CAC Scoring Plus SPECT Provide Long and Short View of Cardiac Risk

Reed Miller

November 03, 2009

November 3, 2009 (Houston, Texas) — Coronary artery calcium (CAC) testing with noncontrast computed tomography can provide a long-term estimate of cardiac-event risk to complement the short-term estimate provided by myocardial perfusion single-photon-emission computed tomography (SPECT), according to the authors of a new trial published in the November 10, 2009 issue of the Journal of the American College of Cardiology [1].

According to authors Dr Su Min Chang (Methodist Hospital, Houston, TX) and colleagues, the study shows that CAC scoring and SPECT results provide independent and complementary predictors of a patient's cardiovascular disease risk. A normal SPECT result predicts short-term event-free survival, but CAC score results showing a severe atherosclerotic plaque burden are associated with greater long-term risk for cardiac adverse events.

"Our results support performing a CAC [score] in patients at intermediate or high clinical risk for CAD who have a normal SPECT result because approximately 20% will have a CAC [score] of at least moderate severity that cannot be predicted from the patient's clinical profile," Chang et al explain. "Calcium scoring allows identification of high-risk individuals among the heterogeneous group of relatively low-risk patients with a normal SPECT result."

Commenting on the significance of study's findings, Dr Leslee Shaw (Emory University, Atlanta, GA) told heart wire , "We have to recall and put this study within the context of prior literature. We have always known that risk in patients with a normal SPECT is variable--that is, a 45-year-old woman with a normal SPECT does not have the same risk as a 70-year-old man with known coronary disease and a normal study. Previously, we used whether or not they could exercise as a threshold. That is, if the patient can exercise, their risk is low, and if they can’t and undergo pharmacologic stress, their risk with a normal study is higher. But this study supports the utility of CACS as a more precise risk stratifier."

CAC Scoring and SPECT Ferret Out More Together

Chang and colleagues evaluated 1126 generally asymptomatic patients without previous cardiovascular disease who had CAC scoring and stress SPECT tests within a short timeframe (median 56 days). The patients were followed for an average of almost seven years.

The risk for total cardiac events and all-cause death and myocardial infarction (MI) increased significantly with increasing CAC score severity and with SPECT abnormalcy. In patients with a normal SPECT result, total cardiac events were under 1% annually and death and MI were under 0.5% annually for the first four years of follow-up. However, patients whose SPECT test showed a >15% stress-induced left-ventricular perfusion defect had significantly more annual cardiac events (9.6% vs 3.8%; p<0.001) and a higher all-cause death/MI rate (6.1% vs 2.3%; p<0.001) than those with smaller defects throughout the follow-up period.

The integration of CAC scoring and SPECT further clarified the patients' risk. Patients with a moderate or severe CAC score with abnormal SPECT results had significantly higher rates of cardiac events and all-cause death or MI compared with patients with similar CAC scores but normal SPECT results. Conversely, among patients with a normal SPECT result, the total cardiac-event and all-cause-death and MI rates significantly increased with CAC score severity.

Subjects With Normal Stress SPECT: Relative-Risk Hazard Ratios Based on CAC Scoring

CAC score Total cardiac events, HR Total cardiac events, p Death or MI, HR Death or MI, p
11–100 1.28 0.52 1.96 0.09
101–400 1.53 0.24 1.72 0.15
>400 3.55 <0.001 2.75 0.008
For CAC score <10, hazard ratio=1

Current ACC/AHA guidelines recommend myocardial perfusion SPECT for patients who are asymptomatic but have a CAC score of over 400, indicating a high atherosclerotic plaque burden. But these new results suggest these patients may be better served if the order of those tests is reversed, study investigator Dr John Mahmarian (Methodist Hospital) told heartwire .

A normal SPECT with a high CAC score indicates a patient who is at high risk for future cardiac adverse events but whose coronary disease is not advanced yet and can therefore be treated. "I don't want to find a high-risk patient I can't do anything about, but there are lots of data that show that it's possible to intervene early in high-risk patients to [avoid a major adverse event].

"The value of CAC [scoring] as a high-risk marker may extend beyond its prognostic implications. Although not specifically addressed in this study, identification of early atherosclerosis, defined by CAC [score], may improve patient outcomes through earlier and more intensive risk-factor modification and treatment of hyperlipidemia," Chang et al explain. "The hypothesis that selectively targeting therapy based on CAC [scoring] and SPECT results may reduce downstream medical and overall healthcare costs is one that warrants further study."

One possible study design to evaluate the benefit of CAC scoring in patients with normal SPECT results would be to randomize these patients to aggressive treatment of hyperlipidemia vs standard therapy, Mahmarian said.

Shaw cautions against changing the testing strategy based on this single-center study. "A major challenge with the current report is that these patients were asymptomatic, and testing asymptomatics is considered inappropriate by current appropriateness criteria," she observed. "Given the concern over radiation, the utility of this type of strategy on testing asymptomatics with both SPECT and CT is limited and requires substantial refining and validation before it would be accepted."

Shaw also pointed out that the ACC/AHA guidelines committee is working on new guidelines for detection of cardiac risk in asymptomatic, high-risk individuals, which it plans to release in 2010. "This will be a definitive review of CT and other modalities on their comparative utility in this type of patient population."

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