Introduction
Arrhythmia means any abnormality in the normal rhythm or rate of the heartbeat. It occurs when electrical impulses that control heart activity are disturbed. These disturbances may involve abnormal impulse generation or abnormal conduction. Some arrhythmias are harmless, but others can be life-threatening and require immediate treatment. Anti-arrhythmic drugs help restore normal rhythm and prevent complications.
Mechanisms of Arrhythmia
- Abnormal automaticity: the heart generates impulses too fast or from abnormal sites.
- After-depolarizations: delayed impulses during repolarization.
- Re-entry circuits: the impulse re-circulates repeatedly in the heart.
- Conduction block: impulse fails to travel through normal pathways.
Classification of Anti-Arrhythmic Drugs
The Vaughan Williams classification divides these medicines into four major classes:
- Class I: Sodium channel blockers
- Class II: Beta blockers
- Class III: Potassium channel blockers
- Class IV: Calcium channel blockers
- Others: Adenosine, digoxin, magnesium sulphate
CLASS I – Sodium Channel Blockers
They block fast sodium channels and reduce the rate of phase 0 depolarization. This slows conduction in cardiac tissue. They are further divided into IA, IB and IC subclasses based on their effect on action potential duration.
Class IA (Quinidine, Procainamide, Disopyramide)
Actions:
- Moderate Na+ channel block
- Prolong action potential duration
- Decrease automaticity and conduction
Uses:
- Atrial flutter, atrial fibrillation
- Ventricular tachycardia
Key Adverse Effects:
- QT prolongation → torsades de pointes
- Hypotension
- Diarrhoea, cinchonism (quinidine)
- Anticholinergic effects (disopyramide)
Class IB (Lidocaine, Mexiletine, Phenytoin)
Actions:
- Shorten repolarization (phase 3)
- Decrease action potential duration
Uses:
- Acute ventricular arrhythmia
- Digitalis toxicity
Side Effects:
- Drowsiness, confusion, slurred speech
- Seizures (with high doses)
Class IC (Flecainide)
Actions:
- Marked slowing of phase 0
- Strong depression of conduction throughout myocardium
Uses:
- Supraventricular arrhythmia
- Ventricular arrhythmia
Side Effects:
- Pro-arrhythmic effect
- Dizziness, visual disturbances
CLASS II – Beta Blockers
Examples: Propranolol, Metoprolol, Esmolol, Acebutolol
These drugs block the effect of catecholamines on β-receptors. They reduce automaticity, slow AV conduction and prolong AV refractory period.
Uses:
- Atrial flutter
- Atrial fibrillation
- Supraventricular tachycardia
- Digitalis-induced arrhythmia
Side Effects:
- Bronchospasm (avoid in asthma)
- Bradycardia
- Hypotension
CLASS III – Potassium Channel Blockers
Examples: Amiodarone, Sotalol, Dofetilide, Ibutilide
These drugs prolong repolarization, increase action potential duration and extend the refractory period. They are powerful anti-arrhythmics used for difficult cases.
Amiodarone
- Blocks K+, Na+, Ca2+ channels and β-receptors
- Long half-life and many interactions
Uses:
- Ventricular tachycardia
- Supraventricular arrhythmia
Important Adverse Effects:
- Pulmonary fibrosis
- Thyroid dysfunction
- Skin discoloration
- Heart block, bradycardia
Sotalol
- Beta-blocking action + K+ channel block
- Used for ventricular and supraventricular arrhythmias
Dofetilide & Ibutilide
- Selective K+ channel blockers
- Used in atrial fibrillation to convert or maintain sinus rhythm
CLASS IV – Calcium Channel Blockers
Examples: Verapamil, Diltiazem
They block L-type calcium channels, reduce SA node automaticity and slow AV node conduction.
Uses:
- Paroxysmal supraventricular tachycardia (PSVT)
- Rate control in atrial flutter and atrial fibrillation
Side Effects:
- Constipation
- Bradycardia
- Heart block
Other Anti-Arrhythmic Drugs
Adenosine
- Very short-acting
- Acts by activating ACh-sensitive K+ channels
Uses:
- Drug of choice for acute PSVT
Side Effects:
- Flushing
- Dyspnoea
- Headache
Detailed Notes:
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