McKenna's Pharmacology for Nursing, 2e

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

TABLE 31.2

DRUGS IN FOCUS Non-selective alpha-adrenergic blocking agent

Drug name

Dosage/route

Usual indications

Adult: 5 mg IV or IM 1–2 hours before surgery; 5–10 mg in 10 mL of saline injected into area of extravasation within 12 hours after extravasation Paediatric: 1 mg IM or IV 1–2 hours before surgery; treat extravasation as in the adult

Prevention of cell death and tissue sloughing after extravasation of intravenous noradrenaline or dopamine, and severe hypertension reactions caused by manipulation of the phaeochromocytoma before and during surgery; diagnosis of phaeochromocytoma

phentolamine (Regitine)

NON-SELECTIVE ALPHA-ADRENERGIC BLOCKING AGENTS Some adrenergic blocking agents have a specific affinity for alpha-receptor sites. Their use is somewhat limited because of the development of even more specific and safer drugs. Only one of this type of drug, phentolamine ( Regitine ), is still used (Table 31.2). Therapeutic actions and indications Phentolamine blocks the postsynaptic alpha 1 -adrenergic receptors, decreasing sympathetic tone in the vascula- ture and causing vasodilation, which leads to a lowering of blood pressure. It also blocks presynaptic alpha 2 - receptors, preventing the feedback control of noradrena- line release. The result is an increase in reflex tachycardia that occurs when blood pressure is lowered. Phentola- mine is most frequently used to prevent cell death and tissue sloughing after extravasation of intravenous noradrenaline or dopamine, causing a local vasodilation and a return of blood flow to the area. Table 31.2 shows usual indications for this agent. Pharmacokinetics Phentolamine is rapidly absorbed after IV or IM injec- tion and is excreted in the urine. There are few data on its metabolism and distribution. Contraindications and cautions Phentolamine is contraindicated in the presence of allergy to this or similar drugs and in the presence of coronary artery disease or MI because of the poten­ tial exacerbation of these conditions ; it should be used cautiously in pregnancy or breastfeeding because of the potential adverse effects on the fetus or neonate. Adverse effects Individuals receiving phentolamine often experi- ence extensions of the therapeutic effects, including hypotension, orthostatic hypotension, angina, MI, cerebrovascular accident, flushing, tachycardia and arrhythmia— all of which are related to vasodilation

and decreased blood pressure. Headache, weakness and dizziness often occur in response to hypotension. Nausea, vomiting and diarrhoea may also occur. Clinically important drug–drug interactions Adrenaline may have decreased hypertensive and vasoconstrictive effects if taken concomitantly with phen- tolamine because these agents work in opposing ways in the body. Increased hypotension may occur if this drug is combined with alcohol, which is also a vasodilator. Prototype summary: Phentolamine Indications: Prevention or control of hypertensive episodes associated with phaeochromocytoma; test for diagnosis of phaeochromocytoma; prevention and treatment of dermal necrosis and sloughing associated with IV extravasation of noradrenaline or dopamine. Actions: Competitively blocks postsynaptic alpha 1 - and presynaptic alpha 2 -receptors, causing a vasodilation and lowering of blood pressure, accompanied by increased reflex tachycardia. Pharmacokinetics: Route Onset Peak Duration Intramuscular Rapid 20 mins 30–45 mins IV Immediate 2 mins 15–30 mins T 1/2 : Metabolism and excretion are unknown. Adverse effects: Acute and prolonged hypotensive episodes, MI, tachycardia, arrhythmias, nausea, flushing.

Care considerations for people receiving non-selective alpha-adrenergic blocking agents

Assessment: History and examination

■ ■ Assess for contraindications or cautions: any known allergies to these drugs to avoid