Anti arrhythmic drug classification
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Antiarrhythmic Drug Classification
Overview
Antiarrhythmics are classified using the Vaughan-Williams (Singh-Vaughan Williams) classification, which groups drugs by their predominant effect on the cardiac action potential. There are four main classes (I-IV), plus miscellaneous agents. Most drugs work by suppressing abnormal automaticity or interrupting reentry circuits.
Important limitation: Many drugs have actions spanning multiple classes (e.g., amiodarone has class I, II, III, and IV actions). The classification reflects the predominant action, not the exclusive one.
- Katzung's Basic & Clinical Pharmacology, 16th Ed., p. 369
Class I - Sodium Channel Blockers
Block fast Na+ channels, slowing phase 0 depolarization (upstroke). Further divided by kinetics and effect on action potential duration (APD):
| Subclass | APD Effect | Kinetics | Drugs |
|---|---|---|---|
| IA | Prolongs APD | Intermediate dissociation | Quinidine, Procainamide, Disopyramide |
| IB | Shortens APD | Fast dissociation | Lidocaine, Mexiletine |
| IC | No effect on APD | Slow dissociation | Flecainide, Propafenone |
Class IA
- Mechanism: Block Na+ channels + block K+ channels (prolonging repolarization) → prolong QT interval
- Drugs: Quinidine, Procainamide, Disopyramide
- Clinical uses: Atrial flutter/fibrillation, ventricular tachycardia
- Key adverse effects:
- Quinidine: Cinchonism (tinnitus, headache, blurred vision), hemolytic anemia, torsades de pointes
- Procainamide: Drug-induced lupus-like syndrome, hypotension
- Disopyramide: Strong anticholinergic effects (dry mouth, urinary retention, blurred vision), negative inotropy
Class IB
- Mechanism: Preferentially bind inactivated Na+ channels (ischemic/depolarized tissue) → shorten APD
- Drugs: Lidocaine (IV only), Mexiletine (oral)
- Clinical uses: Ventricular arrhythmias, post-MI arrhythmias; ineffective for atrial arrhythmias
- Key adverse effects: CNS toxicity (tremor, seizures, confusion at toxic doses); mexiletine causes GI upset
Class IC
- Mechanism: Most potent Na+ channel blockers - markedly slow phase 0; minimal effect on APD
- Drugs: Flecainide, Propafenone
- Clinical uses: Atrial fibrillation/flutter (rhythm control); contraindicated in structural heart disease
- Key adverse effects: Proarrhythmic (can promote reentrant VT/ventricular flutter); propafenone has mild beta-blocker activity
- CAST trial warning: Increased mortality in post-MI patients - avoid in structural heart disease
Class II - Beta-Adrenergic Blockers
- Mechanism: Block beta-1 receptors → reduce sympathetic drive → slow phase 4 spontaneous depolarization (automaticity) in SA and AV nodes; slow AV conduction
- Drugs: Metoprolol, Atenolol, Esmolol (ultra-short IV), Propranolol, Carvedilol
- Clinical uses: Rate control in AF/flutter, SVT, post-MI arrhythmias, exercise-induced arrhythmias
- Key adverse effects: Bradycardia, heart block, bronchospasm, hypotension, fatigue, worsening heart failure (acute)
Class III - Potassium Channel Blockers
- Mechanism: Block K+ channels (predominantly IKr - rapid delayed rectifier) → prolong phase 3 repolarization → prolong APD and effective refractory period (ERP) → widen QT interval
- Effect on ECG: QT prolongation
| Drug | Key Notes |
|---|---|
| Amiodarone | Has Class I + II + III + IV actions; most effective antiarrhythmic drug; multiple organ toxicity with long-term use |
| Sotalol | Also has significant beta-blocker (Class II) activity |
| Dofetilide | Pure IKr blocker; dose adjusted for renal function; risk of torsades |
| Ibutilide | IV only; used for acute cardioversion of AF/flutter |
| Dronedarone | Amiodarone analog without iodine; milder and less toxic; has mild Class I, II, III, IV effects |
Amiodarone adverse effects (extensive due to multi-organ distribution):
- Pulmonary toxicity (pneumonitis, fibrosis)
- Hepatotoxicity
- Hypo- or hyperthyroidism (contains ~37% iodine by weight)
- Corneal microdeposits, optic neuritis
- Blue-gray skin discoloration (with prolonged use)
- Peripheral neuropathy
- Bradycardia, QT prolongation
Class IV - Calcium Channel Blockers (Non-dihydropyridine)
- Mechanism: Block L-type Ca2+ channels → slow phase 0 of SA/AV node action potentials (which are Ca2+-dependent, unlike ventricular myocytes) → slow AV conduction
- Drugs: Verapamil, Diltiazem
- Clinical uses: Rate control in AF/flutter, PSVT (termination and prophylaxis)
- Key adverse effects: Bradycardia, AV block, hypotension, worsening heart failure; verapamil causes constipation
- Contraindication: Do NOT combine with beta-blockers (risk of complete heart block); avoid in WPW syndrome with AF
Miscellaneous / "Other" Agents
These do not fit neatly into the Vaughan-Williams schema:
| Drug | Mechanism | Use |
|---|---|---|
| Adenosine | Activates A1 receptors → hyperpolarizes AV node (opens K+ channels, inhibits Ca2+ current) | First-line for acute PSVT termination |
| Digoxin | Inhibits Na+/K+ ATPase → vagotonic effect on AV node (indirect) | Rate control in AF; narrow therapeutic index |
| Magnesium sulfate | Multiple mechanisms; stabilizes membranes | Torsades de pointes, digitalis toxicity |
| Ranolazine | Late Na+ channel blockade + IKr blockade | Refractory atrial/ventricular arrhythmias |
| Ivabradine | Blocks If (funny current) in SA node | Inappropriate sinus tachycardia, HFrEF |
ECG Effects by Class
| Class | PR interval | QRS duration | QT interval |
|---|---|---|---|
| IA | Slight ↑ | ↑ | ↑↑ |
| IB | No change | No change | ↓ or no change |
| IC | No change | ↑↑ | No change |
| II | ↑ | No change | No change |
| III | No change | No change | ↑↑ |
| IV | ↑↑ | No change | No change |
Proarrhythmia - Critical Concept
All antiarrhythmic drugs can paradoxically cause arrhythmias. Mechanisms include:
- QT prolongation (Classes IA, III) → torsades de pointes
- Reentry promotion (Class IC) → monomorphic VT
- Bradyarrhythmias (Classes II, IV)
- Risk increases with structural heart disease, hypokalemia, hypomagnesemia, fast heart rates, and ischemia.
Lippincott Illustrated Reviews: Pharmacology, p. 388-389 | Katzung's Basic & Clinical Pharmacology, 16th Ed., pp. 369-370 | Harrison's Principles of Internal Medicine 22E, p. 1959
Key Memory Aid
"Some Block Potassium Channels"
- Sodium (Class I)
- Beta-blockers (Class II)
- Potassium channel blockers (Class III)
- Calcium channel blockers (Class IV)
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