Myocardial perfusion and coronary microcirculation: from pathophysiology to clinical application.

The clinical use of radioisotopes in the evaluation of patients with ischemic heart disease is largely based on noninvasive methodology suitable for visualizing myocardial perfusion. Although contrast echocardiography and magnetic resonance imaging have been proposed recently for the same purpose, at present most of the noninvasive tests for assessing myocardial perfusion reside in nuclear cardiology. The most popular radioactive flow tracers are characterized by a rapid myocardial extraction followed by either a sequestration (for technetium 99m–labeled agents) or a very slow washout (for thallium 201). These features make the tracer uptake proportional to blood flow in each myocardial region, but the quantitation of absolute flow is not possible with these techniques. Despite this limitation, myocardial perfusion can be imaged and underperfusion can be detected as a relative uptake defect compared with the better perfused myocardium. As “significant” coronary stenosis regionally impairs flow reserve, myocardial perfusion imaging has become a common tool for the diagnosis of coronary artery disease and coronary angiography the gold standard for defining its sensitivity and specificity. This review will not discuss the diagnostic accuracy of myocardial perfusion scintigraphy in detecting coronary stenosis, but rather the significance of perfusion imaging in the light of recent research documenting the presence of coronary microvascular alterations in various heart diseases and will focus on possible pathophysiologic mechanisms.