and sometimes skin for grafting. (See
also artificial insemination; blood dona-
tion; bone marrow transplant; organ
donation;transplant surgery.)
dopa-decarboxylase inhibitorsDrugs
used in the treatment of Parkinson's dis-
ease. The 2 main dopa-decarboxylase
inhibitors, co-beneldopa and co-carel-
dopa, are a combination of levodopaand
benserazide and levodopa and carbidopa
respectively. These drugs prevent levo-
dopa from being activated except within
the brain, which reduces the incidence of
side effects such as nausea and vomiting.
dopamineA neurotransmitterfound in
the brain and around some blood ves-
sels. It helps control body movements:
a deficiency of dopamine in the basal
ganglia(groups of nerve cells deep in
the brain) causes Parkinson’s disease.
Synthetic dopamine is injected as an
emergency treatment for shock caused
by a myocardial infarction(heart attack)
or septicaemia(blood infection) and as
a treatment for severe heart failure.
Doppler effectA change in the fre-
quency with which sound waves from a
given source reach an observer when the
source is in rapid motion with respect to
the observer. Approaching sounds appear
higher in pitch (frequency) than sounds
that are moving away. This is because
the wavelengths of the sound from an
approaching source are progressively
foreshortened, whereas the wavelengths
from a receding source are stretched. The
Doppler effect is used in ultrasound scan-
ningtechniques. An emitter sends out
pulses of ultrasound (inaudible high-
frequency sound) of a specific frequency.
When these pulses bounce off a moving
object (blood flowing through a blood
vessel, for example), the frequency of the
echoes is changed from that of the emit-
ted sound. A sensor detects the frequency
changes and converts the data into use-
ful information (about how fast the
blood flows, for example). Dopplerultra-
sound techniques are also used to
monitor fetal heartbeat, to detect air bub-
bles in dialysisand heart–lung machines,
and to measure blood pressure.
dorsalRelating to the back, located on or
near the back, or describing the upper-
most part of a body structure when aperson is lying face-down. The opposite
of dorsal is ventral.
doseA term used to refer to the amount
of a drug taken at a particular time, or
to the amount of radiation an individual
is exposed to during a session of radio-
therapy. Drug dose can be expressed in
terms of the weight of its active sub-
stance, the volume of liquid to be
drunk, or its effects on body tissues.
The amount of radiation absorbed by
body tissues during a session of radio-
therapy is expressed in units called
millisieverts (see radiation units).
dosulepinA tricyclic antidepressant drug
used in the treatment of depression.
The drug has a sedative action and is
particularly useful in cases of depression
accompanied by anxietyor insomnia.
Possible adverse effects include blurred
vision, dizziness, flushing, and rash.
dothiepinAnother name for dosulepin.
double-blindA type of controlled trial
that tests the effectiveness of a treatment
or compares the benefits of different
treatments. In double-blind trials, nei-
ther the patients nor the doctors
assessing the treatments know which
patients are receiving which treatment.
This eliminates any expectations about
which treatment will be most effective.
double vision Also known as diplopia,
the seeing of 2 instead of 1 visual image
of a single object. It is usually a symp-
tom of a squint, especially of paralytic
squint, in which paralysis of 1 or more
of the eye muscles impairs eye move-
ment. Other causes includea tumour in
the eyelid or a tumour or blood clot
behind the eye. Double vision can also
occur in exophthalmos, when the eye-
balls protrude because of an underlying
hormonal disorder. A child with squint
needs treatment to prevent amblyopia
(lazy eye). In adults double vision needs
immediate investigation.
doucheThe introduction of water and/or
a cleansing agent into the vagina using a
bag and tubing with a nozzle. Douching
is rarely recommended nowadays. It is
unnecessary for purposes of hygiene
and is an ineffective form of contracep-
tion. It may also introduce infection or
spread an existing vaginal infection into
the uterus or fallopian tubes.DOPA-DECARBOXYLASE INHIBITORS DOUCHED