C H A P T E R 3 0
Adrenergic agonists
463
forms for use to control hypertension and as an injec-
tion for epidural infusion to control pain in people with
cancer. Because of its centrally acting effects, clonidine
is associated with many more CNS effects (bad dreams,
sedation, drowsiness, fatigue, headache) than other sym-
pathomimetics. It can also cause extreme hypotension,
heart failure and bradycardia due to its decreased effects
of the sympathetic outflow from the CNS.
Pharmacokinetics
These drugs are generally well absorbed and reach
peak levels in a short period—20 to 45 minutes. They
are widely distributed in the body, metabolised in the
liver and primarily excreted in the urine. The trans
dermal form of clonidine is slow release and has a 7-day
duration of effects, so it only needs to be replaced once a
week. Phenylephrine can be given intramuscularly (IM),
subcutaneously (SC), IV, orally and as a nasal or an oph-
thalmic solution.
Contraindications and cautions
The
a
-specific adrenergic agonists are contraindicated in
the presence of allergy to the specific drug
to avoid hyper-
sensitivity reactions
; severe hypertension or tachycardia
because of possible additive effects
; and narrow-angle
glaucoma,
which could be exacerbated by arterial con-
striction.
There are no adequate studies about use during
pregnancy and breastfeeding, so use should be reserved
for situations in which the benefit to the mother out-
weighs any potential risk to the fetus or neonate.
They should be used with caution in the presence
of cardiovascular disease or vasomotor spasm
because
these conditions could be aggravated by the vascular
effects of the drug
; thyrotoxicosis or diabetes
because
of the thyroid-stimulating and glucose-elevating effects
of sympathetic stimulation
; or renal or hepatic impair-
ment,
which could interfere with metabolism and
excretion of the drug.
Adverse effects
People receiving these drugs often experience adverse
effects that are extensions of the therapeutic effects
or other sympathetic stimulatory reactions. CNS
effects include feelings of anxiety, restlessness, depres-
sion, fatigue, strange dreams and personality changes.
Blurred vision and sensitivity to light may occur because
of the pupil dilation that occurs when the sympathetic
system is stimulated. Cardiovascular effects can include
arrhythmias, ECG changes, blood pressure changes and
peripheral vascular problems. Nausea, vomiting and
anorexia can occur, related to the depressant effects
of the SNS on the GI tract. Genitourinary effects can
include decreased urinary output, difficulty urinat-
ing, dysuria and changes in sexual function related to
the sympathetic stimulation of these systems. These
drugs should not be stopped suddenly; adrenergic
receptors will be very sensitive to catecholamines and
sudden withdrawal can lead to tachycardia, hyperten-
sion, arrhythmias, flushing and even death. Avoid these
effects by tapering the drug over 2 to 4 days when it is
being discontinued. As with other sympathomimetics,
if phenylephrine is given IV, care should be taken to
avoid extravasation. The vasoconstricting effects of the
drug can lead to necrosis and cell death in the area of
extravasation.
Clinically important drug–drug interactions
Phenylephrine combined with MAOIs can cause severe
hypertension, headache and hyperpyrexia; this combi-
nation should be avoided. Increased sympathomimetic
effects occur when phenylephrine is combined with tri-
cyclic antidepressants (TCAs); if this combination must
be used, the person should be monitored very closely.
Clonidine has a decreased antihypertensive effect if
taken with TCAs and a paradoxical hypertension occurs
if it is combined with propranolol. If these combinations
are used, the person’s response should be monitored
closely and dose adjustment made as needed.
Any adrenergic agonist will lose effectiveness if
combined with any adrenergic antagonist. Monitor the
person’s drug regimen for appropriate use of the drugs.
TABLE 30.2
DRUGS IN FOCUS Alpha-specific adrenergic agonists
Drug name
Dosage/route
Usual indications
clonidine (Catapres)
75 mcg PO b.d. or t.d.s to maximum
900 mcg/day; 150–300 mcg IM or IV in
10 mL NaCl over 5 minutes
Treatment of essential hypertension;
chronic pain; to ease opiate withdrawal;
used only for adults
phenylephrine
(Abalon,
Neo-Synephrine)
1–10 mg SC or IV or 0.1–0.5 mg IV as a starting
dose; 0.5 mg IV by rapid injection to convert
tachycardias; 1–2 gtt in affected eye(s) for
glaucoma
Cold and allergies; shock and shock-like
states; supraventricular tachycardias;
glaucoma; allergic rhinitis; otitis media
