Nuclear cardiology is a widely available, cost-effective, non-invasive methodology in use for more than two decades with which a large amount of experience has been accumulated. The technique is in constant development and its application has been growing to constitute nowadays about 30-40% of all nuclear medicine procedures in an average institution. Most indications are related to coronary artery disease (CAD) evaluation and are supported by large amounts of scientific evidence. The following are some of such indications:
a) Diagnosis of CAD.
This application was probably the first to demonstrate the efficacy of nuclear cardiology studies, specially the investigation of myocardial perfusion together with stress testing. According to Bayes' theorem, patients with intermediate pre-test probability of CAD benefit most from the study (i.e., patients with chest pain and negative exercise test, asymptomatic patients with ECG changes or patients with non-diagnostic ECG).
b) Prognosis and risk stratification in chronic CAD patients.
This is an application of increasing frequency regarding decision-making and rational patient management. Considering the findings of a myocardial perfusion study in terms of number, extension and severity of ischemic defects, the patient can be assigned a low, intermediate or high probability for future cardiac events and the most appropriate therapeutic choice can be selected. This is also useful for non-cardiac preoperative evaluation.
c) Unstable angina.
Most patients admitted with diagnosis of unstable angina are usually derived for immediate invasive evaluation. However, it has been demonstrated that if stabilization is achieved, these patients can be safely submitted for stress myocardial perfusion studies to determine the extent and severity of induced ischemia in order to choose between invasive and non-invasive strategies.
d) Myocardial infarction.
There is strong evidence that patients with the so-called "non-Q wave" myocardial infarction can benefit from non-invasive treatment unless a functional study result indicates high ischemic risk. In patients with transmural, non-complicated myocardial infarction, a non-invasive functional evaluation at discharge is suggested for risk-stratification, which can be done through a stress perfusion study preferably with gated SPECT, in order to determine the need for cardiac catheterization.
e) Evaluation of revascularization procedures.
Percutaneous transluminal coronary angioplasty (PTCA) is a minimal-invasive revascularization technique but has a relatively high rate of restenosis, even with stent implantation. In patients with chest pain and/or positive/indeterminate stress tests post-PTCA, myocardial perfusion studies can determine the presence of restenosis or detect a new affected vascular territory, or even rule out significant ischemia. In patients with previous by-pass surgery, perfusion studies are also useful for assessment of graft patency.
f) Chest pain evaluation in the emergency department.
Chest pain is one of the most frequent cause of emergency admissions worldwide. Many patients are really undergoing a cardiac ischemic episode, but many more have non-cardiac causes of pain that would need no hospitalization. Chest pain units have been installed in some institutions to rationally evaluate these patients, and acute rest myocardial perfusion studies have been demonstrated to represent a cost-effective procedure to be implemented within the diagnostic algorithm, specially in cases with non-interpretable ECGs.
g) Investigation of myocardial viability.
Patients with known CAD with previous myocardial infarction and poor ventricular function are potential candidates for revascularization procedures tending to improve survival and life quality. However, this can only be achieved if myocardial viability is present since non-viable tissue is not expected to improve function after restoration of blood flow. Hypokinetic but viable myocardium is known as "hibernating" myocardium and represents a condition believed to occur as a functional down-regulation in the presence of a diminished blood supply or as a consequence of repetitive acute ischemic episodes. Various nuclear medicine techniques have been developed to detect myocardial viability including PET, thallium perfusion with reinjection and late imaging, and nitrate-enhanced perfusion with technetium-based agents, with similar diagnostic efficacy. Gated SPECT with low-dose dobutamine infusion is also under clinical investigation.
h) Evaluation of medical treatment.
Evolution of coronary atherosclerosis can be influenced by means of aggressive control of risk factors and medication. Coronary flow reserve and endothelial function can be effectively assessed in these patients to evaluate the efficacy of non-invasive therapy. This will probably constitute a main indication for perfusion studies in the near future as new pharmachologic products are released to the market raising the need for objective evaluation of cardiac effects.
i) Functional assessment of known anatomic lesions.
Patients with borderline (30-60%) coronary stenosis are frequently submitted for myocardial perfusion studies in order to evaluate the functional significance of the lesions and eventually identify the one(s) causing symptoms to the patient (culprit lesion). This helps the invasive cardiologist to make a decision on which vessel to attempt revascularization. Also, with the widespread use of electron-beam computerized tomography (EBCT) for detection of coronary calcifications, functional assessment will be indicated to determine the need for further invasive evaluation.
