The relation between mean coronary flow and mean perfusion pressure is under the influence of autonomic, neural and hormonal control. Autoregulation causes the rather constant flow for the physiological range of pressures, and the change in flow with cardiac metabolism. Autonomic coronary flow regulation consists of three mechanisms: metabolic, myogenic and endothelium mediated vasoactivity. The so-called instantaneous pressure-flow relations are obtained in diastole to avoid the effect of cardiac muscle contraction, and describe the state of the coronary bed. Cardiac contraction reduces coronary arterial inflow and augments venous outflow in systole, the ‘intramyocardial pump’. This effect results from three mechanisms: The direct effect of increased muscle stiffening (varying elastance), the indirect effect of increased ventricular pressure producing an intramyocardial (interstitial) pressure in the ventricular wall, and the thickening of the muscle during shortening contractions at the expense of vascular lumen. Cardiac contraction is the main reason why the subendocardial layers are most prone to ischemia.
KeywordsAutoregulation Instantaneous pressure-flow relation Intramyocardial pump Muscle stiffening Intramyocardial pressure Muscle thickening Ischemia Transmural perfusion Waterfall
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