Using radiopharmaceuticals to visualize the regional distribution of myocardial perfusion during rest and stress is a well-established modality for the evaluation of known or suspected coronary artery disease (CAD). In 1964, the first scintigraphic images of myocardial perfusion were acquired by Carrea et al.,1 while Zaret et al. were the first to demonstrate exercise-induced myocardial ischemia using radioactive potassium in 1973.2 Since then, the field of nuclear cardiology has grown dramatically, and numerous studies have validated the utility of both exercise and pharmacologic stress myocardial perfusion imaging (MPI) for risk assessment and the prediction of future cardiac events. With >8 million such studies being performed yearly in the United States alone, understanding the logistics of and options available for radionuclide stress testing is critical.3
Whenever possible, exercise is the preferred modality for stress testing, because it allows for a physiologic assessment of functional capacity, hemodynamics, and symptoms. In addition, when compared to pharmacologic stress testing, exercise is associated with less extensive hepatic and gastrointestinal tracer uptake, which significantly improves image quality.4,5
MPI in conjunction with exercise stress testing enhances diagnostic sensitivity and specificity, particularly among patients with resting electrocardiographic (ECG) abnormalities that preclude the interpretation of ST-segment deviation. Similarly, MPI can differentiate true-positive from false-positive ST-segment depression (STD), which is helpful, because among patients referred for exercise ECG testing with a low to intermediate pretest probability of CAD, approximately 40% of those who develop STD will not have CAD.6 When compared to ECG interpretation in isolation, MPI not only provides a more accurate assessment of the extent and severity of disease, but it can also localize ischemia to a particular vascular distribution. MPI is also useful when patients fail to achieve their target heart rate during exercise, because myocardial perfusion abnormalities in response to stress occur earlier than ECG changes.7 Finally, when combined with exercise, MPI not only improves diagnostic capability, but is also predictive of short- and long-term cardiac events.8 This important prognostic ability does not apply to ECG interpretation without concurrent use of the Duke treadmill score (Table 8-1) or the presence of significant ischemic changes, such as ST-segment elevation (STE). Despite the clear advantages of MPI in conjunction with exercise testing, an important consideration should be made for patients who are able to achieve ≥10 METS on exercise stress testing. A recent study by Bourque et al.9 showed that individuals who are at intermediate risk for CAD or those who have known CAD and are able to accomplish ≥10 METS have an excellent cardiovascular prognosis regardless of peak heart rate achieved, and thus the addition and utility of MPI in this population is questionable.
Table 8-1Duke Treadmill Score