The combined cohort, comprising 44,052 asymptomatic middle-age patients free of known coronary artery disease who underwent electron beam tomography, represents the largest follow-up data set yet studied for the occurrence of all-cause death after CAC screening, notes the study team led by Dr. Michael Blaha of Johns Hopkins Ciccarone Center for the Prevention of Heart Disease, Baltimore, Maryland.
According to the article, 19,898 patients (45%) had no CAC, 5,388 (12%) had low CAC scores (CAC 1 to 10), and 18,766 (43%) had CAC scores greater than 10. There were 104 deaths, 58 deaths and 739 deaths, respectively, in these three groups.
With a mean follow-up of 5.6 years (range 1 to 13 years), a CAC score of 0 was associated with “excellent survival,” with all-cause mortality rates of 0.87 per 1,000 person-years (less than 1% 10-year risk or less than 0.1% per year).
After adjusting for risk factors, individuals with low CAC scores had a nearly twofold increased risk of death (1.92 per 1,000 person-years), compared with those with no CAC, “suggesting that low CAC represents a distinct risk group,” the authors say. Nonetheless, the researchers report that “mortality rates in individuals with low CAC scores remained low” (less than 5% 10-year risk or less than 0.5% per year).
In contrast to the no CAC and low CAC groups, individuals with high CAC scores had annualized all-cause mortality rates of 7.48 per 1,000 person-years.
Based on their findings, Dr. Blaha and colleagues say, “In the appropriately selected non-high-risk patients, the absence of CAC could potentially be used as a rationale to emphasize lifestyle therapy, scale back on costly preventive pharmacotherapy, and refrain from frequent cardiac imaging and testing.”
“Given the low 1% 10-year risk in this population, a drug that produces a 30% relative risk reduction would have to be given to over 300 patients for 10 years to prevent 1 death (number needed to treat, approximately 333 for 10 years),” they point out.
It should be kept in mind, they add, that even in the absence of CAC, relatively more events occur among diabetic patients and smokers, which might be due to mechanisms other than atherosclerotic plaque. “Although absolute mortality rates remain low (3 to 4 deaths per 1,000 person-years, equivalent to 3% to 4% 10-year risk), risk in these patients warrants further study and these patients should continue close follow up and pharmacotherapy according to present guidelines,” Dr. Blaha and colleagues advise.
J Am Coll Cardiol Img 2009;2:692-700.