Classification of Antiarrhythmic Drugs

Cardiac arrhythmias are disorders of impulse generation, conduction, or both, and can lead to symptoms like palpitations, dizziness, syncope, or even sudden cardiac death. Antiarrhythmic drugs are agents used to suppress abnormal rhythms and restore normal heart rhythm and rate.

This blog from Pharmacy Freak outlines the classification, clinical uses, drug of choice insights, and adverse effects of antiarrhythmic drugs using key academic references suitable for pharmacology students and professionals.

What is Antiarrhythmic Drugs

Antiarrhythmic drugs are pharmacological agents used to prevent, suppress, or treat cardiac arrhythmias by altering the electrical conduction pathways of the heart. These agents act on cardiac ion channels or autonomic receptors.

Classification of Antiarrhythmic Drugs (KD Tripathi)

Class I – Membrane stabilizing agents:
(A) Moderately decrease dv/dt of phase 0: Quinidine, Procainamide, Disopyramide
(B) Little decrease in dv/dt of phase 0: Lidocaine, Mexiletine
(C) Marked decrease in dv/dt of phase 0: Propafenone, Flecainide
Class II – Antiadrenergic agents: Propranolol, Sotalol (also has Class III property), Esmolol
Class III – Agents widening action potential: Amiodarone, Dronedarone, Dofetilide, Ibutilide
Class IV – Calcium channel blockers: Verapamil, Diltiazem

Drugs for PSVT (Paroxysmal Supraventricular Tachycardia): Adenosine, Digoxin

Classification (General)

The most widely used classification is the Vaughan Williams Classification, which categorizes antiarrhythmics into five main classes:

Class I – Sodium Channel Blockers

These drugs inhibit fast sodium channels, reducing phase 0 depolarization in myocardial cells. They are further subdivided into:

Class IA: Moderate Na+ channel blockade; prolong action potential duration
Examples: Quinidine, Procainamide, Disopyramide
Use: Atrial and ventricular arrhythmias
Class IB: Weak Na+ channel blockade; shorten action potential duration
Examples: Lidocaine, Mexiletine, Phenytoin
Use: Ventricular arrhythmias, especially post-MI
Class IC: Strong Na+ channel blockade; minimal effect on action potential duration
Examples: Flecainide, Propafenone
Use: Supraventricular arrhythmias in structurally normal hearts

Class II – Beta Blockers
These drugs block beta-adrenergic receptors, reducing sympathetic stimulation.
Examples: Propranolol, Metoprolol, Esmolol
Use: Rate control in atrial fibrillation, supraventricular tachycardia, and prevention of post-MI arrhythmias

Class III – Potassium Channel Blockers
They prolong repolarization by blocking potassium channels, thereby increasing the action potential duration and refractory period.
Examples: Amiodarone, Sotalol, Dofetilide, Ibutilide
Use: Atrial fibrillation, ventricular tachycardia, and re-entrant arrhythmias
Note: Amiodarone is one of the most effective but has significant toxicity

Class IV – Calcium Channel Blockers
These inhibit L-type calcium channels, primarily affecting the SA and AV nodes.
Examples: Verapamil, Diltiazem
Use: Supraventricular tachycardias, rate control in atrial fibrillation

Class V – Miscellaneous Agents
These act through different or multiple mechanisms and do not fall into the above classes.

Adenosine – Inhibits AV nodal conduction; used for acute termination of paroxysmal supraventricular tachycardia
Digoxin – Increases vagal tone, slows AV conduction; used in atrial fibrillation with CHF
Magnesium sulfate – Used in torsades de pointes and digitalis-induced arrhythmias
Ivabradine – Inhibits funny current (If) in SA node; used in heart rate control in heart failure

Uses

Antiarrhythmic drugs are used in the treatment of:

Supraventricular arrhythmias (e.g., atrial fibrillation, AV nodal reentrant tachycardia)
Ventricular arrhythmias (e.g., ventricular tachycardia, ventricular fibrillation)
Rate control in atrial fibrillation
Post-myocardial infarction arrhythmias
Congenital long QT syndrome
Drug-induced arrhythmias

Drug of Choice Highlights

Atrial fibrillation – Rate control: Metoprolol or Diltiazem; Rhythm control: Amiodarone
Acute PSVT – Adenosine (IV push)
Ventricular arrhythmia post-MI – Lidocaine
Digitalis-induced arrhythmias – Phenytoin or Magnesium sulfate
Torsades de pointes – Magnesium sulfate
Prevention of post-MI arrhythmias – Beta blockers
Atrial flutter – Amiodarone or Dofetilide

Side Effects

Class I

IA (Quinidine) – Cinchonism, torsades de pointes
IB (Lidocaine) – CNS toxicity (tremor, confusion)
IC (Flecainide) – Proarrhythmic in structural heart disease

Class II

Bradycardia, hypotension, bronchospasm (non-selective beta blockers)

Class III

Amiodarone – Pulmonary fibrosis, hepatotoxicity, thyroid dysfunction, corneal deposits, photosensitivity
Sotalol – QT prolongation, torsades de pointes

Class IV

Constipation (Verapamil), bradycardia, AV block, hypotension

Class V

Adenosine – Transient asystole, flushing, chest pain
Digoxin – Bradyarrhythmias, AV block, GI symptoms, visual disturbances
Magnesium – Hypotension, respiratory depression at high doses

Updated Clinical Pearls

Amiodarone has properties of all four Vaughan Williams classes but should be used cautiously due to its multi-organ toxicity.
Beta blockers are the most effective drugs for reducing post-MI mortality and should be included in long-term management.
In patients with structural heart disease, Class IC agents should be avoided due to proarrhythmic risk.
Adenosine has a very short half-life and is ideal for emergency use in supraventricular tachycardia.
Torsades de pointes is typically triggered by prolonged QT interval and is best managed with IV magnesium sulfate.

References

Related Links