Webster's New World Medical Dictionary

arm, wrist, and hand bones
See 
bones of the
arm, wrist, and hand.
armed tapeworm
See 
Taenia solium.
Arnold Chiari malformation
See 
Chiari mal-
formation.
aromatherapy
A form of alternative medicine in
which essential oils or other scents are inhaled to
achieve therapeutic benefit. The mechanism of
action in aromatherapy is unknown, but recent
studies have shown that aromatherapy may be ben-
eficial for some health problems.
arrector pili
A microscopic band of muscle tis-
sue that connects a hair follicle to the dermis. When
stimulated, the arrector pili contracts and causes
the hair to become more perpendicular to the skin
surface, thereby erecting the hair (causing the hair
to stand on end). The arrector pili muscle plays a
key role in forming goose bumps. See also 
goose
bumps.
arrhythmia
An abnormal heart rhythm. With an
arrhythmia, the heartbeats may be irregular or too
slow (bradycardia), too rapid (tachycardia), or too
early. When a single heartbeat occurs earlier than
normal, it is called a premature contraction. See
also 
bradycardia; tachycardia.
arrhythmia, atrial
An abnormal heart rhythm
due to electrical disturbances in the upper cham-
bers of the heart (atria) or the atrioventricular (AV)
node “relay station,” leading to fast heart beats.
Examples of atrial arrhythmias include atrial fibril-
lation, atrial flutter, and paroxysmal atrial tachycar-
dia.
arrhythmia, ventricular
An abnormally rapid
heart rhythm that originates in the lower chambers
of the heart (ventricles). Ventricular arrhythmias
include ventricular tachycardia and ventricular fib-
rillation. Both are life-threatening arrhythmias, and
they are most commonly associated with heart
attacks or scarring of the heart muscle from previ-
ous heart attacks.