Impact of myocardial perfusion scintigraphy in general cardiology practice

The study by Høilund-Carlsen and Johansen evaluates the benefits of using myocardial perfusion scintigraphy (MPS) in a new way—as a gatekeeping technique for further invasive diagnostic and therapeutic procedures among patients with stable angina pectoris. As cardiologists in clinical practice, we all use MPS as a standard tool in the evaluation and treatment of patients with angina pectoris and suspected or known coronary artery disease (CAD). Myocardial perfusion scintigraphy is used mainly in 3 ways: for diagnosis, therapeutic management, and risk stratification. By keeping the results of MPS secret from the referring physicians, the present study was able to eliminate the post-test referral bias present in other studies, in which the referring physicians could have been influenced by the results of MPS.

The results of this study are quite enlightening. In a prospective series of 507 of 972 patients referred for coronary angiography, MPS showed normal perfusion in 51% of patients (258/507), reversible perfusion defects in 40%, and fixed defects in 9%. A total of 168 patients eventually underwent a coronary revascularization procedure. Of those patients, 16% had normal perfusion studies and 8% had fixed defects showing no ischemia. Coronary angiography actually was performed in 476 patients who were referred for that procedure. Of those patients, 53% had normal findings or insignificant CAD. The authors conclude that, based on this observational study, 48% of cardiac catheterizations and 19% of revascularization procedures could have been avoided. Although patients at high risk for severe CAD, diabetic patients, patients older than 75 years of age, and patients with known severe CAD being followed in the cardiology clinic were excluded, the authors concluded that the risk of overlooking patients with severe disease was negligible. This was based on analysis of the coronary angiography results after the MPS results were finally made available.

Although this study was observational, its strengths lie in examining the population of patients in which MPS is most useful, namely, the “intermediate”-risk patients being evaluated for angina pectoris and possible CAD. A limitation of this study was that MPS was not performed in the entire consecutive series of patients (507/972). The second limitation noted was that patients at high risk, in particular, patients with known CAD undergoing uptitration of their medicines, elderly patients, and diabetic patients, were excluded. This actually can be viewed as a strength of the study because many of these patients would not be referred for MPS anyway, but instead would be taken to the catheterization laboratory for an invasive procedure. The fact that this study assessed more than 500 patients in the intermediate category makes it very unique.

The clinical message from this study can be summarized as follows. Although data suggest that MPS would be a good gatekeeping technique when evaluating patients for angina pectoris and chest pain syndromes, this may never come to pass. First, cardiologists in general practice regard MPS as a diagnostic tool and a “piece of the puzzle,” along with clinical presentation, noninvasive functional data, and other imaging modalities in the initial and subsequent evaluation of the patient. We are all trained to balance the significance of the MPS results and view them in light of the other data points mentioned. This is especially true when viewing an abnormal MPS study in a patient without ongoing ischemia. A patient with angina pectoris with some atypical features, along with an abnormal MPS (showing “fixed” defects but not ischemia), would surely proceed to the cardiac catheterization more often than not. Secondly, the practice guidelines, as updated by the American Heart Association, the American College of Cardiology, and the American Society for Nuclear Cardiology, do not view MPS strictly as a gatekeeping procedure, even in the intermediate-probability patient population. What we can learn from this study is to place more emphasis on the MPS results when deciding whether to proceed to the cardiac catheterization laboratory and evaluate a patient invasively, especially when there may be other mitigating clinical circumstances. These circumstances would include patients at higher risk for a complication during an invasive procedure, such as those with renal insufficiency or intervascular access problems.

Finally, we should consider the present study in light of the newer noninvasive imaging technologies that are becoming part of our practice, namely, high-resolution computed tomography and magnetic resonance angiography. As these imaging techniques become more widely accepted and available, and with the expected advances in the technologic imaging field, one has to ask the following question: Who will be the gatekeeper of the future?