C H A P T E R 3 2
Cholinergic agonists
491
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Cholinergic agonists stimulate the parasympathetic
nerves, some nerves in the brain and the
neuromuscular junction at the same site that
ACh does.
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Cholinergic agonists are used topically in the eye
to produce miosis (pupillary constriction) and treat
glaucoma.
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Systemically, these agents are used to increase bladder
tone (e.g. postoperative or postpartum) and to
increase secretions to relieve dry mouth associated
with Sjögren’s syndrome.
INDIRECT-ACTING CHOLINERGIC
AGONISTS
The indirect-acting cholinergic agonists do not react
directly with ACh receptor sites; instead, they react
chemically with acetylcholinesterase (the enzyme
responsible for the breakdown of ACh) in the synaptic
cleft to prevent it from breaking down ACh. As a result,
the ACh that is released from the presynaptic nerve
remains in the area and accumulates, stimulating the
ACh receptors for a longer period of time than normally
expected. These drugs work at all ACh receptors, in the
parasympathetic nervous system, in the central nervous
system (CNS) and at the neuromuscular junction. Most
of these drugs bind reversibly to acetylcholinesterase, so
their effects pass with time when the acetylcholinesterase
is released and allowed to break down ACh. However,
there are certain indirect-acting cholinergic agonists that
irreversibly bind to acetylcholinesterase. These drugs are
not used therapeutically; they are being developed as
nerve gas to be used as weapons (Box 32.2). Because
these drugs might be encountered in a war situation,
it is important to have an antidote readily available
to military personnel and any civilian who might be
affected. Pralidoxime is the antidote developed for the
irreversible indirect-acting cholinergic agonists and is
also used to reverse poisoning associated with organo-
phosphate pesticides (Box 32.3).
The reversible indirect-acting cholinergic agonists
fall into two main categories: (1) agents used to
treat myasthenia gravis; and (2) agents used to treat
Alzheimer’s disease.
KEY POINTS
to determine the effectiveness of therapy and
evaluate for any potential adverse effects.
■
■
Assess vital signs, including pulse and blood
pressure, and cardiopulmonary status,
including heart and lung sounds,
to evaluate
for changes related to cardiovascular effects
of parasympathetic activity
; obtain an
electrocardiogram (ECG) as indicated
to evaluate
heart rate and rhythm.
■
■
Assess abdomen,
auscultating for bowel sounds
;
palpate bladder
for distension.
■
■
Monitor intake and output, noting any complaints
of urinary urgency,
to monitor for drug effects on
the urinary system.
Implementation with rationale
■
■
Ensure proper administration of ophthalmic
preparations
to increase the effectiveness of
drug therapy and minimise the risk of systemic
absorption.
■
■
Administer oral drug on an empty stomach
to decrease nausea and vomiting.
■
■
Monitor response closely, including blood pressure,
ECG, urine output and cardiac output,
and
arrange to adjust dose accordingly to ensure the
most benefit with the least amount of toxicity.
Maintain a cholinergic blocking drug on standby
such as atropine
to use as an antidote for excessive
doses of cholinergic drugs
(see Focus on safe
medication administration in discussion of Agents
for myasthenia gravis in this chapter)
to reverse
overdose or counteract severe reactions
(see
Chapter 33 for further discussion of atropine).
■
■
Provide safety precautions if the person reports
poor visual acuity in dim light
to prevent injury.
■
■
Monitor urinary output
to evaluate effects on the
bladder
;
ensure ready access to bathroom facilities
as needed with GI stimulation.
■
■
Provide thorough teaching, including drug
name, dosage and schedule of administration;
administration of oral forms before meals
or without food; proper administration for
ophthalmic preparations as indicated; measures
to prevent or minimise adverse effects; need for
readily available access to toileting facilities;
warning signs of problems; and importance of
follow-up and evaluation.
Evaluation
■
■
Monitor response to the drug (improvement in
bladder function, increased salivation, miosis).
■
■
Monitor for adverse effects (cardiovascular
changes, GI stimulation, urinary urgency,
respiratory distress).
■
■
Evaluate the effectiveness of the teaching plan
(person can name drug, dosage, adverse effects to
watch for and specific measures to avoid them, and
proper administration of ophthalmic drugs).
■
■
Monitor the effectiveness of comfort and safety
measures and compliance with the regimen.
