MAC is a buildup of calcium in the fibrous ring that supports the mitral valve. The mitral valve controls blood flow from the heart's upper left chamber (left atrium) to the lower left chamber (left ventricle), says lead author Caroline S. Fox, M.D., M.P.H., a medical officer with the National Heart, Lung, and Blood Institute's Framingham Heart Study.
After adjusting for known heart disease risk factors such as high blood pressure, high cholesterol, diabetes, obesity, physical inactivity and smoking, MAC was independently associated with increased risk of heart attack or death. The researchers say it is unclear what causes MAC, but atherosclerosis is a likely contributor.
Fox and her colleagues used echocardiography (ultrasound imaging of the heart) to measure MAC in participants in the Framingham Heart Study. MAC was defined as the presence of a dense band of calcification that could be clearly seen when the valve was open and closed. Calcification was measured in millimeter (mm) segments.
The new study is an analysis of data from echocardiograms of 1,197 people – 445 men and 752 women who had echocardiograms during routine examinations from 1979 to 1981. Men were 69 years old on average and women were an average age of 73. Participants were followed for 16 years after their original echocardiogram. Cardiovascular events during the study period included heart attack, coronary insufficiency, congestive heart failure and non-hemorrhagic stroke. Fatal cardiovascular events included sudden cardiac death.
"MAC is fairly common. We found it in 14 percent of the elderly people tested," says study co-author Emelia J. Benjamin, M.D., Sc.M., director of echocardiography and vascular testing for the Framingham Heart Study.
There were 307 new cases of cardiovascular disease and 621 deaths in the study group during follow-up; 213 deaths were attributed to heart disease.
Fox says people with MAC were 50 percent more likely to have developed cardiovascular disease during the follow-up – and 60 percent more likely to have died of cardiovascular causes – than those who had no evidence of MAC.
"MAC was also associated with an increased risk for all-cause mortality," she says. Based on the findings, Fox says MAC may be a marker for "preclinical cardiovascular disease."
Both Fox and Benjamin say there appears to be an association between MAC and traditional heart disease risk factors. "We can speculate that MAC is a test for long-term risk factor exposure," Benjamin says. Thus, they speculate that control of known risk factors such as high blood pressure and high cholesterol could lessen the risk for developing MAC.
The Framingham Heart Study began in 1948 when 5,209 men and women aged 28 to 62 were enrolled. Subjects are examined every two years. Researchers recently began collecting data from the third generation of Framingham participants.
Co-authors are Ramachandran S. Vasan, M.D.; Helen Parise, Sc.D.; Daniel Levy, M.D.; Christopher J. O'Donnell, M.D.; and Ralph B. D'Agostino, Ph.D.
The study was funded by the National Heart, Lung, and Blood Institute.
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