ARC
ARRHYTHMIA, CARDIAC
mites, and ticks. (See also
insects and dis-
ease; mites and disease; ticks and disease).
ARC An abbreviation for
AIDS-related
complex.
(See also
AIDS.)
arcus senilis A grey-white ring near the
edge of the
cornea
overlying the iris
(the coloured part of the eye). Arcus
senilis is caused by degeneration of
fatty material in the cornea and devel-
ops gradually during adult life. The ring
does not affect eyesight. Development
of the condition in early adult life may
be associated with an abnormality of
fats in the blood (see
tysperlipidaemia).
areola The pigmented circular area sur-
rounding the
nipple.
The term is also
used to describe an inflamed area
around a pimple (see
pustule).
aromatherapy A form of
complemen-
tary medicine
that uses aromatic oils
extracted from plants. The oil is applied
in small quantities through massage; or
it is inhaled, incorporated into creams
or lotions, or, very occasionally, taken
internally. There is no conclusive scien-
tific evidence of the benefits,
arousal The awakening of a person from
unconsciousness or semiconsciousness.
The term is also used to describe any
state of heightened awareness, such as
that caused by sexual stimulation or
fear. Arousal is regulated by the reticu-
lar formation in the
brainstem.
arrhenoblastom a A rare tumour of the
ovary that occurs in young women. The
tumour is noncancerous but secretes
androgen hormones
(male sex hormones)
that cause
virilization
(the development
of male characteristics). Treatment is by
surgical removal of the affected ovary,
arrhythmia, cardiac An abnormality
of the rhythm or rate of the
heartbeat.
Arrhythmias, which are caused by a dis-
turbance in the electrical impulses in
the
heart,
can be divided into 2 main
groups: tachycardias, in which the rate
is faster than normal, and bradycardias,
in which the rate is slower.
In
sinus tachycardia,
the rate is raised,
the rhythm is regular, and the beat
originates in the sinoatrial node (see
pacemaker). Supraventricular tachycar-
dia
is faster and the rhythm is regular. It
may be caused by an abnormal electri-
cal pathway that allows an impulse to
circulate continuously in the heart and
take over from the sinoatrial node.
Rapid, irregular beats that originate in
the ventricles are
called
ventricular
tachycardia.
In
atrial flutter,
the atria
(see
atrium)
beat regularly and very
rapidly, but not every impulse reaches
the ventricles, which beat at a slower
rate. Uncoordinated, fast beating of the
atria is called
atrial fibrillation
and pro-
duces totally irregular ventricular beats.
Ventricular fibrillation
is a form of
car-
diac arrest
in which the ventricles twitch
very rapidly in a disorganized manner.
Sinus bradycardia
is a slow, regular
beat. In
heart block,
the conduction
of
electrical
impulses
through
the
heart muscle is partially or completely
blocked, leading to a slow, irregular
heartbeat. Periods of bradycardia may
alternate with periods of tachycardia
due to a fault in impulse generation
(see
sick sinus s^mdrome).
SUPRAVENTRICULAR TACHYCARDIA
A common cause of arrhythmia is
coronary artery disease,
particularly
after
myocardial infarction.
Some tachy-
cardias are due to a
congenital
defect in
the heart's conducting system.
Caffeine
can cause tachycardia in some people.
Amitriptyline
and some other
antidepres-
sant drugs
can cause serious arrhythmias
if they are taken in high doses.
An arrhythmia may be felt as palpita-
tions, but in some cases arrhythmias
49
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