Facts on File Encyclopedia of Health and Medicine

arteries that trail across the left ventricle. The right
coronary artery traverses the right atrium, nour-
ishing the upper right heart. Numerous smaller
arteries branch from it, the largest of which is the
right marginal artery. The arterial network inter-
twines across the front of the heart in a network
called Vieussens’s ring, which is similar in struc-
ture and purpose to theCIRCLE OFWILLISat the
base of the BRAIN. The precise pattern of the coro-
nary arteries is unique for each person, however.
The coronary arteries deliver the largest vol-
ume of blood to the heart during diastole, the
trough phase of the CARDIAC CYCLEduring which
the ventricles fill with blood, pulling blood from
the aortic root as systole draws to a close. About
60 percent of the ventricular ejection goes to the
coronary arteries. During systole, the peak of the
cardiac cycle during which the ventricles pump
blood out, the dynamic forces of the heart’s con-
traction cause the coronary arteries to constrict.
The extensive branching of the coronary arter-
ies provides a fairly substantial level of redun-
dancy for supplying the heart with blood. Even if
damage or occlusion blocks one branch or several
branches, other arterial branches can deliver blood
to the same or nearby areas to cover the deficit. As
well, the heart tends to develop collateral circula-
tion in response to arterial damage, a self-repair
feature in which new small arteries sprout to
extend around the area of damage. These mecha-
nisms can sustain adequate myocardial perfusion
(distribution of blood throughout the heart
MUSCLE) for a considerable time, even in the face
of significant damage to the heart’s circulatory
structures. As occlusion of the coronary arteries
progresses, the collateral circulation of Vieussens’s
ring extends.
The efficiency of coronary circulation is such
that symptoms of oxygen deficiency (notably
ANGINA PECTORISand shortness of breath with exer-
tion) do not become apparent until damage to the
coronary arteries drops blood flow to about 30
percent of normal. At this point a cardiologist may
recommend ANGIOPLASTY with STENT placement.
The standard diagnostic point for the more inva-
sive CORONARY ARTERY BYPASS GRAFT(CABG) is 90 per-
cent or greater occlusion. The symptoms of
restricted blood flow become most apparent when
occlusion affects the larger branches of the coro-

nary arteries, notably the LAD and circumflex.

The primary conditions that affect the coronary

arteries are ATHEROSCLEROSIS, called CORONARY

ARTERY DISEASE(CAD) when it involves the coronary

arteries, and coronary artery spasm, which often

results from CAD though may have other causes.

For further discussion of the coronary arteries

within the context of cardiovascular structure and

function, please see the overview section “The

Cardiovascular System.”

See also COCAINE; HEART ATTACK; LIFESTYLE AND

CARDIOVASCULAR HEALTH.

coronary artery bypass graft (CABG) A surgical

OPERATIONto replace diseased CORONARY ARTERIES

supplying the HEART with BLOOD. Cardiovascular

surgeons began performing CABG to treat severe

CORONARY ARTERY DISEASE(CAD), typically following

HEART ATTACK, in the 1960s. The operation became

feasible with refinements in CARDIOPULMONARY

BYPASStechnology and surgical technique. In the

ensuing decades CABG has become one of the

most frequently performed operations in the

United States, with surgeons performing more

than 300,000 a year. The surgeon may use CABG

to replace one to five coronary arteries; three or

four is most common (triple or quadruple bypass).

The most frequently bypassed coronary arteries

are the left anterior descending (LAD), which

traverses the front of the heart; the circumflex,

which wraps around the heart; and their respec-

tive branches.

Weighing the Benefits and Risks

Whether CABG is an appropriate treatment choice

depends on numerous variables that include the

person’s age and general health status, degree and

extent of occlusion in the coronary arteries, and

the presence of other CARDIOVASCULAR DISEASE

(CVD). Variables that strongly influence the proce-

dure’s success include lifestyle factors such as ciga-

rette smoking, body fat and weight, physical

inactivity, and dietary habits. Other health condi-

tions that affect HEALING, such as DIABETES, are also

important considerations, as are conditions affect-

ing the LUNGSsuch as CHRONIC OBSTRUCTIVE PUL-

MONARY DISEASE(COPD).

Now that several million Americans have had

CABG and researchers have accumulated data

coronary artery bypass graft (CABG) 41