Discuss the drug treatment of arrhythmia
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Drug Treatment of Arrhythmias
Overview
Antiarrhythmic drug (AAD) therapy has two goals: termination of an ongoing arrhythmia or prevention of its recurrence. Critically, AADs can themselves cause arrhythmias (proarrhythmia), even during long-term therapy. Drug selection therefore requires accurate diagnosis of arrhythmia type, identification of precipitating factors, and a careful risk-benefit assessment. The advent of catheter ablation and implantable cardioverter-defibrillators (ICDs) has largely relegated drug therapy to a secondary or adjunctive role for serious arrhythmias, though drugs remain important for symptomatic control and reduction of recurrences. — Braunwald's Heart Disease, p. 457
Electrophysiological Basis
Antiarrhythmic drugs act by modulating ion channel conductance or autonomic function:
- Phase 0 (rapid depolarization): driven by fast Na⁺ influx — target of Class I drugs
- Phase 2 (plateau): driven by L-type Ca²⁺ influx — target of Class IV drugs
- Phase 3 (repolarization): driven by K⁺ efflux — target of Class III drugs
- Phase 4 (spontaneous depolarization in pacemaker cells): driven by "funny" current (Iₓ) and β-adrenergic stimulation — targets of Class 0 (ivabradine) and Class II (β-blockers)
Arrhythmias arise from abnormal automaticity, triggered activity (early or delayed afterdepolarizations), or reentry circuits. Most AADs affect more than one ionic current and exert complex, context-dependent effects. — Goodman & Gilman's, p. 687
The Vaughan Williams Classification
The most widely used framework, though limited by its basis on idealized laboratory conditions. A modified classification (incorporating Classes 0–VII) has been proposed to account for additional targets such as connexins, mechanosensitive channels, and gap junctions.
Class I — Sodium Channel Blockers
All Class I drugs block the fast inward Na⁺ channel (Iₙₐ), reducing Vmax (phase 0 upstroke velocity) and slowing conduction. They are subdivided by their kinetics of channel binding and effects on action potential duration (APD):
Class IA — Moderate Na⁺ block + APD prolongation
Drugs: Quinidine, Procainamide, Disopyramide
Mechanism: Reduce Vmax and prolong APD by also blocking IKr (outward K⁺ current). This lengthens the QT interval.
| Drug | Key Features | Adverse Effects |
|---|---|---|
| Quinidine | Alpha-adrenergic and vagolytic effects; available again for Brugada syndrome and idiopathic VF | Cinchonism (tinnitus, visual disturbances), QT prolongation → Torsades de Pointes (TdP), hemolytic anemia, thrombocytopenia |
| Procainamide | IV/IM route available; active metabolite NAPA (Class III effect) | Drug-induced lupus (up to 30% on long-term use), hypotension, agranulocytosis |
| Disopyramide | Strong negative inotrope; strong anticholinergic | Urinary retention, dry mouth, worsens heart failure — avoid in elderly |
— Braunwald's Heart Disease, p. 457; Lippincott Pharmacology, p. 389
Class IB — Weak/rapid Na⁺ block + APD shortening
Drugs: Lidocaine, Mexiletine
Mechanism: Preferentially block inactivated Na⁺ channels (ischemic or rapidly depolarizing tissue); shorten phase 3 repolarization. Fast on/off kinetics.
| Drug | Key Features | Adverse Effects |
|---|---|---|
| Lidocaine | IV only; first-line for acute ventricular arrhythmias in ischemia; wide therapeutic use in ACLS | CNS toxicity: tremor, confusion, seizures; minimal at therapeutic doses |
| Mexiletine | Oral analogue of lidocaine; used for ventricular arrhythmias and long QT syndrome type 3 | Nausea/vomiting, dyspepsia, neurological symptoms; often used with amiodarone |
Class IC — Marked Na⁺ block, minimal APD change
Drugs: Flecainide, Propafenone
Mechanism: Most potent Na⁺ channel blockers; markedly slow conduction (widen QRS). Propafenone also has β-blocking and weak Class IV activity.
| Drug | Key Features | Adverse Effects |
|---|---|---|
| Flecainide | Highly effective for SVT, paroxysmal AF in structurally normal hearts ("pill-in-pocket") | Contraindicated post-MI / structural heart disease — CAST trial showed increased mortality |
| Propafenone | Useful for AF/SVT in normal hearts | Bronchospasm (β-block), metallic taste, worsening HF, liver toxicity |
CAST Warning: The Cardiac Arrhythmia Suppression Trial (CAST) demonstrated that encainide and flecainide increased mortality (7.7% vs 3.0% placebo) in post-MI patients despite suppressing ambient ventricular ectopy. This landmark finding ended empiric use of Class IC drugs in structural heart disease. — Braunwald's Heart Disease, p. 457
Class II — β-Adrenergic Receptor Blockers
Drugs: Metoprolol, Atenolol, Esmolol (IV), Propranolol, Carvedilol
Mechanism: Block β₁-adrenergic receptors → inhibit cAMP-mediated enhancement of If and ICa-L → slow phase 4 depolarization in SA and AV nodes. Reduce heart rate, prolong AV conduction (lengthen PR interval), reduce myocardial oxygen demand.
