For decades now, heart researchers have been tallying a list of risk factors that can increase the development of heart disease: smoking, being overweight, having high cholesterol, having high blood pressure or being diabetic. And, of this list, half of all Americans have one of the last three and most common culprits, according to a report released by the Centers for Disease Control and Prevention this week.
But while the presence of such risk factors may be helpful for sussing out who is at highest risk of the disease (defined as those who have a 20% or higher chance of having a heart event in the next 10 years) versus those who are at low risk (with less than a 10% chance), they are not a perfect measure of risk overall. Many patients fall within the gray zone of intermediate heart risk, as determined by traditional risk factors and there is no straightforward way for doctors to know whether these otherwise healthy patients should be treated with medications.
So the hunt has been on for additional tests that can help pinpoint these patients' true odds of a heart attack. Now scientists report some intriguing evidence for one such test, which measures calcium deposits in the heart arteries.
Calcium in the blood vessels are the building blocks for the fatty plaques that can accumulate within heart vessel walls; heart attacks occur when plaques rupture and block blood flow to the heart. Previous studies have linked higher levels of coronary artery calcium to a greater risk of heart disease, but doctors have not been convinced that routine testing of a healthy population for calcium levels is necessary. "The American Heart Association (AHA) and the American College of Cardiology have previously addressed this area...and complete embrace with widespread recommendation for use has been reserved pending more compelling data," Dr. Clyde Yancy, president of the AHA, writes in an email comment on the study.
The first pieces of that data came in the new report, which was published Tuesday in the Journal of the American Medical Association. Led by Dr. Philip Greenland and Dr. Tamar Polonsky at Northwestern University, a group of researchers conducted the most thorough analysis yet of how powerful a heart-attack predictor calcium deposits can be.
The authors began by asking whether the addition of a patient's coronary artery calcium (CAC) score to a suite of traditional heart risk factors such as age, smoking history and cholesterol levels would change his or her heart-risk classification. For instance, could a CAC score reclassify a person initially determined to be at intermediate risk into the high-risk category? Or might it move him down to the low-risk group? Such classifications have profound treatment implications, since high-risk patients are commonly given powerful medications to lower blood pressure or cholesterol levels, while low-risk patients can generally rely on modifications of lifestyle, such as diet and exercise, and avoid drugs.
Researchers followed a study population of 5,878 men and women for nearly six years. Each patient's heart attack risk was calculated twice at the outset: once using only the traditional risk factors, and again with the addition of the CAC score. At the start of the study, none of the participants had cardiovascular disease; by the end of the study, 209 heart events were logged, 122 of which were major events including heart attack and heart-related death.
What researchers found was that the addition of a CAC score led to significantly better prediction of heart events than traditional factors alone. Adding CAC scores moved 25% of the participants into a different risk group than where they were placed based on traditional risk factors. It had the largest effect among those initially considered to be at intermediate risk: among these patients, the addition of a calcium score moved 16% into the high-risk group and 39% into the low-risk group.
What's more, with the addition of the calcium score, an additional 23% of participants who experienced heart events, such as angina or a heart attack, or underwent a procedure to open up blocked vessels were reclassified into the high-risk group; an additional 13% of those who did not have heart events were relabeled as low risk.
"I think these data are consistent with the idea of very selective testing," says Greenland. "After you have done an initial risk assessment using the traditional risk factors, if you find somebody who falls into the intermediate risk category, you might get them a coronary calcium study to move them one way or another." Greenland stresses that his group's findings should not be taken as license to recommend widespread screening for everyone; rather, the test should be limited to those patients who fall into the gray area of intermediate risk.
But even within that population, routine CAC testing is not a foregone conclusion. For one thing, the data still do not provide any evidence that regular calcium testing can actually help people prevent future heart events. To prove that, it would require a gold-standard randomized controlled trial, in which patients are randomly divided into groups that either receive regular calcium testing or do not, and are then observed for their rates of heart events.
In addition, the coronary artery calcium test uses a CT scan, which exposes the patient to radiation and could result in increases in cancer risk, says Dr. John Ioannidis, a Tufts University professor and chairman of the department of hygiene and epidemiology at University of Ioannina in Greece. The more patients who get tested, the greater the odds they will develop cancers due radiation exposure.
Also, unlike simple blood analyses for cholesterol and diabetes, cardiac CT scans are expensive costing about $600 or so. "You need to balance the benefit against the need to test more people with an expensive test that has the potential to cause some cancers," says Ioannidis, who wrote an editorial accompanying the new study in JAMA. But if the trend identified in the new study holds, then calcium testing may yet become a part of the arsenal of heart-disease risk factors and help more people to avoid it.