Webster's New World Medical Dictionary

heart, artificial
A human-made heart that is
used to replace a diseased or malfunctioning heart
when a donor organ is not available.
heart, left
The left atrium and left ventricle.
heart, right
The right atrium and right ventricle.
headache, rebound
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heart attack
A sudden blockage of a coronary
artery. Not infrequently, this leads to the death of
part of the heart muscle due to its loss of blood sup-
ply. Typically, the loss of blood supply is caused by a
complete blockage of a coronary artery by a blood
clot. The interruption of blood flow is usually
caused by arteriosclerosis, with narrowing of the
coronary arteries, the culminating event being a
thrombosis (clot). Death of the heart muscle often
causes chest pain and electrical instability of the
heart muscle tissue. Electrical instability of the heart
may cause ventricular fibrillation (chaotic electrical
disturbance), resulting in the inability of the heart to
deliver oxygenated blood to the body. Permanent
brain damage and death can result from heart
attack unless oxygenated blood flow is restored
within 5 minutes. Heart attack deaths can be
avoided if a bystander starts CPR (cardiopulmonary
resuscitation) within 5 minutes of the onset of ven-
tricular fibrillation. When paramedics arrive, med-
ications and/or electrical shock (cardioversion) to
the heart can be administered to convert ventricular
fibrillation to a normal heart rhythm. Therefore,
prompt CPR and rapid paramedic response can
improve the survival chances after a heart attack.
Also known as a myocardial infarction (MI). See
also 
cardiac arrest.
heart block
A blockage in the conduction of the
normal electrical impulses in the heart. Heart block
is not uncommon and is detected with an electro-
cardiogram. Heart block occurs from degeneration
or scarring of the electrical pathways in the heart
muscle, either naturally or as a result of disease.
Heart block typically requires no treatment, but can
be a factor in the decision of whether or not to put
a pacemaker in a heart that is failing or irregularly
beating.