Classification of antiarrythmic drug’s
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Classification of Antiarrhythmic Drugs
The standard system is the Vaughan-Williams (Singh-Vaughan Williams) classification, which groups drugs by their predominant effect on the cardiac action potential. It has four classes, with Class I subdivided into three subclasses.
Note: Many antiarrhythmic drugs have actions spanning more than one class. The classification is a useful shorthand but does not fully capture the complexity of drug action. - Katzung's Basic and Clinical Pharmacology, 16th Ed.
Class I - Sodium (Na⁺) Channel Blockers
These drugs block voltage-sensitive Na⁺ channels and show use-dependence (state-dependence): they bind more avidly to open or inactivated channels, so they selectively suppress abnormally rapidly firing cells without affecting normal cardiac rhythm. - Lippincott Illustrated Reviews: Pharmacology
They are subdivided by their kinetics of dissociation from the Na⁺ channel and their effect on action potential duration (APD):
Class IA - Intermediate dissociation kinetics | APD prolonged
| Feature | Detail |
|---|---|
| Mechanism | Block Na⁺ channels (phase 0) + also block K⁺ channels → slow conduction AND prolong repolarization |
| Effect on AP | Slows phase 0 upstroke; also prolongs phase 3 repolarization (widens QRS + prolongs QT) |
| Drugs | Quinidine, Procainamide, Disopyramide |
| Uses | Atrial fibrillation/flutter, SVT, ventricular tachyarrhythmias |
| Key ADRs | QT prolongation, torsades de pointes, proarrhythmia |
Drug-specific toxicities:
- Quinidine: Cinchonism (tinnitus, headache, visual disturbances), hemolytic anemia, esophagitis, anticholinergic effects
- Procainamide: Drug-induced lupus syndrome (anti-histone antibodies), hypotension
- Disopyramide: Marked anticholinergic effects (urinary retention, constipation), strong negative inotropic effect - Lippincott Illustrated Reviews: Pharmacology
Class IB - Rapid dissociation kinetics | APD shortened or unchanged
| Feature | Detail |
|---|---|
| Mechanism | Block Na⁺ channels; rapidly dissociate from resting channels → preferentially act in ischemic/depolarized tissue |
| Effect on AP | Shortens phase 3 repolarization (shortens APD and QT interval) |
| Drugs | Lidocaine, Mexiletine |
| Uses | Ventricular arrhythmias (lidocaine IV for acute/refractory VT/VF); mexiletine orally for chronic ventricular arrhythmias |
| Key ADRs | CNS effects: nystagmus (early toxicity sign), drowsiness, slurred speech, paresthesia, seizures |
- Lidocaine: IV only (extensive first-pass metabolism); metabolized by CYP1A2/CYP3A4
- Mexiletine: Oral; narrow therapeutic index; GI side effects (nausea, vomiting, dyspepsia) - Lippincott Illustrated Reviews: Pharmacology
Class IC - Slow dissociation kinetics | No effect on APD
| Feature | Detail |
|---|---|
| Mechanism | Potent Na⁺ channel blockade; slowly dissociate → marked slowing of conduction in all cardiac tissue |
| Effect on AP | Markedly slows phase 0 upstroke; prominent QRS widening with little change to APD |
| Drugs | Flecainide, Propafenone |
| Uses | Atrial fibrillation/flutter and SVT in patients WITHOUT structural heart disease |
| Key ADRs | Strongly proarrhythmic; can promote reentrant VT (CAST trial: increased mortality post-MI); negative inotropy |
| Contraindication | Structural heart disease (LV dysfunction, CAD, heart failure) - Harrison's Principles of Internal Medicine 22E; Lippincott Illustrated Reviews: Pharmacology |
- Propafenone: Also has mild β-blocking activity; can cause bronchospasm, agranulocytosis
Class II - β-Adrenergic Blockers
| Feature | Detail |
|---|---|
| Mechanism | Inhibit catecholamine-mediated stimulation; suppress phase 4 spontaneous depolarization in SA and AV nodes |
| Effect on AP | Decreased automaticity, slowed AV conduction (↑ PR interval), reduced heart rate |
| Drugs | Atenolol, Esmolol (IV, ultra-short acting), Metoprolol, Propranolol, Bisoprolol |
| Uses | Rate control in AF/AFL; SVT; post-MI ventricular arrhythmias; thyrotoxic arrhythmias |
| Key ADRs | Bradycardia, heart block, hypotension, bronchospasm, fatigue, worsening heart failure |
- Lippincott Illustrated Reviews: Pharmacology; Katzung's Basic and Clinical Pharmacology
Class III - Potassium (K⁺) Channel Blockers
| Feature | Detail |
|---|---|
| Mechanism | Block delayed rectifier K⁺ current (IKr) → prolong phase 3 repolarization → widen APD and refractory period |
| Effect on AP | Prolongs QT interval; increases effective refractory period (ERP) without slowing conduction velocity |
| Drugs | Amiodarone*, Sotalol*, Dofetilide, Ibutilide, Dronedarone* |
* These drugs have multiple class actions
Individual drug profiles:
| Drug | Route | Special Features | Key ADRs |
|---|---|---|---|
| Amiodarone | PO/IV | Has Class I+II+III+IV actions; drug of choice for VT/VF | Pulmonary toxicity (pneumonitis/fibrosis), hepatotoxicity, thyroid dysfunction (hypo- AND hyperthyroid), corneal deposits, blue-gray skin discoloration, peripheral neuropathy, optic neuritis |
| Sotalol | PO | Also a non-selective β-blocker (Class II) | QT prolongation, torsades de pointes, bradycardia |
| Dofetilide | PO | Pure IKr blocker; used in AF/AFL | QT prolongation; must be initiated in hospital |
| Ibutilide | IV only | Used for acute cardioversion of AF/AFL | QT prolongation; torsades de pointes |
| Dronedarone | PO | Amiodarone analog without iodine; mild effects across all classes | Contraindicated in severe HF; hepatotoxicity |
- Lippincott Illustrated Reviews: Pharmacology; Harrison's Principles of Internal Medicine 22E; Katzung's 16th Ed.
