have FIBROSIS (scar tissue). Treatment targets
relieving arrhythmias and HEART FAILURE, and may
include IMMUNOSUPPRESSIVE THERAPY. HEART TRANS-
PLANTATIONmay become an option for end-stage
heart failure.
See also AUTOIMMUNE DISORDERS; BACTERIA; COLDS;
ENDOCARDITIS; PERICARDITIS; VIRUS.
myocardium The MUSCLEtissue that forms the
walls of the HEART. Myocardial cells are unique in
their structure, blending muscle and NERVEstruc-
tures so they can both contract and conduct elec-
trical impulses. Myocardial cells thus can contract
independent of external stimulation. The myocar-
dial fibers of the atria have a different configura-
tion from those of the ventricles. The CORONARY
ARTERIESprovide an extensive network to supply
blood to the myocardium, which requires about
70 percent of the blood’s oxygen. With increasing
age, fibrous and fatty tissue tends to infiltrate the
myocardium, somewhat reducing its effectiveness.
CONDITIONS THAT CAN AFFECT THE MYOCARDIUM
CARDIOMYOPATHY CONGENITAL ANOMALY
ISCHEMIC HEART DISEASE MICROINFARCTION
MYOCARDIAL INFARCTION MYOCARDITIS
CORONARY ARTERY DISEASE(CAD) is the most sig-
nificant threat to the myocardium, as occlusions
in the coronary arteries deprive the myocardium
of blood and thus oxygen. When the consequence
is MYOCARDIAL INFARCTION, CARDIAC ARREST can
result.
For further discussion of the myocardium
within the context of cardiovascular structure and
function, please see the overview section “The
Cardiovascular System.”
See also ENDOCARDIUM; HEART FAILURE; PERI-
CARDIUM.myxoma A nonmalignant tumor that grows in
the HEART, nearly always in one of the atria and
most commonly in the left atrium. Myxoma arises
from the ENDOCARDIUMand may be either firm or
soft in consistency. The tumor can block the flow
of BLOODthrough the atrium, interfere with the
function of the heart valves, or break apart to send
fragments into the blood circulation that cause
embolism (sudden blockage of an ARTERY) else-
where in the body. Soft myxomas are more likely
to fragment; firm myxomas are more likely to be
occlusive. As a myxoma grows it causes increasing
turbulence in the blood as it flows through the
chamber, presenting a significant risk for the for-
mation of blood clots. ECHOCARDIOGRAMgenerally
provides definitive diagnosis. Treatment is OPEN
HEART SURGERY to remove the tumor. Once
removed, the tumor results in no residual conse-
quences. Myxoma is most common in people
between the ages of 30 years and 60 years.
See also ATRIAL FIBRILLATION; TRANSIENT ISCHEMIC
ATTACK.88 The Cardiovascular System