Clinical uses:
- Rate control in atrial fibrillation/flutter
- Post-MI arrhythmia prevention (proven mortality benefit)
- Supraventricular tachycardias (AVNRT, AVRT)
- Catecholaminergic polymorphic VT (CPVT) — first-line
- Long QT syndrome types 1 and 2
Adverse effects: Bradycardia, heart block, bronchospasm, worsening HF acutely, fatigue, sexual dysfunction, hyperlipidaemia. — Lippincott Pharmacology, p. 389
Class III — Potassium Channel Blockers (Repolarization Prolongers)
Mechanism: Block outward K⁺ currents (predominantly IKr) → prolong phase 3 repolarization → lengthen APD and effective refractory period (ERP) → widen QT interval.
| Drug | Key Features | Adverse Effects |
|---|---|---|
| Amiodarone | Multi-class: I + II + III + IV; broad-spectrum; drug of choice for many sustained ventricular arrhythmias; half-life 40–55 days | Pulmonary toxicity (pneumonitis/fibrosis), hypo/hyperthyroidism, hepatotoxicity, corneal deposits, blue-grey skin, peripheral neuropathy, optic neuritis |
| Dronedarone | Non-iodinated analogue of amiodarone; shorter half-life; less thyroid/pulmonary toxicity; inferior efficacy | Worsens outcomes in HF with reduced EF or permanent AF (PALLAS trial) — contraindicated in decompensated HF |
| Sotalol | Also a β-blocker (Class II + III); racemic mixture | QT prolongation → TdP (dose-dependent); hypotension; requires renal dose adjustment |
| Dofetilide | Pure IKr blocker; used for AF/flutter; hospital initiation required due to QT monitoring | TdP; requires careful renal monitoring |
| Ibutilide | IV only; acute conversion of AF/flutter; very short use window | TdP (4–8% incidence); requires ECG monitoring post-administration |
Amiodarone dosing: Loading 800–1200 mg/day for 1–3 weeks, tapering to maintenance 100–400 mg/day. IV: 150 mg bolus + 1 mg/min × 6 hours, then 0.5 mg/min. Therapeutic serum level: 0.5–1.5 μg/mL. — Braunwald's Heart Disease, p. 465
Class IV — Calcium Channel Blockers (Non-dihydropyridines)
Drugs: Verapamil, Diltiazem
Mechanism: Block L-type Ca²⁺ channels → inhibit slow-response action potentials in SA and AV node tissue → slow rate and prolong AV conduction.
Clinical uses:
- Rate control in AF/flutter
- Termination of AVNRT (AV nodal reentry tachycardia)
- Idiopathic fascicular VT (verapamil-sensitive VT)
Adverse effects: Bradycardia, heart block, hypotension, worsening HF, peripheral oedema, constipation (verapamil).
Note: Verapamil is contraindicated in pre-excitation (WPW) with AF — it may accelerate accessory pathway conduction and cause VF.
Class 0 — HCN Channel Blockers (If Blockers)
Drug: Ivabradine
Mechanism: Selectively blocks the "funny current" (If) in sinus node → pure heart rate reduction without affecting contractility or blood pressure.
