McKenna's Pharmacology for Nursing, 2e

723

C H A P T E R 4 6 Antianginal agents

reach peak levels in 60 to 90 minutes and have varying duration of effects, ranging from 6 to 19 hours. Food has been found to increase the bioavailability of pro- pranolol, but this effect has not been found with other beta-adrenergic blocking agents. Contraindications and cautions The beta-blockers are contraindicated in individuals with bradycardia, heart block and cardiogenic shock because blocking of the sympathetic response could exacerbate these diseases. They are also contraindicated with pregnancy and breastfeeding because of the poten- tial for adverse effects on the fetus or neonate. Caution should be used in individuals with diabetes, peripheral vascular disease, asthma, chronic obstructive pulmonary disease (COPD) or thyrotoxicosis because the blockade of the sympathetic response blocks normal reflexes that are necessary for maintaining homeostasis in people with these diseases. Many people with these complicating disorders receive beta-blockers, and these individuals need to be monitored carefully to avoid serious adverse effects. Adverse effects Beta-blockers have many adverse effects associated with the blockade of the sympathetic nervous system. However, the dose used to prevent angina is lower than doses used to treat hypertension. Therefore, there is a decreased incidence of adverse effects associated with this specific use of beta-blockers. Adverse effects do occur. CNS effects include dizziness, fatigue, emotional depression and sleep dis- turbances. GI problems include gastric pain, nausea, vomiting, colitis and diarrhoea. Cardiovascular effects can include heart failure, reduced cardiac output and arrhythmias. Respiratory effects can include broncho­ spasm, dyspnoea and cough. Decreased exercise tolerance and malaise are also common complaints. Clinically important drug–drug interactions A paradoxical hypertension occurs when clonidine is given with beta-blockers, and an increased rebound hypertension with clonidine withdrawal may also occur; it is best to avoid this combination. A decreased antihypertensive effect occurs when beta-blockers are given with non-steroidal anti- inflammatory drugs; if this combination is used, the person should be monitored closely and a dose adjust- ment made. An initial hypertensive episode followed by bradycardia occurs if these drugs are given with noradrenaline, and a possibility of peripheral ischaemia exists if beta-blockers are taken in combination with ergot alkaloids.

There also is a potential for a change in blood glucose levels if these drugs are given with insulin or antidiabetic agents, and the individual will not have the usual signs and symptoms of hypoglycaemia or hyper- glycaemia to alert them to potential problems. If this combination is used, the person should monitor blood glucose frequently throughout the day and be alert to new warnings about glucose imbalance. Prototype summary: Metoprolol Indications: Treatment of stable angina pectoris; also used for treatment of hypertension, prevention of reinfarction in people with MI, and treatment of stable, symptomatic HF. Actions: Competitively blocks beta-adrenergic receptors in the heart and kidneys, decreasing the influence of the sympathetic nervous system on these tissues and the excitability of the heart; decreases cardiac output, which results in a lowered blood pressure and decreased cardiac workload. Pharmacokinetics: Route Onset Peak Duration Oral 15 mins 90 mins 15–19 hours IV Immediate 60–90 mins 15–19 hours T 1/2 : 3 to 4 hours; metabolised in the liver and excreted in urine. Adverse effects: Dizziness, vertigo, HF, arrhythmias, gastric pain, flatulence, diarrhoea, vomiting, impotence, decreased exercise tolerance.

Care considerations for people receiving beta-blockers

See Chapter 31 for the care considerations associated with beta-blockers.

KEY POINTS

■■ Beta-blockers are used in the treatment of angina to help restore the balance between supply of oxygen and demand for oxygen. ■■ Beta-blockers prevent the activation of sympathetic receptors, which normally would increase heart rate, increase blood pressure and increase cardiac contraction. All of these actions would increase the demand for oxygen; blocking these actions decreases the demand for oxygen.