Role of nuclear cardiology in evaluating the total ischemic burden in coronary artery disease

Abstract

Goals of exercise radionuclide imaging are to: (1) enhance sensitivity, specificity and predictive value of coronary artery disease (CAD) detection; (2) noninvasively assess extent and severity of functionally significant CAD; (3) determine prognosis so that specific therapeutic strategies can be more rationally implemented; (4) detect silent ischemia in asymptomatic subjects or in patients with known CAD with a higher degree of specificity than can be accomplished by electrocardiogram stress testing alone; (5) evaluate the response to therapeutic interventions aimed at enhancing coronary blood flow. Two major radionuclide techniques are currently used in evaluating the total ischemic burden in patients with CAD. These are myocardial perfusion imaging with either thallium-201 or rubidium-82, and radionuclide angiography performed after administration of technetium-99m. Areas of diminished thallium-201 activity on early postexercise images are abnormal and represent either areas of stress-induced ischemia or myocardial scar. To differentiate between the two, delayed images are obtained to determine if the initial postexercise defect either persists or demonstrates redistribution (i.e., delayed defect disappearance or improvement). Defects demonstrating redistribution represent transient ischemia, whereas areas of previous infarction or scar usually appear as persistent defects. Patients with left main or 3-vessel CAD usually show multiple thallium-201 redistribution defects in more than 1 vascular supply region, a phenomenon often associated with abnormal lung thallium-201 uptake. In terms of radionuclide angiography, the normal ventricular response to exercise has been defined as an absolute increment of at least 5 % in the left ventricular ejection fraction without the development of regional wall motion abnormalities. In response to exercise, patients with multivessel CAD have a more profound decrease in ejection fraction than do patients with less high-risk anatomy. The observation that electrocardiographic and radionuclide markers of ischemia are more common during exercise testing than angina in patients with CAD appears to indicate a significant prevalence of asymptomatic ischemia. Patients who demonstrate thallium-201 redistribution remote from an infarct region, or who have a decrease in left ventricular ejection fraction from rest to exercise during predischarge testing after an uncomplicated myocardial infarction, should be considered for early catheterization. Certain patients demonstrate these abnormal perfusion or functional findings at variable thresholds of double-product response, suggesting that vasoconstriction may sometimes be a component in the ischemic response to stress.