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P A R T 8
Drugs acting on the cardiovascular system
V
asodilators
If other drug therapies do not achieve the desired reduc-
tion in blood pressure, it is sometimes necessary to
use a direct vasodilator. Most of the vasodilators are
reserved for use in severe hypertension or hypertensive
emergencies. These include diazoxide (generic), hydrala-
zine (
Alphapress, Apresoline
), minoxidil (
Loniten
) and
sodium nitroprusside (generic).
Therapeutic actions and indications
The vasodilators act directly on vascular smooth muscle
to cause muscle relaxation, leading to vasodilation and
drop in blood pressure. They do not block the reflex
tachycardia that occurs when blood pressure drops.
They are indicated for the treatment of severe hyper
tension that has not responded to other therapy (see
Table 43.2).
Pharmacokinetics
Diazoxide and sodium nitroprusside are used intra
venously; hydralazine is available for oral and
intravenous use; and minoxidil is available as an oral
agent only. These drugs are rapidly absorbed and widely
distributed. They are metabolised in the liver and prim
arily excreted in urine. They cross the placenta and enter
breast milk (see Contraindications and cautions).
Contraindications and cautions
The vasodilators are contraindicated in the presence
of known allergy to the drug
to prevent hypersensi-
tivity reactions
and with any condition
that could be
exacerbated by a sudden fall in blood pressure
, such as
cerebral insufficiency. Caution should be used in people
with peripheral vascular disease, CAD, heart failure or
tachycardia,
all of which could be exacerbated by the
fall in blood pressure.
Diazoxide must be used with
extreme caution in people with functional hypogly
caemia
because this drug increases blood glucose levels
by blocking insulin release
.
These drugs are also contraindicated with preg-
nancy unless the benefit to the mother clearly outweighs
the potential risk
because of the potential for adverse
effects on the fetus or neonate.
If they are needed by
a breastfeeding woman, another method of feeding the
baby should be selected,
because of the potential for
adverse effects on the baby.
Adverse effects
The adverse effects most frequently seen with these
drugs are related to the changes in blood pressure. These
include dizziness, anxiety and headache; reflex tachy-
cardia, heart failure, chest pain, oedema; skin rash and
lesions (abnormal hair growth with minoxidil); and GI
upset, nausea and vomiting. Cyanide toxicity (dyspnoea,
headache, vomiting, dizziness, ataxia, loss of conscious-
ness, imperceptible pulse, absent reflexes, dilated pupils,
pink colour, distant heart sounds and shallow breath-
ing) may occur with nitroprusside, which is metabolised
to cyanide and also suppresses iodine uptake and can
cause hypothyroidism.
Clinically important drug–drug interactions
Each of these drugs works differently in the body, so
each drug should be checked for potential drug–drug
interactions before use.
• Be extremely careful in any situation that might
lead to a drop in blood pressure (e.g. excessive
sweating, vomiting, diarrhoea, dehydration). If you
experience light-headedness or dizziness in any of
these situations, consult your healthcare provider
immediately.
• Keep this drug, and all medications, out of the reach
of children.
Prototype summary: Sodium nitroprusside
Indications:
Severe hypertension, maintenance of
controlled hypotension during anaesthesia, acute
heart failure.
Actions:
Acts directly on vascular smooth muscle to
cause vasodilation and drop of blood pressure; does
not inhibit cardiovascular reflexes and tachycardia;
renin release will occur.
Pharmacokinetics:
Route Onset
Peak
Duration
IV 1–2 mins
Rapid
1–10 mins
T
1/2
:
2 mins; metabolised in the liver and excreted in
urine.
Adverse effects:
Apprehension, headache, retrosternal
pressure, palpitations, cyanide toxicity, diaphoresis,
nausea, vomiting, abdominal pain, irritation at the
injection site.
Care considerations for
people receiving vasodilators
Assessment: History and examination
■
■
Assess for the following conditions,
which
could be cautions or contraindications to use
