McKenna's Pharmacology for Nursing, 2e

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P A R T 5  Drugs acting on the autonomic nervous system

The drugs carbachol and pilocarpine are available as ophthalmic agents. They are used to induce miosis , or pupil constriction; to relieve the increased intraocular pressure of glaucoma; and to allow surgeons to perform certain surgical procedures. Pilocarpine, which binds to muscarinic receptors throughout the system, is used to increase secretions in the mouth and GI tract and relieve the symptoms of dry mouth that are in seen in Sjögren’s syndrome. It is approved for use in adults and is given three times a day, often with meals. Pharmacokinetics The direct-acting cholinergic agonists are generally well absorbed after oral administration and have relatively short half-lives, ranging from 1 to 6 hours. The meta­ bolism and excretion of these drugs is not known but is believed to occur at the synaptic level using normal processes similar to the way that ACh is handled. Drugs used topically are not generally absorbed systemically. Contraindications and cautions These drugs are used sparingly because of the potential undesirable systemic effects of parasympathetic stimu- lation. They are contraindicated with hypersensitivity to any component of the drug to avoid hypersensitiv- ity reaction and in the presence of any condition that would be exacerbated by parasympathetic effects, such as bradycardia, hypotension, vasomotor instability and coronary artery disease, which could be made worse by the cardiac- and cardiovascular-suppressing effects of the parasympathetic system. Peptic ulcer, intestinal obstruction or recent GI surgery could be negatively affected by the GI stimulating effects of the parasym- pathetic nervous system. Asthma could be exacerbated by the increased parasympathetic effect, overriding the protective sympathetic bronchodilation. Bladder obstruction or impaired healing of sites from recent bladder surgery could be aggravated by the stimulatory effects on the bladder. Epilepsy and parkinsonism could be affected by the stimulation of ACh receptors in the brain. Caution should be used during pregnancy and breastfeeding because of the potential adverse effects on the fetus or neonate. Adverse effects Individuals should be cautioned about the potential adverse effects of these drugs. Even if the drug is being given as a topical ophthalmic agent, there is always a possibility that it can be absorbed systemically. The adverse effects associated with these drugs are related to parasympathetic nervous system stimulation. Cardio­ vascular effects can include bradycardia, heart block, hypotension and even cardiac arrest related to the

cardiac-suppressing effects of the parasympathetic nervous system. GI effects can include nausea, vomiting, cramps, diarrhoea, increased salivation and involuntary defaecation related to the increase in GI secretions and activity. Dehydration is possible due to the increase in GI motility and resultant diarrhoea. Urinary tract effects can include a sense of urgency related to the stimulation of the bladder muscles and sphincter relaxation. Other effects may include flushing and increased sweating sec- ondary to stimulation of the cholinergic receptors in the sympathetic nervous system. Clinically important drug–drug interactions There is an increased risk of cholinergic effects if these drugs are combined or given with acetylcholinesterase inhibitors, such as neostigmine or tacrine. The person should be monitored and appropriate dose adjustments made. Prototype summary: Bethanechol Indications: Acute postoperative or postpartum non-obstructive urinary retention; neurogenic atony of the bladder with retention. Actions: Acts directly on cholinergic receptors to mimic the effects of ACh; increases tone of detrusor muscles and causes emptying of the bladder. Pharmacokinetics: Route Onset Peak Duration Oral 30–90 mins 60–90 mins 1–6 hours T 1/2 : Metabolism and excretion unknown; thought to be synaptic. Adverse effects: Abdominal discomfort, salivation, nausea, vomiting, sweating, flushing.

Care considerations for people receiving direct-acting cholinergic agonists

Assessment: History and examination

■ ■ Assess for contraindications or cautions: known allergies to these drugs to avoid hypersensitivity reactions ; bradycardia, vasomotor instability, peptic ulcer, obstructive urinary or GI diseases; recent GI or genitourinary surgery; asthma; parkinsonism or epilepsy, which could be exacerbated or complicated by parasympathetic stimulation ; and current status of pregnancy and breastfeeding because of the potential for adverse effects to the fetus or neonate. ■ ■ Perform a physical assessment to establish a baseline status before beginning therapy,