The heart has the highest metabolic demands when compared to other organs. The vast majority of the energy substrate utilization is expended during unrelenting periodic contraction of the myocardium. Blood flow is delivered at a rate of 1 mL per gram of cardiac tissue per minute at rest. Increases in myocardial oxygen consumption, via adenosine diphosphate and adenosine-mediated arteriolar vasodilation, can result in a reciprocal increase in blood flow, up to five times normal. This increased blood flow is accommodated by recruitment of an extensive capillary bed within the myocardium, with nearly one capillary per myocyte. Between 70% and 80% of available oxygen is extracted from coronary blood flow at rest. Thus, the metabolic needs of the heart are tightly coupled to the availability of coronary blood flow, since additional extraction is limited. In addition, blood flow through the left ventricular epicardial arteries is phasic. During myocardial contraction, extravascular compression of intramyocardial capillaries prevents forward flow during systole, limiting flow to the diastolic phase of the cardiac cycle. This is even more pronounced in the subendocardial region where myocardial oxygen demands are greatest as a result of increased wall tension and greater sarcomere shortening. Because of the elevated and insistent myocardial oxygen consumption, the restriction of blood flow to diastole, and the high basal level of oxygen consumption, the heart is particularly susceptible to ischemic injury related to stenosis of the epicardial coronary arteries.
The heart receives its blood supply from the left and right coronary arteries (Figure 20–1). These epicardial vessels originate as the first branches off of the aortic root, in their respective sinuses of Valsalva. The coronary circulation is traditionally divided into three territories or regions: the left anterior descending and the circumflex (arising from the left coronary artery) and the right (from the right coronary artery). The dominance of the heart refers to which major artery terminates as the posterior descending branch. Ninety percent of individuals are right dominant, as the right coronary artery supplies the posterior descending artery. The remaining 10% are left dominant, as the terminal branch of the circumflex artery supplies the posterior descending artery.
Anatomy of the coronary circulation. AV, atrioventricular; SA, sinoatrial.
The left coronary artery is referred to as the left main coronary artery. After its origin in the left sinus of Valsalva, it courses between the left atrial appendage and the pulmonary artery. The left main coronary artery varies in length but is typically less than 2 cm long. It terminates in two branches: the left anterior descending (LAD) and the circumflex coronary arteries. In less than 1% of patients, the left main artery is absent, with the LAD and circumflex originating as separate ostia from the left sinus of Valsalva.