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Coronary Calcification and Perfusion Imaging
Abstract & Commentary
By Michael H. Crawford, MD, Professor of Medicine, and Chief of Clinical Cardiology, at the University of California, San Francisco. Dr. Crawford is on the speaker's bureau for Pfizer.
Source: Ramakrishna, G, et al. Relationship and prognostic value of coronary artery calcification by electron beam computed tomography to stress-induced ischemia by single photon emission computed tomography. Am Heart J 2007;153:807-814.
After a positive coronary CT scan, patients often undergo stress myocardial perfusion scanning, but the relationship between these two tests in relation to prognosis is poorly understood. Thus, these investigators from the Mayo Clinic identified 1230 patients who had undergone electron beam computerized tomography (EBCT) and stress single photon emission computed tomography (SPECT) within 3 months, over a 4-year period. After excluding patients with known coronary artery disease (CAD), valvular disease and paced rhythm or left bundle branch block the final population was 835 who consented to participate. They were followed for an average of 5 years to assess the primary endpoint of all-cause mortality. The secondary endpoints were myocardial infarction and coronary revascularization after 3 months post testing. Coronary calcium was scored as normal (0), minimal (1-10), mild (11-100), moderate (100-400) and severe (>400). SPECT results were similarly categorized using the Cedars Sinai criteria.
Results: EBCT scores tended toward more abnormal studies (26% >400), whereas, SPECT scores tended toward low-risk studies (64% were normal). There was a weak correlation between the 2 studies; 4% of patients with a normal EBCT and 18% with an EBCT score >400 had a high-risk SPECT. Conversely, 70% of patients with EBCT >400 had a normal SPECT. EBCT better predicted a high-risk perfusion scan than any clinical variable. Adverse outcomes were correlated with both tests, but only EBCT predicted mortality in asymptomatic patients. The highest of mortality was in those with an EBCT >400 and a high-risk perfusion study. The authors concluded that EBCT and SPECT are weakly correlated; both predict mortality in symptomatic patients; but only EBCT predicts mortality in asymptomatic patients.
Interestingly, I am seeing patients referred because of an abnormal coronary calcium score on fast CT imaging ordered by their primary care doctor or by themselves. It is difficult not to perform a stress test on these patients, since they and their doctors are concerned about the results. This report of the Mayo Clinic experience in patients who have had both tests performed provides some interesting insights that may be of value in managing such patients. First, 96% of their patients with a calcium scan of zero had a negative SPECT. We are not told if the 4% with positive SPECT are true positives, but I suspect most were false positives. Second, even among those with a calcium score >400, 70% had a negative SPECT. These results are not different from other reports in the literature.
Unique to this study was the outcome of patients with a high-risk SPECT and a calcium score >400. They had a 10-year mortality of 42%. This is much higher than a calcium score of >400 alone of 27% and a high-risk SPECT alone of 31%. High-risk SPECT or calcium scans predicted higher mortality and morbidity in symptomatic patients, but only EBCT predicted outcomes in asymptomatic patients. However, the death rate in asymptomatic patients was 0.4% over 5 years. It is difficult to imagine that any testing or treatment strategy could be shown to reduce this low mortality.
This study has several limitations. It is retrospective. There is a referral bias toward patients with abnormal tests and 20% of their patients were lost to follow-up. Regardless, there are clinical implications to note. It is hard to imagine what benefit an asymptomatic patient derives from either test. Also, in patients with a markedly positive calcium scan, stress SPECT is usually negative. Therefore, it would seem that one could be selective in whom with a positive calcium score to do a stress test. Certainly symptomatic patients, but there would have to be other compelling reasons in an asymptomatic patient, given their excellent prognosis.