29/10/2026
The Vaughan Williams Classification of antiarrhythmic drugs.
The 4 Classes
• Class I: Sodium (Na+) Channel Blockers
• Targets: Phase 0 (Rapid Depolarization).
• Action: Slows the "upstroke" of the heartbeat.
• Examples: Procainamide (Ia), Lidocaine (Ib), Flecainide (Ic).
• Class II: Beta Blockers
• Targets: Phase 4 (Resting Potential/Pacemaker slope).
• Action: Decreases heart rate and conduction speed by suppressing the "drift" toward firing.
• Examples: Metoprolol, Atenolol (drugs ending in "-olol").
• Class III: Potassium (K+) Channel Blockers
• Targets: Phase 3 (Rapid Repolarization).
• Action: Delays repolarization. This prolongs the "reset" time (Refractory Period).
• Examples: Amiodarone, Sotalol, Dofetilide.
• Class IV: Calcium (Ca2+) Channel Blockers
• Targets: Phase 2 (Plateau) and Phase 0 of pacemaker cells.
• Action: Shortens the action potential and slows the AV node.
• Examples: Verapamil, Diltiazem (Non-dihydropyridines).
To remember the order (I, II, III, IV):
> "S-B-P-C"
> Some Bad Potassium Channels
> (Sodium, Beta, Potassium, Calcium)
To remember Class I Subclasses (Ia, Ib, Ic):
> "Double Quarter Pounder, Lettuce Mayo, Fries Please"
> Ia: Disopyramide, Quinidine, Procainamide
> Ib: Lidocaine, Mexiletine
> Ic: Flecainide, Propafenone
The mechanism of action
• Class I: "Slows the Upstroke."
• Class II: "Decreases Slope of Phase 4."
• Class III: "Prolongs Phase 3" (Increases the QT interval on EKG).
• Class IV: "Slows the AV node."
Quick Note
• Amiodarone (Class III) is the "wildcard" because it actually has properties of all four classes, but it is officially categorized as Class III because its primary effect is blocking Potassium channels.
• Phase 0 = Class I
• Phase 2 = Class IV
• Phase 3 = Class III
• Phase 4 = Class II