I
Arcus senilis
The arcus senilis is the lighter ring that overlies the
edge ofthe iris (the coloured part of the eye).
Arcus senilis, which is caused by degen-
eration of fatty material in the cornea,
develops gradually during adult life.The
ring never spreads to the centre and
does not affect
eyesight. The
deve-
lopment
of the
condition
in
early
adulthood may be associated with an
abnormally high level of fats in the
blood (see
hyperlipidaemia).
areola
The pigmented circular area surround-
ing the
nipple.
The term is also used to
describe an inflamed area around a
pimple (see
pustule).
Aricept
A brand name for donepezil, an
acetyl-
cholinesterase inhibitor
used
to
treat
Alzheimer’s disease.
aromatherapy
A form of
complementary medicine
that
uses aromatic oils extracted from plants
to treat a wide range of disorders. Prac-
titioners
claim
that aromatherapy is
particularly effective in treating stress-
related and
psychosomatic
conditions.
The oil is applied in small quantities
through
massage;
it
may
also
be
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 reticular
formation in the
brainstem.
arrhenoblastoma
A rare tumour of the ovary that occurs
in young women. The tumour is non-
cancerous,
but
it
secretes
androgen
ARRHYTHMIA, CARDIAC
hormones
(male
sex
hormones)
that
cause
virilization
(development of male
characteristics). Treatment of arrheno-
blastoma is by surgical removal
of
the affected ovary.
arrhythmia, cardiac
An abnormality of the rhythm or the
rate of the
heartbeat
. Arrhythmias are
the result of a disturbance in the elec-
trical impulses within the
heart
(see
Cardiac arrhythmia
box, below). Any iso-
lated irregular beat is known as an
ectopic heartbeat.
Ectopic beats do not
necessarily indicate the presence of an
abnormality, however.
TYPES
Arrhythmias can be divided into two
main groups: tachycardias, in which
the rate of the heartbeat is faster than
normal,
and bradycardias,
in which
the rate is slower. The rhythm may be
regular, with each beat of the atria
(upper chambers, see
atrium
) being fol-
lowed by one beat of the
ventricles
(lower chambers).
Tachycardias
In sinus tachycardia, the
rate is raised, the rhythm is regular, and
the beat originates in the sinoatrial
node
(see
pacemaker). Supraventricular
tachycardia
is faster and the rhythm is
regular. It may be caused by an abnor-
mal electrical pathway that allows an
impulse to circulate continuously in the
heart and take over from the sinoatrial
node. A rapid, irregular beat that origi-
nates in the ventricles is known as a
ventricular tachycardia.
In
atrial flutter,
the
atria beat regularly and very rapidly, but
not every impulse reaches the ven-
tricles, which beat at a slower rate.
Uncoordinated, fast beating of the atria
is known as
atrial fibrillation
and pro-
duces totally irregular ventricular beats.
Ventricular fibrillation
is a form of
cardiac
arrest
in which the heart does not
pump
blood
because
the
ventricles
are twitching very rapidly in a dis-
organized manner.
Bradycardias
Sinus bradycardia
is a slow,
regular beat. In
heart block,
the conduc-
tion of electrical impulses through the
A
CARDIAC ARRHYTHMIA
Any disorder that interferes with the generation or transmission of impulses
through the heart’s electrical conducting system (below) can lead to a disturbance
of cardiac rate or rhythm. These ECG recordings show two kinds of arrhythmia:
sinus bradycardia and atrial fibrillation.
Rightventricle
Sinoatrial node
This is the heart’s natural
pacemakerfrom which
electrical impulses
originate. The impulses
spread over the atria,
causing them
to contract.
Leftventricle
Rightatrium
Leftatrium
Atrioventricular node
Impulsesfrom the
sinoatrial node travel to
this second node, from
which theyspread to the
ventricles, causing
contractions that follow
the atrial beats.
A
w
_
1
.
y \
________«J
L _
Sinus bradycardia
The heart rate is slow, butthe rhythm normal,
with each atrial beat (small rise) followed by
a ventricular beat (spike). Sinus bradycardia
is common in athletes but can also be caused
by hypothyroidism.
Atrial fibrillation
The atria beat rapidly and irregularly.
Ventricular beats (spikes) do not follow each
atrial beat and are irregularlyspaced. This
arrhythmia is common in the elderly and in
people with hyperthyroidism.
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