Analysis of coronary haemodynamics by coronary circulation model with three layered myocardium

The purpose of this study was to explain the following three characteristics of the phasic intramyocardial microvascular diameter changes during a cardiac cycle observed with our needle-probe CCD intra-vital videomicroscope by a model simulation. (i) The arteriolar diameter decrease was much greater in subendocardium and midmyocardium than in subepicardium. (ii) The venular diameter in the subendocardium decreased while the diameter in subepicardium increased. The midmyocardial venular diameter change was in between. (iii) Subendocardial arteriolar and venular diameter changes were similar (10-20% systolic narrowing). The coronary model is composed of three layers, subepicardium, midmyocardium and subendocardium. The vascular compartment of each layer consists of arterioles, capillaries and venules. The extravascular compressive force was represented by both hydrostatic intramyocardial pressure and myocardial elastance caused by cardiac contractility. The left ventricular, aortic and coronary venous pressures were input into the model. The characteristics of each vessel were determined from published data by others or our own data. The transmural difference of intramyocardial hydrostatic pressure was most important to simulate the experimental results (i) and (ii). The systolic increase in the subepicardial venular diameter was explained by the introduction of epicardial venous capacitance and resistance in addition to the low myocardial pressure. The uniform myocardial elastance was necessary to explain the experimental results (iii).