Physiologic and clinical significance of myocardial blood flow quantitation: What is expected from these measurements in the clinical ward and in the physiology laboratory?

In this essay we review data on absolute quantitation of myocardial blood flow (MBF) in humans. Earlier work established that coronary heart disease (CAD) can be detected by coronary angiography and that this disease has characteristic features at rest and during stress, which indicate the linkage between regional metabolic needs and myocardial perfusion. In the 1970s myocardial perfusion was mapped in patients with radioxenon, but this method had significant technical limitations. About the same time, radioactive microspheres were introduced for cardiovascular research and investigations; these particles provided insights on MBF in acute infarction and ischemia, myocardial reper-fusion, collateral circulation, myocardial blood flow during exercise, coronary flow reserve (CFR), and layer-to-layer distribution of MBF. Studies with microspheres also permitted investigators to establish the presence in the heart of MBF heterogeneity. Currently, there are several techniques that aim at extending these concepts into clinical investigation. Two of these techniques, i.e. Doppler coronary flow velocity and fast magnetic resonance imaging assess epicardial flow dynamics and CFR. Contrast myocardial echocardiography is another novel technique which has been useful in mapping the area at risk, reperfusion, myocardial viability and collateral circulation. This essay also considers the emerging technique of intracoronary ultrasound which has shown evidence of disease underestimation by conventional contrast angiography. Positron emission tomography (PET) is a noninvasive technique that uniquely and quantitatively maps myocardial perfusion and CFR. The latter can be computed before and after angioplasty. PET studies have further demonstrated that chronic myocardial ischemia does not exist as a distinct state in patients with CAD. From the above investigations the concept has arisen that not only is CAD an entity involving epicardial vessels but also, in a significant portion of patients, an abnormal microcirculation plays an important role in the pathogenesis of ischemic syndromes. PET studies have relatively low spatial resolution since they cannot resolve layer-to-layer absolute MBF.