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 1 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 19–1). 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
Anatomy of the coronary circulation.
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 2 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.