C H A P T E R 3 0
Adrenergic agonists
459
Contraindications and cautions
The
a
- and
b
-agonists are contraindicated in people with
known hypersensitivity to any component of the drug
to prevent hypersensitivity reactions
; phaeochromocy-
toma
because the systemic overload of catecholamines
could be fatal
; with tachyarrhythmias or ventricular
fibrillation
because the increased heart rate and oxygen
consumption usually caused by these drugs could exac-
erbate these conditions
; with hypovolaemia,
for which
fluid replacement would be the treatment for the asso-
ciated hypotension
; and with halogenated hydrocarbon
general anaesthetics (and several industrial solvents),
which sensitise the myocardium to catecholamines and
could cause serious cardiac effects.
Caution should be
used with any kind of peripheral vascular disease (e.g.
atherosclerosis, Raynaud’s disease, diabetic endarteritis),
which could be exacerbated by systemic vasoconstric-
tion.
Because the sympathomimetic drugs stimulate
the SNS, they should be used during pregnancy and
breastfeeding only if the benefits to the mother clearly
outweigh any potential risks to the fetus or neonate.
Adverse effects
The adverse effects associated with the use of
a
- and
b
-adrenergic agonists may be associated with the drugs’
effects on the sympathetic nervous system: arrhythmias,
hypertension, palpitations, angina and dyspnoea related
to the effects on the heart and cardiovascular (CV)
system; nausea, vomiting and constipation related to
the depressant effects on the gastrointestinal (GI) tract;
and headache, sweating, feelings of tension or anxiety
and piloerection related to the sympathetic stimulation.
Because all of these drugs cause vasoconstriction, care
must be taken to avoid extravasation of any infused
drug. The vasoconstriction in the area of extravasation
can lead to cell death in that area.
Clinically important drug–drug interactions
Increased effects of tricyclic antidepressants and mono-
amine oxidase (MAO) inhibitors can occur because of
the increased noradrenaline levels or increased receptor
stimulation that occurs with both drugs. There is an
increased risk of hypertension if
a
- and
b
-adrenergic
agonists are given with any other drugs that cause
hypertension, including herbal therapies and OTC
preparations (Box 30.2). Any adrenergic agonist will lose
effectiveness if combined with any adrenergic antago-
nist. Monitor the person’s drug regimen for appropriate
use of the drugs.
Safe medication administration
Ephedra, a herb that acts like ephedrine, has been in
headlines in recent years because people who were using
the herb to promote weight loss died suddenly. The US Food
and Drug Administration (FDA) has banned ephedra as a drug
and in Australia the TGA imposes a restriction on ephedra
which stipulates the ephedrine concentration from all
ingredients must not exceed 0.001%. Importation to Australia
also requires an import permit-licence. People should be
taught about the potential danger of this product. Any person
who is at risk for serious reactions to the stimulatory effects
of a sympathomimetic—people with narrow-angle glaucoma,
dehydration, cerebral or peripheral vascular disease, cardiac
disease or arrhythmias, hypertension, renal dysfunction,
thyroid disease, diabetes, prostatic disorders, pregnancy or
breastfeeding—should receive direct instruction about the
dangers of this product.
Prototype summary: Dopamine
Indications:
Correction of haemodynamic
imbalances present in shock.
Actions:
Acts directly and by the release of
noradrenaline from sympathetic nerve terminals;
mediates dilation of vessels in the renal and
splanchnic beds to maintain renal perfusion while
stimulating the sympathetic response.
Pharmacokinetics:
Route Onset
Peak
Duration
IV 1–2 mins 10 mins Length of infusion
T
1/2
:
2 minutes; metabolised in the liver, excreted in
the urine.
Adverse effects:
Tachycardia, ectopic beats, anginal
pain, hypotension, dyspnoea, nausea, vomiting,
headache.
People being treated with any adrenergic agonists who
are also taking ma huang, guarana or caffeine are at
increased risk for overstimulation, including increased
blood pressure, stroke and death. Counsel people to
avoid these combinations.
Herbal and alternative therapies
BOX 30.2
Care considerations for people receiving
α
- and
β
-adrenergic agonists
Assessment: History and examination
■
■
Assess for contraindications or cautions:
any known allergies to these drugs
to avoid
hypersensitivity reactions
; phaeochromocytoma,
which could lead to fatal reactions due to systemic
overload of catecholamines
; tachyarrhythmias or
ventricular fibrillation,
which could be exacerbated