Class IV - Calcium (Ca²⁺) Channel Blockers (Non-dihydropyridine)
| Feature | Detail |
|---|---|
| Mechanism | Block L-type Ca²⁺ channels in SA and AV nodes (where AP upstroke is Ca²⁺-dependent) |
| Effect on AP | Slowed AV conduction, decreased automaticity of SA/AV nodes |
| Drugs | Verapamil, Diltiazem |
| Uses | Rate control in AF/AFL; termination of AVNRT (paroxysmal SVT) |
| Key ADRs | Bradycardia, heart block, acute heart failure (especially verapamil), hypotension, constipation |
| Contraindication | Ventricular tachycardia (can cause CV collapse); Wolff-Parkinson-White syndrome with AF |
- Lippincott Illustrated Reviews: Pharmacology
Other/Unclassified Antiarrhythmic Agents
These drugs do not fit neatly into the Vaughan-Williams scheme:
| Drug | Mechanism | Uses | Notes |
|---|---|---|---|
| Adenosine | Activates adenosine A₁ receptors → hyperpolarizes AV node cells | Drug of choice for acute termination of AVNRT/SVT | Ultra-short half-life (~10 sec); causes transient AV block; flushing, dyspnea, chest discomfort |
| Digoxin | Inhibits Na⁺/K⁺-ATPase; increases vagal tone → slows AV conduction | Rate control in AF | Narrow therapeutic index; digitalis toxicity |
| Magnesium | Mechanism not fully clear | Torsades de pointes; digoxin toxicity arrhythmias | First-line for torsades de pointes |
| Atropine | Muscarinic (M₂) antagonist | Symptomatic bradycardia | Blocks vagal tone on SA/AV node |
- Katzung's Basic and Clinical Pharmacology 16th Ed.; Goodman & Gilman's The Pharmacological Basis of Therapeutics
Summary: Effect on Action Potential Phases
| Class | Target | Phase Affected | ECG Change |
|---|---|---|---|
| IA | Na⁺ channel (moderate block) + K⁺ channel | Slows phase 0; prolongs phase 3 | ↑ QRS width + ↑ QT |
| IB | Na⁺ channel (weak, rapid) | Shortens phase 3 | ↓ QT (minimal QRS change) |
| IC | Na⁺ channel (strong, slow) | Markedly slows phase 0 | ↑↑ QRS width |
| II | β-receptor (sympatholytic) | Suppresses phase 4 | ↓ HR, ↑ PR interval |
| III | K⁺ channel | Prolongs phase 3 | ↑↑ QT interval |
| IV | Ca²⁺ channel | Slows phase 0 of nodal tissue | ↓ HR, ↑ PR interval |
Sicilian Gambit (Alternative Classification)
A newer classification - the Sicilian Gambit - classifies antiarrhythmic drugs based on their specific channel and receptor targets (Na⁺, K⁺, Ca²⁺ channels, α- and β-receptors, and other targets) rather than grouping them into broad classes. It is more mechanistically precise but less commonly used clinically than the Vaughan-Williams system. - Textbook of Family Medicine 9e
Key clinical caveat: Amiodarone is unique in sharing Class I, II, III, and IV actions simultaneously. Despite being classified as Class III, it is the broadest-spectrum antiarrhythmic available, and is the drug of choice for hemodynamically unstable VT/VF. However, its long-term toxicity profile (pulmonary, thyroid, hepatic, ocular) demands careful monitoring. - Harrison's Principles of Internal Medicine 22E; Katzung's Basic and Clinical Pharmacology 16th Ed.
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