McKenna's Pharmacology for Nursing, 2e

476

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

Therapeutic actions and indications The therapeutic effects of the alpha 1 -selective adren- ergic blocking agents come from their ability to block the postsynaptic alpha 1 -receptor sites. This causes a decrease in vascular tone and vasodilation, which leads to a fall in blood pressure. Because these drugs do not block the presynaptic alpha 2 -receptor sites, the reflex tachycardia that accompanies a fall in blood pressure does not occur. They also block smooth muscle recep- tors in the prostate, prostatic capsule, prostatic urethra and urinary bladder neck, which leads to a relaxation of the bladder and prostate, and improved flow of urine in males. These drugs are available in oral form and can be used to treat benign prostatic hypertrophy (BPH) (see Chapter 52 for further discussion on BPH) and hyper- tension. The drugs may be used alone or as part of a combination therapy. Table 31.3 shows usual indica- tions for each of these agents. Pharmacokinetics The alpha 1 -selective adrenergic blocking agents are well absorbed after oral administration and undergo extens­ ive hepatic metabolism. They are excreted in the urine. Contraindications and cautions The alpha 1 -selective adrenergic blocking agents are contraindicated in the presence of allergy to any of these drugs to avoid hypersensitivity reactions and also with breastfeeding because the drugs cross into breast milk and could have adverse effects on the neonate. They should be used cautiously in the presence of HF or renal failure because their blood pressure–lowering effects could exacerbate these conditions and with hepatic impairment, which could alter the metabolism of these drugs. Caution also should be used during pregnancy because of the potential for adverse effects on the fetus. Adverse effects The adverse effects associated with the use of these drugs are usually related to their effects of SNS blockage. CNS effects include headache, dizziness, weakness, fatigue, drowsiness and depression. Nausea, vomiting, abdominal pain and diarrhoea may occur as a result of direct effects on the GI tract and sympathetic blocking. Anticipated cardiovascular effects include arrhythmias, hypotension, oedema, HF and angina. The vasodilation caused by these drugs can also cause flushing, rhinitis, reddened eyes, nasal congestion and priapism. Clinically important drug–drug interactions Increased hypotensive effects may occur if these drugs are combined with any other vasodilating or antihypertens­ ive drugs, such as nitrates, calcium-channel blockers and angiotensin-converting enzyme (ACE) inhibitors.

Prototype summary: Doxazosin Indications: Treatment of mild to moderate

hypertension as monotherapy or in combination with other antihypertensives; treatment of BPH. Actions: Reduces total peripheral resistance through alpha blockade; does not affect heart rate or cardiac output; increases high-density lipoproteins while lowering total cholesterol levels. Pharmacokinetics: Route Onset Peak Duration Oral Varies 2–3 hours Not known T 1/2 : 22 hours, with hepatic metabolism and excretion in the bile, faeces and urine. Adverse effects: Headache, fatigue, dizziness, postural dizziness, vertigo, tachycardia, oedema, nausea, dyspepsia, diarrhoea, sexual dysfunction. Care considerations for people receiving alpha 1 -selective adrenergic blocking agents Assessment: History and examination ■ ■ Assess for contraindications or cautions: any known allergies to either drug t o avoid hypersensitivity reactions ; HF or renal failure, which could be exacerbated by drug use ; hepatic dysfunction, which could alter the drug’s metabolism ; and current status of pregnancy or breastfeeding because of unknown or adverse effects to the fetus or neonate. ■ ■ Perform a physical assessment to establish baseline data for determining the effectiveness of drug therapy and the occurrence of any adverse effects. ■ ■ Monitor the level of orientation, affect and reflexes to monitor for CNS changes related to drug therapy. ■ ■ Monitor vital signs and assess cardiovascular status, including pulse, blood pressure, peripheral perfusion and cardiac output, to evaluate for possible cardiac effects ; obtain an ECG as ordered to assess for possible irregularities in rate or rhythm. ■ ■ Assess renal function, including urinary output, to evaluate effects on the renal system and assess BPH and its effects on urinary output. ■ ■ Monitor renal and hepatic function tests to evaluate potential need for dose adjustment. Implementation with rationale ■ ■ Monitor blood pressure, pulse, rhythm and cardiac output regularly to evaluate for changes that may