ANATOMY & PHYSIOLOGY OF THE CORONARY CIRCULATION
The right coronary artery (RCA) normally supplies the right atrium, most of the right ventricle, and a variable portion of the left ventricle (inferior wall). In 85% of persons, the RCA gives rise to the posterior descending artery (PDA), which supplies the superior–posterior interventricular septum and inferior wall—a right dominant circulation; in the remaining 15% of persons, the PDA is a branch of the left coronary artery—a left dominant circulation.
The left coronary artery normally supplies the left atrium and most of the interventricular septum and left ventricle (septal, anterior, and lateral walls). After a short course, the left main coronary artery bifurcates into the left anterior descending (LAD) artery and the circumflex artery (CX); the LAD supplies the septum and anterior wall, and the CX supplies the lateral wall. In a left dominant circulation, the CX wraps around the AV groove and continues down as the PDA to also supply most of the posterior septum and inferior wall.
The arterial supply to the SA node may be derived from either the RCA (60% of individuals) or the LAD (the remaining 40%). The AV node is usually supplied by the RCA (85–90%) or, less frequently, by the CX (10–15%); the bundle of His has a dual blood supply derived from the PDA and LAD. The anterior papillary muscle of the mitral valve also has a dual blood supply that is fed by diagonal branches of the LAD and marginal branches of the CX. In contrast, the posterior papillary of the mitral valve is usually supplied only by the PDA and is therefore much more vulnerable to ischemic dysfunction.
2. Determinants of Coronary Perfusion
Coronary perfusion is unique in that it is intermittent rather than continuous, as it is in other organs. During contraction, intramyocardial pressures in the left ventricle approach systemic arterial pressure. The force of left ventricular contraction almost completely occludes the intramyocardial part of the coronary arteries. Coronary perfusion pressure is usually determined by the difference between aortic pressure and ventricular pressure. The left ventricle is perfused almost entirely during diastole. In contrast, the right ventricle is perfused during both systole and diastole. Moreover, as a determinant of left heart myocardial blood flow, arterial diastolic pressure is more important than MAP. Therefore, left coronary artery perfusion pressure is determined by the difference between arterial diastolic pressure and left ventricular end-diastolic pressure (LVEDP).
Coronary perfusion pressure = Arterial diastolic pressure – LVEDP
Because the endocardium is subjected to the greatest intramural pressures during systole, it tends to be most vulnerable to ischemia during decreases in coronary perfusion pressure.
3. Myocardial Oxygen Balance
Myocardial oxygen demand is usually the most important determinant of myocardial blood flow. The myocardium usually extracts ...