464
P A R T 5
Drugs acting on the autonomic nervous system
Prototype summary: Phenylephrine
Indications:
Treatment of vascular failure in
shock or drug-induced hypotension; to overcome
paroxysmal supraventricular tachycardia; to
prolong spinal anaesthesia; as a vasoconstrictor in
regional anaesthesia; to maintain blood pressure
during anaesthesia; topically for symptomatic relief
of nasal congestion and as adjunctive therapy in
middle ear infections; ophthalmically to dilate
pupils and as a decongestant to provide temporary
relief of eye irritation.
Actions:
Powerful postsynaptic alpha-adrenergic
receptor stimulant causing vasoconstriction and
raising systolic and diastolic blood pressure with
little effect on the beta-receptors in the heart.
Pharmacokinetics:
Route
Onset
Duration
IV
Immediate
15–20 mins
IM, SC 10–15 mins
30–120 mins
Topically Very little systemic
absorption occurs
T
1/2
:
47 to 100 hours; metabolised in the tissues and
liver; excreted in urine and bile.
Adverse effects:
Fear, anxiety, restlessness, headache,
nausea, decreased urine formation, pallor.
Care considerations for people receiving
α
-specific adrenergic agonists
Assessment: History and examination
■
■
Assess for contraindications or cautions:
any known allergies to the drug
to avoid
hypersensitivity reactions
; presence of any
cardiovascular diseases,
which could be
exacerbated by the vascular effects of these drugs;
thyrotoxicosis or diabetes,
which would lead
to an increase in thyroid stimulation or glucose
elevation
; chronic renal failure,
which could
be exacerbated by drug use
; renal or hepatic
impairment,
which could interfere with drug
excretion or metabolism
;
and current status of
pregnancy and breastfeeding.
■
■
Perform a physical assessment to establish
baseline
status before beginning therapy to determine
effectiveness and during therapy to evaluate for
any potential adverse effects.
■
■
Assess level of orientation, affect, reflexes and
vision
to monitor for CNS changes related to drug
therapy.
■
■
Monitor blood pressure and pulse, assess
peripheral perfusion and obtain ECG, if indicated,
to determine drug effectiveness and evaluate for
adverse cardiovascular effects.
■
■
Assess urinary output
to evaluate renal function
and monitor for adverse effects of the drug.
■
■
Evaluate person for nausea and constipation
to
assess adverse effects of the drug and establish
appropriate interventions.
■
■
Monitor laboratory test results, such as renal and
liver function tests,
to determine drug effects on
renal and hepatic systems.
Implementation with rationale
■
■
Do not discontinue the drug abruptly
because
sudden withdrawal can result in rebound
hypertension, arrhythmias, flushing and even
hypertensive encephalopathy and death
; taper
drug over 2 to 4 days.
■
■
Do not discontinue the drug before surgery;
document on the person’s chart and monitor blood
pressure carefully during surgery.
Sympathetic
stimulation may alter the normal response to
anaesthesia, as well as recovery from anaesthesia.
■
■
Monitor blood pressure, orthostatic blood
pressure, pulse, rhythm and cardiac output
regularly, even with ophthalmic preparations,
to adjust dose or discontinue the drug if
cardiovascular effects are severe.
■
■
When giving phenylephrine intravenously, ensure
that an
α
-blocking agent is readily available to
counteract the effects
in case severe reaction
occurs
; infiltrate any area of extravasation with
phentolamine within 12 hours after extravasation
to preserve tissue.
■
■
Arrange for supportive care and comfort
measures, including rest and environmental
control,
to decrease CNS irritation
; analgesics for
headache
to relieve discomfort
; safety measures,
such as use of side rails and assistance with
ambulation if CNS effects occur,
to protect the
person from injury
; and protective measures
if
CNS effects are severe.
■
■
Provide thorough teaching about drug name, dose
and schedule for administration; technique for
administration if appropriate; measures to prevent
potential adverse effects such as voiding before
taking the drug and use of bowel training activities
if constipation is a problem; safety measures such
as avoiding driving and operating dangerous
machinery if CNS effects occur, and getting up and
down slowly if orthostatic hypotension is an issue;
warning signs of problems; and importance of
monitoring and follow-up.