Clinical use: Rate control in sinus tachycardia (e.g., heart failure with elevated resting HR), as an adjunct when β-blockers are contraindicated or insufficient. Not a primary antiarrhythmic in the traditional sense but classified under the revised Vaughan Williams scheme. — Braunwald's Heart Disease
Unclassified Antiarrhythmic Drugs
Adenosine
- Mechanism: Activates K⁺ channels (IKACh) in atria and AV node → hyperpolarisation → transient complete AV block
- Use: First-line IV drug for acute termination of AVNRT and AVRT (reentrant SVTs); diagnostic tool for wide-complex tachycardia
- Dose: 6 mg rapid IV bolus; repeat with 12 mg if needed
- Half-life: < 10 seconds (metabolised by endothelial cells and erythrocytes)
- Adverse effects: Transient flushing, dyspnoea, chest pain, bronchospasm (caution in asthma) — Goodman & Gilman's, p. 702
Digoxin
- Mechanism: Inhibits Na⁺/K⁺-ATPase → raises intracellular Ca²⁺ → enhanced vagal tone at AV node (rate-slowing effect); mild positive inotropy
- Use: Rate control in AF (especially with HF and reduced EF); not for acute conversion
- Therapeutic level: 0.5–2.0 ng/mL (narrow therapeutic window)
- Toxicity: Nausea, visual disturbances (yellow halos), bradycardia, AV block, ventricular arrhythmias; toxicity worsened by hypokalaemia
Magnesium (IV)
- Use: Treatment of Torsades de Pointes (regardless of QT interval), digoxin-induced arrhythmias
- Mechanism: Blocks early afterdepolarizations; stabilises the cell membrane
Proarrhythmia — A Critical Concern
All antiarrhythmic drugs can cause arrhythmias:
| Mechanism | Example Drugs | Arrhythmia Caused |
|---|---|---|
| QT prolongation | Class IA, III | Torsades de Pointes (TdP) |
| Conduction slowing | Class IC | Atrial flutter with 1:1 conduction; VT (CAST) |
| Bradycardia / AV block | Class II, IV | Sinus arrest, complete heart block |
| Triggered activity | Digoxin toxicity | DAD-mediated VT |
The CAST trial and subsequent studies fundamentally changed practice: empirical suppression of ventricular ectopy with Class I drugs increases mortality in patients with structural heart disease. — Braunwald's Heart Disease, p. 457
Drug Selection by Arrhythmia Type
| Arrhythmia | First-line Drug(s) | Notes |
|---|---|---|
| AVNRT / AVRT (acute) | Adenosine → Verapamil/Diltiazem → β-blockers | Avoid verapamil in WPW-AF |
| Atrial Fibrillation — rate control | β-blockers, diltiazem, digoxin | Combination often needed |
| AF — rhythm control | Flecainide/propafenone (no SHD), amiodarone, sotalol, dofetilide | CAST precludes IC drugs post-MI |
| Atrial Flutter | Ibutilide (acute conversion); amiodarone, dofetilide (maintenance) | Ablation preferred long-term |
| VT — acute (stable) | Amiodarone IV, lidocaine IV, procainamide IV | ICD preferred for recurrent VT |
| VF / pulseless VT | Amiodarone IV (ACLS algorithm), lidocaine as alternative | After defibrillation |
| TdP / long QT | Magnesium IV, isoproterenol (increase rate), pacing; stop offending drug | |
| CPVT | β-blockers (nadolol preferred), flecainide adjunct | ICD + flecainide in refractory cases |
| Brugada syndrome (VF storms) | Quinidine, isoproterenol | ICD remains primary therapy |
| Digoxin toxicity VT | Digoxin-specific Fab antibodies, magnesium | Avoid Class IA drugs |
Special Populations
- Post-MI / Structural heart disease: Only β-blockers and amiodarone have evidence of not increasing mortality; Class IC drugs are contraindicated
- Heart failure (HFrEF): β-blockers reduce SCD; amiodarone or dofetilide are acceptable for rhythm control; dronedarone contraindicated
- Elderly: Avoid disopyramide (anticholinergic + negative inotropy); reduce lidocaine loading dose; quinidine/procainamide clearance reduced with age — Katzung's Pharmacology, p. 1656
- Pregnancy: Flecainide, sotalol, and digoxin used for fetal arrhythmias; most AADs carry teratogenic risk — Braunwald's Heart Disease, p. 457
- WPW syndrome: Class IA, IC, or III drugs preferred (targeting accessory pathway); avoid AV nodal agents (digoxin, verapamil, adenosine) in pre-excited AF
Summary Table: Vaughan Williams Classification
| Class | Target | Effect on AP | Key Drugs |
|---|---|---|---|
| 0 | HCN (Iₓ) | Slows phase 4 (SA node) | Ivabradine |
| IA | Na⁺ channel (moderate) | ↓ Vmax, prolongs APD/QT | Quinidine, Procainamide, Disopyramide |
| IB | Na⁺ channel (fast off) | ↓ Vmax in ischaemic tissue, shortens APD | Lidocaine, Mexiletine |
| IC | Na⁺ channel (slow off) | Markedly ↓ Vmax, minimal APD change | Flecainide, Propafenone |
| II | β₁-adrenoceptor | Slows phase 4, ↓ HR, prolongs PR | Metoprolol, Esmolol, Atenolol |
| III | K⁺ channel (IKr) | Prolongs phase 3, APD, QT, ERP | Amiodarone, Sotalol, Dofetilide, Ibutilide, Dronedarone |
| IV | L-type Ca²⁺ channel | Slows SA/AV node, prolongs PR | Verapamil, Diltiazem |
| Unclassified | Various | — | Adenosine, Digoxin, Magnesium |
Sources: Goodman & Gilman's Pharmacological Basis of Therapeutics; Braunwald's Heart Disease, 12th Ed.; Lippincott Illustrated Reviews: Pharmacology; Katzung's Basic & Clinical Pharmacology, 16th Ed.
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