Antiarrhythmic drugs are among the most concept-heavy and exam-oriented topics in pharmacology and cardiology. Understanding how each drug affects ion channels, action potential, and ECG is essential for MBBS, nursing, pharmacy students, and emergency care.
Overview: Vaughan Williams Classification (Foundation)
Antiarrhythmic drugs are classified based on their primary effect on cardiac action potential.
| Class | Main Action |
|---|---|
| Class I | Na⁺ channel blockers |
| Class II | β-blockers |
| Class III | K⁺ channel blockers |
| Class IV | Ca²⁺ channel blockers |
Drugs in this article:
1. Procainamide – Class IA
Cardiac Action Potential: Why These Drugs Matter
1. Phase 0 → Na⁺ influx (depolarization)Antiarrhythmics work by modifying these phases.
Procainamide
Drug Class
Class IA antiarrhythmic
Mechanism of Action
- Blocks fast Na⁺ channels
- Also blocks K⁺ channels
- Slows conduction + prolongs action potential
ECG effect:
↑ QRS, ↑ QT interval
Clinical Uses
- Atrial arrhythmias
- Ventricular arrhythmias
- Drug of choice in WPW with atrial fibrillation
Important Adverse Effects
- Drug-induced lupus erythematosus
- QT prolongation → torsades de pointes
- Hypotension (IV)
Exam Pearl
Procainamide = WPW + lupus risk
Quinidine
Drug Class
Class IA antiarrhythmic
Mechanism of Action
- Blocks Na⁺ channels
- Blocks K⁺ channels
- Prolongs action potential & refractory period
ECG effect:
↑ QT interval
Clinical Uses
- Atrial arrhythmias
- Ventricular arrhythmias (rare today)
Major Adverse Effects (Very High-Yield)
1. Cinchonism:
- Tinnitus
- Headache
- Dizziness
- Blurred vision
Exam Pearl
Quinidine causes cinchonism
Lidocaine (Parenteral)
Drug Class
Class IB antiarrhythmic
Mechanism of Action
- Weak Na⁺ channel blocker
- Acts preferentially on ischemic or depolarized ventricular tissue
- Shortens action potential
ECG effect:
↓ QT interval (or minimal change)
Clinical Uses
- Acute ventricular arrhythmias
- Post-myocardial infarction arrhythmias
- Digitalis-induced arrhythmias
Used only IV (parenteral)
Adverse Effects
CNS toxicity:
- Drowsiness
- Confusion
- Seizures (high dose)
Exam Pearl
Lidocaine = post-MI ventricular arrhythmias
Amiodarone
Drug Class
Primarily Class III, but has Class I, II, III & IV actions
Mechanism of Action
1. Blocks K⁺ channels → prolongs repolarization
2. Also blocks:
- Na⁺ channels
- β-receptors
- Ca²⁺ channels
ECG effect:
↑ QT interval (but low torsades risk)
Clinical Uses (Very Important)
- Atrial fibrillation
- Ventricular tachycardia
- Ventricular fibrillation
- Most effective broad-spectrum antiarrhythmic
Used in cardiac arrest algorithms
Adverse Effects (Classic Exam Topic)
| System | Effect |
|---|---|
| Lungs | Pulmonary fibrosis |
| Thyroid | Hypo- or hyperthyroidism |
| Liver | Hepatotoxicity |
| Eyes | Corneal deposits |
| Skin | Blue-gray discoloration |
| Heart | Bradycardia |
Iodine-rich drug
Exam Pearl
Amiodarone = effective but toxic
High-Yield Comparison Table
| Drug | Class | AP Duration | Key Use | Key Toxicity |
|---|---|---|---|---|
| Procainamide | IA | ↑ | WPW | Lupus |
| Quinidine | IA | ↑ | Atrial arrhythmia | Cinchonism |
| Lidocaine | IB | ↓ | Post-MI VT | CNS toxicity |
| Amiodarone | III | ↑ | AF, VT, VF | Multi-organ |
ECG Effects Summary (Exam Gold)
| Drug Class | QT Interval |
|---|---|
| Class IA | ↑ |
| Class IB | ↓ / same |
| Class III | ↑ |
Torsades de Pointes Risk
High risk with:
- Quinidine
- Procainamide
Low risk with:
- Amiodarone (important exception)
Important Exam-Oriented Pearls
- Class IA → QT prolongation
- Class IB → ischemic tissue selective
- Amiodarone has the longest half-life
- Lidocaine is IV only
- Procainamide is best in WPW + AF
Easy Memory Tricks
- “IA = Increase Action potential”
- “IB = Ischemic Blocks”
- “Quini-dine → Ringing ears (cinchonism)”
- “Amio = Almost everything blocked”
FAQs
1. Which drug is best for WPW with atrial fibrillation?
Procainamide.
2. Which antiarrhythmic causes cinchonism?
Quinidine.
3. Which drug is used in post-MI ventricular arrhythmias?
Lidocaine.
4. Why is amiodarone widely used despite toxicity?
Because it is highly effective against multiple arrhythmias.
5. Which drugs prolong QT interval?
Procainamide, quinidine, and amiodarone.
6. Which antiarrhythmic has iodine?
Amiodarone.
7. Can lidocaine be given orally?
No, only parenteral use.
8. Which drug causes drug-induced lupus?
Procainamide.
9. Which antiarrhythmic has the longest half-life?
Amiodarone.
10. Which drug is safest in heart failure?
Amiodarone.
