Antiarrhythmic drugs

Antiarrhythmic drugs:

Drugs that suppress selected arrhythmias by inhibiting the abnormal transmission of electrical impulses through the heart. Classified into four groups, depending on which part of the cardiac cycle they influence (but several drugs have more than one effect).

Class 1: (action is by blocking sodium channels and slowing conduction of electrical impulses in the atria, ventricles and His-Pukinje system)
• Class 1A:
• eg disopyramide, procainamide, quinidine
• slow/delay depolarisation and prolong polarisation
• disopyramide  peripheral vasoconstriction, but has more side effects
• procainamide and quinidine (used most often)  decrease peripheral resistance. Also anticholinergic  blocks vagal input to heart
• indication  life threatening ventricular arrythmias and symptomatic non life threatening arrhythmias.
• tinnitus, headache, nausea, vertigo, diarrhoea, cardiotoxicity (in high doses)
• Class 1B:
• eg lidocaine, tocainide, mexiletine
• normalise depolarisation and shorten/accelerate repolarisation
• lidocaine blocks sodium channels (slows conduction); reduces automacity in ventricles; accelerates repolarisation – has not anticholinergic properties (class 1B do)
• indication – life threatening ventricular arrythmias (lidocaine); symptomatic non life threatening arrthymias (mexiletine); suppress frequency of ventricular ectopy (tocainide)
• adverse effects – drowsiness, confusion, parathesias
• Class 1C:
• eg flecainide, encuinide
• slow depolarisation and normalise repolarisation
• indication  life threatening ventricular arrythmias

Class 2:
• beta-adrenergic blockers
• eg acebutol, esmolol, propranolol, atenolol
• action is by decreasing sympathetic activity as sinoatrial and AV nodes – decreases automaticity and prolongs the refractory period
• indication – sinus tachycardia, atrial fibrillation, atrial flutter
• Propranolol  ventricular arrhythmias
• adverse effects – dizziness, fatigue, hypertension, congestive heart failure, bradycardia, GI symptoms

Class 3:
• potassium channel blockers
• eg amiodarone, bretylium, ibutilide, sotalol
• action is by prolonging action potential and the absolute refractory period by blocking potassium channels
• indications – ventricular fibrillation, ventricular tachycardia
• adverse effects – hypotension, bradycardia, nausea, vomiting

Class 4:
• calcium channel blockers
• eg verapamil, diltiazem
• block the slow inward calcium channel  slows conduction at AV node
• indications – atrial fibrillation, atrial flutter, supraventricular tachycardias
• adverse effects – dizziness, hypotension, bradycardia, oedema, constipation

Other
• adenosine  slows conduction at AV node by interrupting re-entrant pathways. Indicated for paroxysmal supraventricular tachycardia
• atropine  blocks muscarnic cholinergic receptors in the SA and AV nodes – also block effects of vagus nerve on cardiac conduction. indicated for sinus bradycardia
• digoxin  used for supraventricular dysrhythmias

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