McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 4 5 Antiarrhythmic agents

Pharmacokinetics Diltiazem is administered intravenously. When used as an antiarrhythmic, verapamil is used intravenously. These drugs are well absorbed after intravenous admini­ stration. They are highly protein bound, metabolised in the liver and excreted in the urine. They cross the placenta and enter breast milk. Contraindications and cautions These drugs are contraindicated with known allergy to any calcium channel blocker to avoid hypersensitiv­ ity reactions ; with sick sinus syndrome or heart block (unless an artificial pacemaker is in place) because the block could be exacerbated by these drugs ; with preg- nancy or breastfeeding because of the potential for adverse effects on the fetus or neonate ; and with HF or hypotension because of the hypotensive effects of these drugs. Caution should be used in cases of idiopathic hypertrophic subaortic stenosis (IHSS), which could be exacerbated , or impaired renal or liver function, which could affect the metabolism or excretion of these drugs. Adverse effects The adverse effects associated with these drugs are related to their vasodilation of blood vessels through- out the body. CNS effects include dizziness, weakness, fatigue, depression and headache. GI upset, nausea and vomiting can occur. Hypotension, HF, shock, arrhyth- mias and oedema have also been reported. Prototype summary: Diltiazem Indications: Treatment of paroxysmal supraventricular tachycardia, atrial fibrillation and atrial flutter. Actions: Blocks the movement of calcium ions across the cell membrane, depressing the generation of action potentials, delaying phases 1 and 2 of repolarisation, and slowing conduction through the AV node. Pharmacokinetics: Route Onset Peak Duration Oral 30–60 mins 2–3 hours 6–8 hours IV Immediate 2–3 mins unknown T 1/2 : 3.5 to 6 hours; metabolised in the liver and excreted in urine. Adverse effects: Dizziness, light-headedness, headache, asthenia, peripheral oedema, bradycardia, AV block, flushing, nausea, hepatic injury.

Clinically important drug–drug interactions Verapamil has been associated with many drug–drug interactions, including increased risk of cardiac depres- sion with beta blockers; additive AV slowing with digoxin; increased serum levels and toxicity of digoxin, carbamazepine, prazosin and quinidine; increased res- piratory depression with atracurium, pancuronium and vecuronium; and decreased effects if combined with calcium products or rifampicin. There is a risk of severe cardiac effects if these drugs are given IV within 48 hours of IV beta-adrenergic drugs. The combination should be avoided. Diltiazem can increase the serum levels and toxicity of cyclosporin if the drugs are taken concurrently. ■■ Class IV antiarrhythmics are calcium channel blockers that shorten the action potential, disrupting ineffective rhythms and rates. ■■ Whichever type of antiarrhythmic is used, the person receiving an antiarrhythmic drug needs to be constantly monitored while being stabilised and throughout the course of therapy to detect the development of arrhythmias or other adverse effects associated with alteration of the action potentials of other muscles or nerves. O ther antiarrhythmics Drugs other than those classified as class I, II, III or IV may be used to treat arrhythmias. Table 45.2 provides a summary of types of arrhythmias and the specific drugs used to treat each type. Additional antiarrhythmics include adenosine ( Adenocor , Adenoscan ) and digoxin ( Lanoxin ). For dosages and usual indications see also Table 45.1. Adenosine is another antiarrhythmic agent that is used to convert supraventricular tachycardia to sinus rhythm if vagal manoeuvres have been ineffective. It is often the drug of choice for terminating supraventricu- lar tachycardias, including those associated with the use of alternative conduction pathways around the AV node (e.g. Wolff–Parkinson–White syndrome), for two reasons: (1) it has a very short duration of action (about 15 seconds), after which it is picked up by circulating red blood cells and cleared through the liver; and (2) it is associated with very few adverse effects (headache, flushing and dyspnoea of short duration). This drug slows conduction through the AV node, prolongs the refractory period and decreases automaticity in the AV node. It is given IV with continuous monitoring of the person. Digoxin (see Chapter 44) is also used at times to treat arrhythmias. This drug slows calcium from KEY POINTS