PHYSIOLOGICAL 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 bl ood flow (MBF) in humans. Earlier work established that coronary heart disease (CAD) can be detected by coronary angiography and that this d isease has characteristic features at rest and during stress, which in dicate the linkage between regional metabolic needs and myocardial per fusion. In the 1970s myocardial perfusion was mapped in patients with radioxenon, but this method had significant technical limitations. Abo ut the same time, radioactive microspheres were introduced for cardiov ascular research and investigations; these particles provided insights on MBF in acute infarction and ischemia, myocardial reperfusion, coll ateral circulation, myocardial blood flow during exercise, coronary fl ow reserve (CFR), and layer-to-layer distribution of MBF, Studies with microspheres also permitted investigators to establish the presence i n the heart of MBF heterogeneity. Currently, there are several techniq ues 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. Co ntrast myocardial echocardiography is another novel technique which ha s been useful in mapping the area at risk, reperfusion, myocardial via bility and collateral circulation, This essay also considers the emerg ing 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 an d quantitatively maps myocardial perfusion and CFR, The latter can be computed before and after angioplasty. PET studies have further demons trated that chronic myocardial ischemia does not exist as a distinct s tate in patients with CAD. From the above investigations the concept h as arisen that not only is CAD an entity involving epicardial vessels but also, in a significant portion of patients, an abnormal microcircu lation plays an important role in the pathogenesis of ischemic syndrom es. PET studies have relatively low spatial resolution since they cann ot resolve layer-to-layer absolute MBF.