Pharmacology · Cardiovascular

Cardiac Electrophysiology and the Vaughan Williams Classification

Action potential phases, arrhythmia mechanisms, and the four-class antiarrhythmic framework

The cardiac action potential has five distinct ionic phases — rapid sodium influx (Phase 0, Class I target), calcium plateau (Phase 2, Class IV target), potassium-driven repolarization (Phase 3, Class III target), and spontaneous phase 4 depolarization in nodal cells (Class II target) — and each antiarrhythmic class exploits a different phase.
Arrhythmias arise through three electrophysiologic mechanisms: abnormal automaticity (non-pacemaker cells develop spontaneous firing, suppressed by Class II); triggered activity via early or delayed afterdepolarizations (magnesium for EADs, beta-blockers for DADs); and re-entry (closed circuit with unidirectional block, terminated by Class I or III).
Class Ic agents (flecainide, propafenone) are absolutely contraindicated in structural heart disease — the Cardiac Arrhythmia Suppression Trial demonstrated approximately 2.5-fold increased mortality despite arrhythmia suppression, because slowed conduction in scarred myocardium converts non-sustained arrhythmias into fatal ones.
Amiodarone defies classification by spanning all four Vaughan Williams classes simultaneously; its 40–55-day elimination half-life requires loading protocols and produces toxicities that emerge over months to years; Class III agents exhibit reverse use-dependence, with the greatest QT prolongation at slow heart rates, the exact condition most favorable for torsades de pointes.
Every antiarrhythmic drug can also cause arrhythmias — proarrhythmia is inherent to altering cardiac electrophysiology, and rational prescribing requires weighing arrhythmia risk against drug risk for each individual patient and substrate.

The Cardiac Action Potential — Five Phases

Phase 0

Rapid Depolarization

Fast sodium channels open; large inward sodium current

Class I target

Phase 1

Early Repolarization

Sodium channels inactivate; brief outward potassium current

—

Phase 2

Plateau

Inward calcium current balanced by outward potassium; triggers contraction

Class IV target

Phase 3

Rapid Repolarization

Outward potassium current dominant; calcium current declines

Class III target

Phase 4

Resting Potential

Stable in working muscle; spontaneous in nodal cells (If, Ca)

Class II target

Mechanisms of Arrhythmogenesis

Abnormal Automaticity

Non-pacemaker cells acquire spontaneous phase 4 depolarization

Triggers: ischemia, catecholamine excess, hypokalemia, digitalis toxicity

Class II suppresses

Triggered Activity

EADs (phases 2–3): bradycardia, hypokalemia, QT-prolonging drugs → torsades de pointes

DADs (phase 4): calcium overload, rapid rates, digitalis toxicity

Mg for EADs · Beta-blockers for DADs

Re-entry

Requires: closed circuit + unidirectional block + slow conduction

Most supraventricular tachycardias and ventricular tachycardia in structural disease

Class I or III terminates

The Vaughan Williams Classification

Class	Mechanism	Prototype Drugs	ECG Effect	Key Caution
Ia	Sodium channel block (intermediate kinetics); also prolongs repolarization	Quinidine, procainamide, disopyramide	Wide QRS + prolonged QT	Torsades de pointes risk
Ib	Sodium channel block (rapid kinetics); shortens action potential duration	Lidocaine, mexiletine	Minimal at rest	Ischemia-selective; ineffective in atrial tissue
Ic	Sodium channel block (slow kinetics); profound conduction slowing	Flecainide, propafenone	Wide QRS	Contraindicated in structural heart disease (Cardiac Arrhythmia Suppression Trial)
II	Beta-adrenergic receptor blockade; reduce phase 4 slope, slow atrioventricular node	Metoprolol, atenolol, propranolol	Prolonged PR	Bradycardia, atrioventricular block, bronchospasm
III	Potassium channel block; prolong action potential duration and effective refractory period	Amiodarone, sotalol, dofetilide, ibutilide	Prolonged QT	Torsades de pointes; amiodarone multi-organ toxicity
IV	Calcium channel block (L-type); slow atrioventricular nodal conduction	Verapamil, diltiazem	Prolonged PR	Contraindicated in ventricular tachycardia; negative inotropy

Agents Outside the Vaughan Williams Framework

Unclassified

Adenosine

Adenosine receptor agonist → atrioventricular nodal block
Half-life under 10 seconds
First-line for supraventricular tachycardia termination
Contraindicated in Wolff-Parkinson-White with atrial fibrillation

Unclassified

Digoxin

Increases vagal tone; inhibits sodium-potassium pump
Rate control in atrial fibrillation
Narrow therapeutic index
Toxicity: ectopy plus atrioventricular block

Unclassified

Magnesium Sulfate

Suppresses early afterdepolarization-driven oscillations
First-line for torsades de pointes
Effective regardless of serum magnesium level
Also used in digoxin toxicity and refractory ventricular fibrillation

Core Principle

Every antiarrhythmic drug can also cause arrhythmias. Proarrhythmia is inherent to altering cardiac electrophysiology. Structural heart disease constrains drug selection: Class Ic agents are contraindicated when myocardial scarring is present. Class III agents carry QT prolongation and torsades de pointes risk, greatest at slow heart rates (reverse use-dependence). Rational prescribing requires weighing arrhythmia risk against drug risk for every patient.

Suggested References

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