Bma Illustrated Medical Dictionary

mites, and ticks. (See also insects and dis-
ease; mites and disease; ticks and disease).
ARCAn abbreviation for AIDS-related
complex. (See also AIDS.)
arcus senilisA grey-white ring near the
edge of the corneaoverlying 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 hyperlipidaemia).
areolaThe pigmented circular area sur-
rounding the nipple. The term is also
used to describe an inflamed area
around a pimple (see pustule).
aromatherapyA form of complemen-
tary medicinethat 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.
arousalThe 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.
arrhenoblastomaA 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, cardiacAn 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 fibrillationand pro-

duces totally irregular ventricular beats.

Ventricular fibrillation is a form of car-

diac arrestin which the ventricles twitch

very rapidly in a disorganized manner.

Sinus bradycardiais 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 syndrome).

A common cause of arrhythmia is

coronary artery disease, particularly

after myocardial infarction. Some tachy-

cardias are due to a congenitaldefect in

the heart's conducting system. Caffeine

can cause tachycardia in some people.

Amitriptylineand some other antidepres-

sant drugscan cause serious arrhythmias

if they are taken in high doses.

An arrhythmia may be felt as palpita-

tions, but in some cases arrhythmias

ARC ARRHYTHMIA, CARDIAC

A

ARRHYTHMIA, CARDIAC

Sinoatrial node (pacemaker)

Electrical pathway

to left atrium

Atrioventricular

node

Ventricles

Normal

electrical

pathway

Abnormal

electrical

pathway

SUPRAVENTRICULAR TACHYCARDIA

Atria