McKenna's Pharmacology for Nursing, 2e

705

C H A P T E R 4 5 Antiarrhythmic agents

Adverse effects The adverse effects associated with class II antiarrhyth- mics are related to the effects of blocking beta-receptors in the sympathetic nervous system. CNS effects include dizziness, insomnia, nightmares and fatigue. Cardiovas- cular symptoms can include hypotension, bradycardia, AV block, arrhythmias and alterations in peripheral per- fusion. Respiratory effects can include bronchospasm and dyspnoea. GI problems frequently include nausea, vomiting, anorexia, constipation and diarrhoea. Other effects to anticipate include a loss of libido, decreased exercise tolerance and alterations in blood glucose levels. Clinically important drug–drug interactions The risk of adverse effects increases if these drugs are taken with verapamil; if this combination is used, dose adjustment will be needed. There is a possibility of increased hypoglycaemia if these drugs are combined with insulin; individuals should be monitored closely. Other specific drug interactions may occur with each drug; check a drug reference before combining these drugs with any others. Prototype summary: Propranolol Indications: Treatment of cardiac arrhythmias, especially supraventricular tachycardia; treatment of ventricular tachycardia induced by digitalis or catecholamines. Actions: Competitively blocks beta-adrenergic receptors in the heart and kidney, has a membrane- stabilising effect, and decreases the influence of the sympathetic nervous system. Pharmacokinetics: Route Onset Peak Duration Oral 20–30 mins 60–90 mins 6–12 hours IV Immediate 1 min 4–6 hours T 1/2 : 3 to 5 hours; metabolised in the liver and excreted in urine. Adverse effects: Bradycardia, HF, cardiac arrhythmias, heart blocks, cerebrovascular accident (CVA), pulmonary oedema, gastric pain, flatulence, nausea, vomiting, diarrhoea, impotence, decreased exercise tolerance, antinuclear antibody (ANA) development.

Effects of Class III antiarrhythmics: K + channel blocker, prolongs repolarisation

phase 1

phase 2

+20

0

0

–40 –60 –20

phase 3

0

Effects of Class IV calcium channel blockers: slow phase 4, spontaneous depolarisation, slow conduction

phase 4

–80 –100

Effects of Class II antiarrhythmic blockers of beta-adrenergic receptors: diminish phase 4, cell less excitable, slower conducting

FIGURE 45.7  The cardiac action potentials, showing the effects of class II, III and IV antiarrhythmics.

decrease in heart rate, cardiac excitability and cardiac output, a slowing of conduction through the AV node and a decrease in the release of renin. These effects stabilise excitable cardiac tissue and decrease blood pressure, which decreases the heart’s workload and may further stabilise hypoxic cardiac tissue. These drugs are indicated for the treatment of supraventricular tachy­ cardias and PVCs. See Table 45.1 for usual indications for each drug. Pharmacokinetics Esmolol is administered intravenously. Propranolol may be administered orally or intravenously. These drugs are absorbed from the GI tract or have an immediate effect when given intravenously and undergo hepatic metabolism. They are excreted in the urine. Food has been found to increase the bioavailability of proprano- lol, although this effect has not been found with other beta-adrenergic blocking agents. Contraindications and cautions The use of these drugs is contraindicated in the presence of sinus bradycardia (rate less than 45 beats/minute) and AV block, which could be exacerbated by the effects of these drugs ; with cardiogenic shock, HF, asthma or res- piratory depression, which could worsen due to blockage of beta receptors ; and with pregnancy and breastfeeding because of the potential for adverse effects on the fetus or neonate. Caution should be used in people with diabetes and thyroid dysfunction, which could be altered by the blockade of the beta-receptors , and in people with renal and hepatic dysfunction, which could alter the metabo­ lism and excretion of these drugs.

KEY POINTS

■■ Class II antiarrhythmics are beta-adrenergic receptor blockers that prevent sympathetic stimulation. ■■ Adverse effects related to class II antiarrhythmics are associated with blocking of the sympathetic response.