occur in previously undiagnosed patients. Doctors need a way to identify patients at risk from heart attacks before they occur. Toward that end, clinicians have identified a number of risk factors that increase the likelihood of heart disease. These include factors that cannot be changed, including male gender and family history, as well as factors that can be modified, including tobacco use, high blood pressure and high cholesterol level. Screening patients for these risk factors allows doctors to calculate an overall risk assessment, which is an important tool for predicting heart disease. However, as many as 50% of heart attacks are not predicted or explained by these risk factors alone.
Testing for coronary artery disease
Cardiologists (heart doctors) often perform a study called a stress test to evaluate the coronary arteries. The heart is ‘stressed’ (forced to work hard) either with exercise on a treadmill or with the administration of a drug which forces the heart to beat harder and faster. The doctors then monitor the patient’s symptoms and the activity of the heart with an electric tracing called an ECG (electrocardiogram). While this test has been proven to provide useful information in patients with known heart disease or symptoms of heart disease such as chest pain, it is not able to reliably identify so-called ‘silent’ coronary artery disease in seemingly healthy individuals.
Coronary angiography is an imaging exam that involves the administration of x-ray dye, usually via a large artery in the leg, to obtain pictures of the coronary arteries directly. This allows doctors to visualize the degree of narrowing in the coronary arteries, and assess the need for intervention or surgery. However, the test is expensive, time-consuming and invasive, and is not an appropriate tool for scanning apparently healthy individuals.
Coronary calcium CT
The ideal screening test to identify apparently healthy individuals at risk for coronary artery disease would be rapid, noninvasive, painless and relatively cost efficient. Enter coronary calcium CT. CT (computed tomography) scanning is an imaging technique in wide clinical use that allows physicians to obtain detailed images of the internal structures of the body. As CT technology has evolved, it has become possible to obtain images very quickly, allowing clear depiction of the beating heart. This allows physicians to accurately measure the amount of calcium in the coronary arteries; calcium is a component of the atheromatous plaques that narrow the coronary arteries. The total amount of calcium is a good predictor of the overall severity of atherosclerotic disease (plaque burden).
This result can be reported as an Agatston score, which is a numerical score between 0 and 1000. Scores below 10 are considered normal; scores above 400 are considered high risk.
Does coronary calcium CT identify asymptomatic patients at risk for heart attack?
A meta-analysis (combined analysis of multiple clinical studies) published in the Archives of Internal Medicine provided convincing evidence that an elevated calcium score is an independent risk factor for coronary heart disease. Patients with only mildly elevated calcium scores (up to 100) were twice as likely to suffer from a heart attack or sudden death than patients with no evidence of coronary calcification. Patients with high coronary calcium scores (greater than 400) were anywhere from 4 to 17 times more likely to suffer from a heart attack or sudden death.
The overall conclusion? Coronary calcium CT appears to be an effective screening tool for identifying seemingly healthy individuals at increased risk for heart attack and sudden death.