Choice of imaging modality for evaluation of myocardial ischemia—what is the bottom line?

In Jahnke et al's study, the authors showed the prognostic value of yet another imaging modality—perfusion magnetic resonance (MR) imaging and dobutamine stress MR. Both imaging modalities, done in quick succession one after the other, provided prognostic information in addition to that provided by clinical risk factors and resting wall motion. What do the results of this study mean to a physician caring for patients with stable coronary artery disease (CAD)? Which imaging modality should physicians use when referring patients for evaluation of ischemia?

Exercise electrocardiography, nuclear perfusion imaging, and stress echocardiography with bicycle or treadmill stress, dobutamine, or dipyridamole/adenosine have been shown by several studies to provide high sensitivity and specificity in detecting myocardial ischemia. In addition, a large body of evidence has accumulated on the prognostic utility of these tests.^1,2 Low cost, portability, and evaluation of exercise capacity, wall motion, valvular function, and pulmonary artery pressures make stress echocardiography a very attractive imaging modality. However, technically difficult imaging windows still remain a limitation in those with chronic obstructive pulmonary disease (COPD) or with a heavy-set chest wall, and in the obese.

Nuclear imaging is able to detect wall motion and myocardial perfusion in those with technically difficult echocardiographic images; however, despite supine and prone imaging, it is still associated with technical artifacts as well as requires injection of a radioisotope. Magnetic resonance imaging may be useful in select patients who are not suitable candidates for the aforementioned modalities, those unable to exercise, and those with COPD who have a contraindication to receiving dipyridamole or adenosine because of advanced heart block. Magnetic resonance imaging stress testing is not widely available, carries increased costs, and cannot be performed in patients with claustrophobia or with implanted medical devices. Magnetic resonance imaging, stress echocardiography, and nuclear perfusion imaging will, however, remain in close competition for functional imaging.

Detection of ischemia on stress echocardiography and perfusion imaging is associated with increased cardiovascular risk. However, rupture of non—flow-limiting plaques that do not produce ischemia on stress imaging accounts for a large burden of this risk.³ Revascularization of areas with ischemia resulting from flow-limiting lesions in patients with stable coronary heart disease (CHD)—although providing symptomatic relief of angina—has not been shown to reduce the incidence of acute myocardial infarction (MI) or death. The Clinical Outcomes Utilizing Revascularization and Aggressive Drug Evaluation (COURAGE) study showed no difference in outcome in subjects with stable CHD who were randomly assigned to receive medical therapy compared with subjects who underwent percutaneous revascularization with stents.⁴ Thus, these imaging modalities detect significant atherosclerotic disease burden. However, acute correction of the problem by revascularization does not reduce MI or death unless unstable ischemic syndrome, left main artery disease, severe triple-vessel disease, or reduced left ventricular function are present.

Aggressive risk factor modification, including treatment of low-density lipoprotein cholesterol to a target level of 60 to 70 mg/dL, effective reduction of blood pressure to < 130/80 mm Hg, control of blood glucose levels, exercise, and diet, remains the cornerstone of treatment for cardiovascular disease in which flow-limiting lesions are only the tip of the iceberg.⁵ This study does not provide information on the clinical presentation of patients. Changes in medical therapy, in particular, the use of lipid-lowering, antiplatelet, and antihypertensive agents, were not tracked over the course of the study. In addition, data on whether the outcome of these patients was any different from those who underwent revascularization for comparable disease severity, such as the 84 patients who underwent a revascularization procedure after MR imaging, was not provided.