McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 3 2 Cholinergic agonists

Drug therapy across the lifespan

BOX 32.1

Cholinergic agonists CHILDREN Children may be more susceptible to the adverse effects associated with the cholinergic agonists, including gastrointestinal (GI) upset, diarrhoea, increased salivation that could lead to choking, and loss of bowel and bladder control, a problem that could cause stress in the child. Children should be monitored closely if these agents are used and should receive appropriate supportive care. Bethanechol is approved for the treatment of neurogenic bladder in children older than 8 years of age. Neostigmine and pyridostigmine are used in the control of myasthenia gravis and for reversal of neuromuscular junction blocker effects in children. Care should be taken in determining the appropriate dose based on weight. Edrophonium is used for diagnosis of myasthenia gravis only. ADULTS Adults should be cautioned about the many adverse effects that can be anticipated when using a cholinergic agonist. Flushing, increased sweating, increased salivation and GI upset, and urinary urgency often occur.The person also needs to be aware that dizziness, drowsiness and blurred vision may occur, and that driving and operating dangerous machinery should be avoided.

PREGNANCY AND BREASTFEEDING In general, there are no adequate studies about the effects of these drugs during pregnancy and breastfeeding. Therefore, the cholinergic agonist should be used only in those situations in which the benefit to the mother is greater than the risk to the fetus or neonate. Breastfeeding women who require one of these drugs should find another way to feed the baby. OLDER ADULTS Older people are more likely to experience the adverse effects associated with these drugs—central nervous system, cardiovascular, GI, respiratory and urinary effects. Because older people often have renal or hepatic impairment, they are also more likely to have toxic levels of the drug related to changes in metabolism and excretion. The older person should be started on lower doses of the drugs and should be monitored very closely for potentially serious arrhythmias or hypotension. Safety precautions should be established if the drug causes dizziness or drowsiness. Special efforts may also be needed to help the person maintain fluid intake and nutrition if the GI effects become uncomfortable.Taking the drug with food and eating several small meals throughout the day may alleviate some of these problems.

TABLE 32.1

DRUGS IN FOCUS Direct-acting cholinergic agonists

Drug name

Dosage/route

Usual indications

10–30 mg PO or SL t.d.s. or q.i.d.

Treatment of non-obstructive postoperative and postpartum urinary retention, neurogenic bladder atony in adults and children >8 years; diagnosis and treatment of reflux oesophagitis in adults, and used orally in infants and children for treatment of oesophageal reflux Induction of miosis to relieve increased intraocular pressure of glaucoma; allows surgeons to perform certain surgical procedures Induction of miosis to relieve increased intraocular pressure of glaucoma; allows surgeons to perform certain surgical procedures

bethanechol (Urocarb)

carbachol (Isopto Carbachol, Miostat)

1–2 drops (gtt) in affected eye(s) as needed, up to three times a day

pilocarpine (Isopto Carpine)

1–2 gtt in affected eye(s) as needed, up to six times per day

Therapeutic actions and indications The direct-acting cholinergic agonists act at choliner- gic receptors in the peripheral nervous system to mimic the effects of ACh and parasympathetic stimulation. These parasympathetic effects include slowed heart rate and decreased myocardial contractility, vasodilation, bronchoconstriction and increased bronchial mucus secretion, increased GI activity and secretions, increased bladder tone, relaxation of GI and bladder sphincters, and pupil constriction (see Figure 32.1).

The agent bethanechol, which has an affinity for the cholinergic receptors in the urinary bladder, is available for use orally and subcutaneously to treat non- obstructive postoperative and postpartum urinary reten- tion and to treat neurogenic bladder atony. It directly increases detrusor muscle tone and relaxes the sphinc- ters to improve bladder emptying. Because this drug is not destroyed by acetylcholinesterase, the effects on the receptor site are longer lasting than with stimulation by ACh. See Table 32.1 for additional indications.