McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 3 0 Adrenergic agonists

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 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

Treatment of essential hypertension; chronic pain; to ease opiate withdrawal; used only for adults Cold and allergies; shock and shock-like states; supraventricular tachycardias; glaucoma; allergic rhinitis; otitis media

phenylephrine (Abalon, Neo-Synephrine)

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.