Antiarrhythmic Drugs: Clinical Pharmacology and Therapeutic Uses

Summary

The effective therapy of cardiac arrhythmias requires accurate cardiac diagnosis, identification and treatment of specific initiating factors, and appropriate drug therapy. If antiarrhythmic drug therapy is required, therapeutic goals should be determined in advance and treatment that is based on sound physiological and pharmacokinetic rationale should be initiated. Careful follow-up should be undertaken, including monitored documentation of drug effects, observation for side-effects, and drug plasma concentration-response correlations. Antiarrhythmic drugs differ in specific patterns of absorption, volumes of distribution, plasma protein binding and elimination (metabolism and excretion). Disease states may alter these properties. Only by applying a thorough understanding of each drug’s individual pharmacological characteristics can maximum benefit be obtained.

Lignocaine is effective against ventricular arrhythmias of many aetiologies, and in the acute situation is usually the agent of first choice. Recently, prophylactic lignocaine has been advocated for younger patients with acute myocardial infarction. Quinidine has a long history of usefulness in both ventricular and supraventricular arrhythmias. It is effective in maintaining sinus rhythm after cardioversion from atrial fibrillation or flutter, and is perhaps the most widely used oral agent in the management of chronic ventricular arrhythmias. Procainamide is a versatile drug with uses comparable with those of quinidine. In addition, it may be administered parenterally in an acute or subacute setting. The anticonvulsant drug phenytoin remains an agent of somewhat limited utility. However, it is recognised as a primary agent for the treatment of digitalis-induced arrhythmias.

β-Adrenoceptor blocking drugs are useful for treating arrhythmias due to excessive catecholamine release, for digitalis excess, for therapy of acute and chronic supraventricular arrhythmias, for ventricular arrhythmias unresponsive to first-line agents, and potentially as prophylaxis against sudden death in ambulatory patients with coronary artery disease. These agents have also been useful in preventing tachyarrhythmias associated with Wolff-Parkinson-White syndrome, and in prophylaxis of delayed repolarisation arrhythmias. Propranolol is better tolerated but less effective than quinidine or procainamide in treating chronic ventricular arrhythmias.

Bretylium may be useful in the treatment of ventricular fibrillation that is refractory to conventional therapy. Tocainide and mexiletine are relatively new, orally effective agents with structural similarities to lignocaine. Disopyramide, another new antiarrhythmic agent, has actions and versatility which resemble those of quinidine, but it may be better tolerated. Verapamil, whose mechanism of action involves blockade of slow ionic currents, appears to be a highly effective drug for therapy of acute supraventricular tachycardia. Aprindine, amiodarone, and acebutolol are examples of other new antiarrhythmic agents whose properties are discussed.

Benefits of antiarrhythmic therapy must always be weighed against the risk of side-effects. For lignocaine, these are primarily neurological and are related to plasma concentration. An initial loading dose or doses followed by constant infusion represents the correct mode of administration. Diseases which alter hepatic blood flow affect lignocaine metabolism, and thus patients with heart failure should receive both smaller initial boluses and smaller infusion rates than normals. The major problems with quinidine therapy are the high incidence of side-effects, such as diarrhoea, leading to intolerance, and the risk, if small, of sudden death from idiosyncratic quinidine-induced ventricular fibrillation. Intravenous administration of procainamide must be carefully monitored to avoid hypotension. In patients with heart failure, smaller boluses should be given and maintenance doses may need to be reduced. The necessity for frequent (3- to 4-hourly) doses and the high incidence of drug-induced lupus after long-term usage have decreased the utility of procainamide for chronic arrhythmia control.

With intravenous administration, phenytoin may cause hypotension if administered rapidly. Toxicity with oral therapy may be manifested by nystagmus, ataxia and lethargy. Propranolol is contraindicated where cardiac function depends upon sympathetic stimulation; congestive heart failure has been precipitated in this setting. Sudden withdrawal of propranolol in non-hospitalised patients with severe anginal symptoms may be associated with serious rebound phenomena. When used intravenously, much smaller dosages than for oral use are used, and slow incremental administration with blood pressure and electrocardiographic monitoring is indicated.

Side-effects with mexiletine and aprindine have been problematic and include cardiovascular and neurological toxicity. The most frequent side-effects of oral disopyramide are anticholinergic, including dry mouth, constipation and urinary hesitancy. The myocardial depressant potential of verapamil contraindicates its use in patients with marked underlying cardiac dysfunction.