The primary system for classifying antiarrhythmic agents is the Vaughan Williams classification, which categorizes drugs based on their main electrophysiological effect on the cardiac action potential.
The original system consists of four main classes, modern updates have expanded it to include newer targets.
Vaughan Williams Classification System
Class I — Sodium Channel Blockers Slow phase 0 depolarization; subdivided by kinetics: Ia — Moderate Na⁺ block + K⁺ block; prolongs action potential Quinidine, Procainamide, Disopyramide, Sodium Channel Blockers, QuinidineLidocaine, Flecainide -Atrial and ventricular tachycardias Ib — Weak Na⁺ block; shortens action potential; preferentially affect ischemic tissue Lidocaine, Mexiletine, Phenytoin Ic — Strong Na⁺ block; minimal effect on action potential duration Flecainide, Propafenone Most potent Na⁺ blockers; contraindicated post-MI
II Beta-Blockers | Metoprolol, Atenolol, Esmolol,Carvedilol, Propranolol, Esmolo late control for AFib; post-MI
Block sympathetic stimulation; slow SA/AV node automaticity and conduction Metoprolol, Atenolol, Carvedilol, Propranolol, Esmolol Used in: rate control (AF/flutter), SVT, post-MI, heart failure
III Potassium Channel Blockers-| Maintenance of sinus rhythm; ventricular tachycardia
Block K⁺ efflux → prolong repolarization and action potential duration → prolong QT • Amiodarone — also has Class I, II, IV properties; most commonly used • Sotalol — also has beta-blocking (Class II) properties • Dofetilide, Ibutilide — pure Class III • Dronedarone — amiodarone derivative without iodine; safer side effect profile but less effective
IV Calcium Channel Blockers | Verapamil, Diltiazem | Supraventricular tachycardia (SVT)
Sub-Classifications Class I: Sodium Channel Blockers
This class is subdivided based on how they affect the duration of the action potential (APD) and their binding kinetics:
Class IA: Moderate sodium channel block; prolongs APD. (Quinidine,Procainamide).
Class IB: Weak sodium channel block; shortens APD. (e.g., Lidocaine, Mexiletine).
Class IC: Strong sodium channel block; no effect on APD. (e.g., Flecainide, Propafenone).
Modernized & Miscellaneous Classes Modern research, has introduced additional categories for drugs that did not fit the original model:
Class 0: HCN channel blockers-Ivabradine which target the “funny current” in pacemaker cells.
Class V: Miscellaneous agents including Digoxin, Adenosine, and Magnesium Sulfate
Class VI: Gap junction modulators (Carbenoxolone).
Class VII: Upstream target modulators like ACE inhibitors and statins that affect structural remodeling.
Class IV —Channel Blockers (non-dihydropyridine)
Block L-type Ca²⁺ channels → slow SA/AV nodal conduction Verapamil, Diltiazem Used in: rate control, SVT termination Avoid in heart failure with reduced ejection fraction
Class V/ Unclassified
Agents that don’t fit neatly into the original system:
Adenosine — transiently blocks AV node; terminates SVT; very short half-life (~10 sec)
Digoxin — vagotonic + Na⁺/K⁺-ATPase inhibition
Magnesium — used in torsades de pointes and refractory VF
Ivabradine — blocks funny current (If) in SA node; pure rate reduction
Many drugs span multiple classes (amiodarone is the prime example)
Doesn’t capture reverse use-dependence, tissue selectivity, or clinical outcomes