CORONARY VASCULAR ANATOMY AND AUTOREGULATION OF MYOCARDIAL BLOOD FLOW
The coronary vasculature may be artificially divided into 3 compartments: the epicardial vessels, the coronary microvasculature, and the venous system1 (Figure 11-1).
Schematic representation of the 3 coronary vascular compartments.
The epicardial coronary arteries (diameter >500 μm) are predominantly capacitance vessels and, in the normal state, offer little resistance to blood flow. The main epicardial coronary arteries are comprised of the right coronary artery (RCA), the circumflex artery, and the left anterior descending (LAD) artery. The RCA arises from the right aortic sinus, descending in the right atrioventricular groove, giving off a marginal branch. The RCA passes to the inferior surface of the heart and continues in the posterior interventricular groove as the posterior descending artery (PDA). The RCA may give off a further branch supplying the lateral left ventricular wall—the posterior left ventricular branch. The left coronary artery arises from the left aortic sinus as the left main stem, which divides into the circumflex artery and the LAD artery. The circumflex artery runs laterally in the left atrioventricular groove and gives off obtuse marginal braches. The LAD runs in the anterior interventricular groove to the apex of the heart. The LAD gives off septal perforators and diagonal branches.2 In approximately 70% of people, the RCA continues as the PDA (right-dominant circulation); in 10%, the circumflex artery forms the PDA (left-dominant circulation); and in 20%, the PDA is formed by an anastomosis of the circumflex artery and RCA (co-dominant circulation) (Figure 11-2).
Ex vivo arteriogram of a normal human heart demonstrating the normal coronary epicardial anatomy: RCA (red contour), LAD (green contour), and circumflex artery (blue contour). The extensive anastomotic communications of the coronary microvasculature are well visualized but are not demonstrated on standard invasive coronary angiography. (Used with permission from Fulton WF. Immersion Radiography of Injected Specimens. The British Journal of Radiology. 1963;36:685-688.)
Epicardial coronary blood flow occurs predominantly in diastole, due to the effect of systolic myocardial compression on the coronary microvasculature. The subendocardium is most susceptible to ischemia (in comparison to the subepicardial myocardium) due to the greater effect of left ventricular systolic compression and higher resting oxygen consumption. Myocardial oxygen extraction at rest is high (60%-80%); therefore, increased oxygen consumption during exercise is predominantly met by increases in myocardial blood flow rather than increased oxygen extraction.3 The coronary microvasculature is the predominant site of autoregulation of myocardial blood flow and resistance to coronary blood flow. Pre-arterioles (diameter 100-500 μm) are comprised of proximal and distal vessels, which are most responsive to changes in flow and pressure, respectively. Pre-arterioles regulate perfusion pressure into the subtended arteriolar compartment. ...