8.3 Antiarrhythmic Agents: Vaughan Williams Classes I-IV

Electrophysiological Foundations & Vaughan Williams Class I

Antiarrhythmic pharmacotherapy targets specific ion channels, pumps, and receptors governing the cardiac action potential. Understanding Phase 0 (rapid depolarization via Na⁺ influx), Phase 1 (early repolarization), Phase 2 (plateau via Ca²⁺ influx), Phase 3 (repolarization via K⁺ efflux), and Phase 4 (resting membrane potential / pacemaker If current) is essential for predicting clinical efficacy and proarrhythmic risks.

Vaughan Williams Class I: Fast Sodium Channel Blockers

Class I antiarrhythmics bind to and block fast voltage-gated sodium channels (INa), depressing Phase 0 depolarization velocity (dV/dt) and slowing conduction velocity through atrial and ventricular myocardium.

• Class IA (Procainamide, Quinidine, Disopyramide):
  • Electrophysiology: Moderate INa channel blockade combined with Phase 3 potassium channel (IKr) blockade. Slows conduction velocity, prolongs Action Potential Duration (APD), and extends the Effective Refractory Period (ERP). Produces widening of the QRS complex and prolongation of the QTc interval.
  • Procainamide Indications & Dosing: Indicated for acute termination of stable monomorphic Ventricular Tachycardia (VT) and pre-excited Atrial Fibrillation in Wolff-Parkinson-White (WPW) syndrome. Administered as a continuous IV infusion at 20 to 50 mg/min until: (1) arrhythmia is suppressed; (2) hypotension occurs; (3) QRS complex widens by >50% from baseline; or (4) cumulative maximum dose of 17 mg/kg is reached. Maintenance drip: 1 to 4 mg/min.
  • Active Metabolite & Monitoring: Hepatically acetylated to N-acetylprocainamide (NAPA), an active Class III metabolite cleared renally. High NAPA levels prolong QTc and induce Torsades de Pointes. Long-term use causes drug-induced systemic lupus erythematosus (positive ANA antibodies).
• Class IB (Lidocaine, Mexiletine):
  • Electrophysiology: Weak sodium channel blockade with fast unbinding kinetics; shortens APD and ERP. Selectively targets depolarized, ischemic, or rapidly firing ventricular myocardium.
  • Indications & Limitations: Second-line therapy for acute VT/VF refractory to amiodarone and post-MI ventricular ectopy. Completely INEFFECTIVE for atrial arrhythmias due to short atrial action potential durations.
  • Lidocaine Toxicity Spectrum: High serum levels (>5 mcg/mL) produce central nervous system toxicity: perioral numbness, metallic taste, auditory tinnitus, slurred speech, confusion, muscle twitching, tremors, visual hallucinations, and grand mal seizures.
• Class IC (Flecainide, Propafenone):
  • Electrophysiology: Potent, slow-binding INa channel blockade. Markedly slows Phase 0 depolarization and cardiac conduction (significant QRS prolongation) with minimal effect on APD or ERP.
  • Indications: "Pill-in-the-pocket" strategy for paroxysmal Atrial Fibrillation in patients with structurally normal hearts.
  • Critical AACN Safety Warning: Absolutely CONTRAINDICATED in structural heart disease (prior MI, CAD, LVH, HFrEF). The landmark Cardiac Arrhythmia Suppression Trial (CAST) proved that Class IC agents significantly increase proarrhythmic cardiac mortality in patients with underlying ischemic structural heart disease.

