Cardiology > Antiarrhythmic Drugs

Background 

Antiarrhythmic drugs are pharmacological agents used to prevent, suppress, or treat cardiac arrhythmias—abnormal heart rhythms that arise from disturbances in impulse formation, conduction, or both. These drugs act by modifying electrophysiologic properties of cardiac tissues, typically by blocking ion channels or altering autonomic influences on the heart. Antiarrhythmics are utilized in both acute and chronic management of supraventricular and ventricular arrhythmias. 

Classification 

By Vaughan-Williams Classification 

• • Class I: Sodium channel blockers 

• • IA: Quinidine, Procainamide, Disopyramide 

• • IB: Lidocaine, Mexiletine 

• • IC: Flecainide, Propafenone 

• • Class II: Beta-blockers 

• • Metoprolol, Esmolol, Propranolol 

• • Class III: Potassium channel blockers 

• • Amiodarone, Dofetilide, Sotalol, Ibutilide 

• • Class IV: Calcium channel blockers 

• • Verapamil, Diltiazem 

Other Agents 

• • Adenosine: AV nodal blocker used for acute supraventricular tachycardia 

• • Digoxin: Enhances vagal tone, used for rate control in atrial fibrillation 

Mechanism of Toxicity 

Arrhythmias result from abnormalities in impulse generation (automaticity), triggered activity (early or delayed afterdepolarizations), or impulse conduction (reentry). Antiarrhythmic drugs target these mechanisms by: 

• • Modifying ion channel activity (Na⁺, K⁺, Ca²⁺) 

• • Altering refractory periods 

• • Suppressing ectopic pacemakers 

• • Slowing conduction through the AV node or accessory pathways 

Class I agents primarily affect depolarization; Class III drugs prolong repolarization. Class II and IV agents modulate AV nodal conduction and refractoriness via autonomic influence. 

Indications for Use 

• • Atrial fibrillation/flutter (rate or rhythm control) 

• • Paroxysmal supraventricular tachycardia (PSVT) 

• • Ventricular tachycardia/fibrillation 

• • Arrhythmias post-MI or in structural heart disease 

 Indications/Goals 

• • Termination or prevention of arrhythmias 

• • Symptom relief: palpitations, dizziness, syncope 

• • Stroke prevention (via rhythm control) 

• • Hemodynamic stabilization in acute settings 

Drug-Specific Side Effects 

• • Amiodarone: Pulmonary fibrosis, thyroid dysfunction, liver toxicity, corneal deposits 

• • Sotalol: Bradycardia, torsades de pointes 

• • Flecainide: Proarrhythmia, especially in structural heart disease 

• • Adenosine: Transient flushing, chest discomfort 

• • Digoxin: Nausea, visual changes, AV block, toxicity in renal impairment 

Essential Tests 

Baseline/Monitoring 

• • ECG: Rhythm type, PR/QRS/QT intervals 

• • Electrolytes: Potassium, magnesium 

• • Thyroid function tests: For amiodarone users 

• • Liver and pulmonary function tests: Baseline for amiodarone 

• • Digoxin level: In suspected toxicity 

Drug-Specific Indications 

• • PSVT: Vagal maneuvers → Adenosine 

• • AFib with RVR: IV beta-blockers, calcium channel blockers, digoxin 

• • VT/VF: Amiodarone, sotalol or lidocaine (in post-MI); defibrillation if pulseless 

• • Torsades de pointes: IV magnesium sulfate, pacing 

• • WPW syndrome: Procainamide or ablation 

Medications 

Patient Education 

• • • Educate on medication side effects and need for regular ECG/labs 

• • • Avoid QT-prolonging drugs in patients on Class III agents 

• • • Inform about signs of toxicity (amiodarone: cough; digoxin: vision changes) 

• • • Encourage medication adherence and electrolyte monitoring 

• • • Advise vaccination against influenza, pneumococcus, and COVID-19 

Consults 

• • Cardiologist: For medication selection in structural heart disease 

• • Pharmacist: Drug interaction review, especially in polypharmacy 

• • Endocrinologist: For amiodarone-induced thyroid disease 

Monitoring 

Short-Term 

• • Monitor ECG, renal/hepatic function, drug levels (e.g., digoxin) 

• • Recheck QT interval with initiation or dose changes of Class III agents 

Long-Term 

• • Periodic ECG, echo (in structural disease) 

• • Lung, thyroid, liver surveillance for amiodarone 

• • Holter monitoring for rhythm recurrence