Antiarrhythmics
Medicines that slow, convert or prevent abnormal heart rhythms by acting on ion channels, beta receptors or the AV node. Most can also cause arrhythmias.
What they are
A mixed group sorted by the Vaughan Williams classes: class I blocks sodium channels (IA quinidine, procainamide, disopyramide; IB lidocaine, mexiletine; IC flecainide, propafenone), class II are beta blockers, class III blocks potassium channels and lengthens repolarisation (amiodarone, sotalol, dofetilide, ibutilide, dronedarone) and class IV are the non-dihydropyridine calcium channel blockers (verapamil, diltiazem). Adenosine and digoxin act on the AV node outside the four classes.
Mechanism of action
Sodium channel blockers slow the upstroke of the action potential and conduction (wider QRS); IA drugs also lengthen repolarisation, IB drugs act mostly on depolarised or ischaemic tissue, and IC drugs slow conduction the most. Potassium channel (IKr, hERG) blockers lengthen the action potential and refractory period (longer QT). Beta blockers and calcium channel blockers slow the sinus node and AV conduction. Adenosine opens potassium channels in the AV node through A1 receptors and blocks it for seconds.
Target: Cardiac sodium (Nav1.5), potassium (hERG/IKr) and L-type calcium channels, beta-1 receptors, adenosine A1 receptors
Drugs in this class
- AmiodaroneA broad antiarrhythmic for ventricular and atrial arrhythmias. Very effective, but it builds up in tissues and can…Cardiac potassium (hERG/IKr), sodium and L-type calcium channels; β-adrenergic receptors (the L-type calcium channel Cav1.2 is shown)
- AdenosineA rapid intravenous push that blocks the AV node for a few seconds and stops most re-entrant supraventricular…Adenosine A1 receptor (AV node)
- FlecainideA class IC sodium channel blocker that keeps sinus rhythm in paroxysmal atrial fibrillation and SVT, but only in…Cardiac sodium channel Nav1.5
- ProcainamideA class IA antiarrhythmic given intravenously for ventricular tachycardia. Long use can cause a lupus-like syndrome…Cardiac sodium channel Nav1.5 (and potassium channels, through its metabolite NAPA)
- PropafenoneA class IC antiarrhythmic with mild beta-blocking action, used to keep sinus rhythm in atrial fibrillation in hearts…Cardiac sodium channel Nav1.5 (and β-adrenergic receptors)
- QuinidineThe oldest class IA antiarrhythmic, a relative of quinine. It prolongs the QT, causes diarrhoea and cinchonism, and…Cardiac sodium channel Nav1.5 (and potassium channels)
- DofetilideA pure potassium channel blocker that converts and prevents atrial fibrillation. It is started in hospital because it…hERG potassium channel (IKr)
- IbutilideAn intravenous class III drug for rapid conversion of recent-onset atrial fibrillation or flutter. Torsades de pointes…Cardiac repolarising currents (slow inward sodium current and hERG/IKr)
- MexiletineAn oral class IB antiarrhythmic, a cousin of lidocaine, for life-threatening ventricular arrhythmias. Nausea, tremor…Cardiac sodium channel Nav1.5
- DisopyramideAn oral class IA antiarrhythmic with strong anticholinergic and negative inotropic effects. It can worsen heart…Cardiac sodium channel Nav1.5 (and muscarinic receptors)
- DronedaroneAn amiodarone relative without iodine that helps keep sinus rhythm after atrial fibrillation. Dangerous in severe…Cardiac potassium (hERG/IKr and others), sodium and L-type calcium channels; also beta-adrenergic receptors
- VernakalantAn intravenous, relatively atrial-selective antiarrhythmic for fast conversion of recent-onset atrial fibrillation…Atrial potassium currents (IK,ACh; Ito through Kv4.3) and hERG, plus rate-dependent sodium-channel block
Others in the class
- Dronedarone
- Dofetilide
- Ibutilide
- Mexiletine
- Disopyramide
- Vernakalant
Test yourself
Think of the answer, then tap the question to see it.
Which Vaughan Williams class do flecainide and propafenone belong to, and what is their main ECG effect?
Class IC: strong sodium channel block that slows conduction and widens the QRS, with little effect on repolarisation.
Which two classes most often prolong the QT and cause torsades de pointes?
Class IA (quinidine, procainamide, disopyramide) and class III (sotalol, dofetilide, ibutilide; amiodarone less often).
Why are class IC drugs avoided after a myocardial infarction?
In the CAST trial they increased deaths and arrests in post-infarction patients, through proarrhythmia in scarred myocardium.
Why is an AV nodal blocker given with a class I drug in atrial flutter?
Slowing the flutter rate can let the AV node conduct every beat (1:1), so the ventricular rate can rise sharply.
Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.
Sources: DailyMed - AMIODARONE HYDROCHLORIDE tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - FLECAINIDE ACETATE tablet (DailyMed (U.S. National Library of Medicine), 2026); DailyMed - PROPAFENONE HYDROCHLORIDE tablet, film coated (DailyMed (U.S. National Library of Medicine), 2026).
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Representative model, for educational purposes only.