Classes II–IV, Amiodarone Protocols & Miscellaneous Agents

Vaughan Williams Classes II, III & IV

• Class II: Beta-Adrenergic Blockers (Esmolol, Metoprolol, Carvedilol): Depress Phase 4 pacemaker depolarization in SA and AV nodes. Decrease sinus rate, prolong AV nodal conduction, and suppress sympathetic-driven automaticity. Esmolol has an ultra-short half-life of 9 minutes (metabolized by erythrocyte esterases), allowing rapid IV titration in acute SVT, AFib with Rapid Ventricular Response (RVR), and aortic dissection.
• Class III: Potassium Channel Blockers (Amiodarone, Sotalol, Dofetilide, Dronedarone): Inhibit outward delayed rectifier potassium currents (IKr), prolonging Phase 3 repolarization, APD, ERP, and QTc interval.
  • Amiodarone (Cordarone):
    • Multi-channel Properties: Possesses Class I (sodium block), Class II (beta block), Class III (potassium block), and Class IV (calcium block) actions.
    • ACLS & Clinical Dosing: Pulseless VT/VF arrest: 300 mg IV push, optional second dose 150 mg IV push. Stable VT or AFib rhythm control: 150 mg IV over 10 min, followed by 1 mg/min for 6 hours, then 0.5 mg/min for 18 hours (1000 mg total over 24 hours). Oral transition: 400 mg TID until 10 g cumulative load, then 200 mg daily.
    • Comprehensive Organ Toxicities & Nursing Surveillance:
      • Pulmonary Toxicity: Subacute interstitial pneumonitis and pulmonary fibrosis (2–5% incidence). Symptoms: dry cough, progressive dyspnea, pleuritic chest pain. Requires baseline Chest X-ray and Pulmonary Function Tests (PFTs with DLCO).
      • Thyroid Dysfunction: Contains 37% iodine by weight (75 mg organic iodine per 200 mg tablet). Triggers hypothyroidism (elevated TSH) or hyperthyroidism (thyrotoxicosis, suppressed TSH). Baseline and 6-month TSH/free T4 monitoring required.
      • Hepatic Toxicity: Elevated AST/ALT transaminases; monitor baseline and periodic LFTs. Discontinue if transaminases exceed 3 times upper limit of normal.
      • Ocular Toxicity: Corneal microdeposits present in >90% of patients (asymptomatic); rare optic neuritis causing permanent blindness.
      • Dermatologic & Cardiac: Photosensitivity and slate-blue skin discoloration. Bradycardia, AV block, and QTc prolongation (notably lower Torsades risk than other Class III agents due to concurrent calcium/beta blockade).
  • Sotalol (Betapace): Non-selective beta-blocker with Class III potassium channel blocking action. Prolongs QTc; renally eliminated. High Torsades risk. Requires mandatory 3-day in-hospital telemetry monitoring during initiation.
  • Dofetilide (Tikosyn): Pure IKr blocker for AFib conversion/maintenance. Mandatory 3-day in-hospital telemetry initiation; initial dosing strictly calculated using baseline QTc and Cockcroft-Gault creatinine clearance. After the first dose, a QTc rise of more than 15% or a QTc above 500 ms (550 ms with intraventricular conduction delay) requires halving the dose, not stopping it; discontinuation is required if the QTc exceeds those limits at any point after the second dose.
  • Dronedarone (Multaq): Non-iodinated amiodarone analog; contraindicated in NYHA Class III–IV heart failure (ANDROMEDA trial showed doubled mortality).
• Class IV: Non-Dihydropyridine Calcium Channel Blockers (Diltiazem, Verapamil): Block L-type calcium channels, slowing Phase 0 depolarization in SA and AV nodes. Primary indication: Rate control in AFib/AFl and SVT conversion. Absolutely contraindicated in wide-complex tachycardia of unknown origin, pre-excited AFib in WPW, and severe HFrEF.

Miscellaneous Antiarrhythmic Agents: Adenosine & Digoxin

• Adenosine (Adenocard):
  • Mechanism of Action: Endogenous purine nucleoside. Binds to myocardial and nodal A₁ adenosine receptors, activating inward rectifier potassium channels (IK,Ado) and hyperpolarizing the cell membrane while inhibiting cAMP. Produces complete, transient AV nodal conduction block.
  • Pharmacokinetics: Elimination half-life is <10 seconds due to rapid enzymatic uptake by erythrocytes and vascular endothelial cells.
  • Administration Protocol: Rapid IV push via proximal IV site (antecubital or central line) at 6 mg rapid bolus over 1–2 seconds, followed immediately by a rapid 20 mL 0.9% saline flush. If no conversion within 1–2 minutes, administer a second rapid bolus of 12 mg with a 20 mL flush.
  • Clinical Guidance: Patient experiences transient severe chest tightness, flushing, dyspnea, and impending doom. Transient sinus pause/asystole on monitor for 2–6 seconds is expected. Reduce dose to 3 mg if given via central line or to patients taking dipyridamole; increase dose in patients consuming caffeine or theophylline (A₁ receptor antagonists).
• Digoxin (Lanoxin):
  • Mechanism of Action: Inhibits cardiac cell membrane Na⁺/K⁺ ATPase pump → increases intracellular sodium → reduces Na⁺/Ca²⁺ exchanger activity → increases intracellular calcium available for sarcoplasmic reticulum storage → positive inotropy. Simultaneously enhances central vagal tone, slowing SA node rate and AV node conduction (negative chronotropy/dromotropy).
  • Therapeutic Drug Monitoring: Target serum concentration for Heart Failure is 0.5 to 0.9 ng/mL. Serum levels >1.2 ng/mL offer no extra benefit and increase mortality. Draw trough blood levels at least 6–8 hours post-dose.
  • Digoxin Toxicity Manifestations:
    • Visual: Pathognomonic xanthopsia (yellow-green halos around light sources), blurred vision, scotomas.
    • GI & Neurological: Anorexia, nausea, vomiting, abdominal pain (earliest non-cardiac signs), delirium, confusion.
    • Cardiac Dysrhythmias: Increased automaticity plus AV conduction block: PVCs, bigeminy, paroxysmal atrial tachycardia with AV block, junctional escape rhythms, and bidirectional VT.
  • Precipitating Electrolytes: Hypokalemia, hypomagnesemia, and hypercalcemia markedly potentiate digoxin toxicity.
  • Reversal Therapy: Digoxin Immune Fab (Digibind / DigiFab). Antigen-binding fragments bind intravascular free digoxin. Indicated for life-threatening dysrhythmias, severe hyperkalemia (K⁺ > 5.0 mEq/L in acute ingestion), or acute ingestion of >10 mg in adults.