Antiarrhythmic Drugs

The Vaughan-Williams classification, first proposed in 1970 and subsequently modified, remains the most widely used framework for categorizing antiarrhythmic drugs by their predominant electrophysiologic mechanism. While an oversimplification — many agents act on multiple ion channels — it provides a clinically useful schema for understanding drug effects on cardiac action potential duration, conduction velocity, and refractoriness.

The Sicilian Gambit (1991) proposed an alternative, channel-receptor-specific classification that more accurately reflects the multifaceted pharmacology of antiarrhythmic agents. Rather than assigning drugs to a single class, it maps each drug's effects on specific ion channels (Na⁺, K⁺, Ca²⁺), receptors (alpha, beta, muscarinic), and transporters. While academically superior, the Sicilian Gambit has not supplanted the Vaughan-Williams system in daily clinical practice due to its complexity.

Class I antiarrhythmics block voltage-gated sodium channels, reducing the rate of phase 0 depolarization and slowing conduction velocity. They are subdivided by their kinetics of binding and dissociation from the sodium channel, which determines their relative effects on action potential duration, QRS width, and use-dependence.

Class Ia — Intermediate Kinetics

Procainamide is the most commonly used Class Ia agent in the acute setting. IV procainamide is guideline-recommended for hemodynamically stable monomorphic VT and is used for pharmacologic provocation of Brugada pattern on ECG (Class I sodium channel challenge). It blocks I_Na and I_Kr, prolonging both QRS duration and QTc interval. Chronic oral use has largely fallen out of favor due to the risk of drug-induced lupus (positive ANA in up to 80% of patients on long-term therapy, with clinical lupus in 20–30%).

Quinidine remains clinically relevant for Brugada syndrome (suppresses I_to-mediated arrhythmogenic substrate), short QT syndrome, and idiopathic VF refractory to other therapies. It blocks I_Na, I_Kr, I_to, and I_K1. Notable for vagolytic (anticholinergic) effects that can paradoxically accelerate AV conduction in atrial flutter. GI side effects (diarrhea, nausea) limit tolerability. Monitor QTc closely; quinidine syncope is a historical term for TdP.

Disopyramide is uniquely useful for hypertrophic obstructive cardiomyopathy (HOCM) due to its potent negative inotropic effect, which reduces dynamic LVOT obstruction. Strong anticholinergic side effects (urinary retention, dry mouth, constipation) limit use. Contraindicated in uncompensated HF and glaucoma.

Class Ib — Fast Kinetics

Lidocaine is the prototypical Class Ib agent, available only as IV formulation. It preferentially binds to inactivated sodium channels and has rapid binding/unbinding kinetics, making it most effective in ischemic or depolarized tissue. Indicated for VT storm (especially ischemic VT) and pulseless VT/VF as a second-line agent after amiodarone. Undergoes extensive first-pass hepatic metabolism (CYP1A2, CYP3A4); dose reduction required in hepatic impairment and low cardiac output states. CNS toxicity (tremor, seizures, confusion) is the primary dose-limiting side effect.

Mexiletine is the oral congener of lidocaine. Used as adjunctive therapy for ventricular arrhythmias (often combined with amiodarone or sotalol) and for LQT3 (gain-of-function sodium channel mutations) where it can shorten the QTc. Hepatically metabolized (CYP2D6, CYP1A2). GI side effects and tremor are common; take with food to improve tolerability.

Class Ic — Slow Kinetics

Flecainide is a potent sodium channel blocker with slow dissociation kinetics and marked use-dependence (greater effect at faster heart rates). It prolongs QRS duration without significantly affecting repolarization. Highly effective for AF suppression, atrial flutter, and SVT. The CAST trial (1989) demonstrated increased mortality in post-MI patients with asymptomatic PVCs treated with flecainide or encainide, leading to a strict contraindication in structural heart disease (ischemic cardiomyopathy, significant LVH, HFrEF).

The “pill-in-pocket” strategy allows selected patients with infrequent, well-tolerated paroxysmal AF to self-administer a single loading dose of flecainide (200–300 mg) at symptom onset, after initial supervised trial in a monitored setting. A concomitant AV nodal blocker (beta-blocker or non-dihydropyridine CCB) is mandatory to prevent 1:1 conduction of organized atrial flutter (Class Ic agents can slow atrial rate to a range that allows 1:1 AV conduction).

Propafenone shares Class Ic sodium channel blockade with additional mild beta-blocking (Class II) and calcium channel blocking (Class IV) properties. Available in immediate-release (TID dosing) and sustained-release (BID) formulations. Similar contraindications to flecainide. The beta-blocking component may cause bronchospasm in asthmatics. Metabolized by CYP2D6; 7% of the population are poor metabolizers with higher drug levels.

Beta-blockers (Class II) and non-dihydropyridine calcium channel blockers (Class IV) are the most commonly used agents for rate control in atrial fibrillation/flutter, acute management of SVT, and reduction of ventricular arrhythmia burden. Beta-blockers are the only antiarrhythmic class with demonstrated mortality benefit in post-MI and HFrEF populations.

Class II — Beta-Blockers

Metoprolol (selective beta-1) is the most commonly used beta-blocker in EP practice. IV metoprolol (2.5–5 mg q5min, max 15 mg) is used for acute rate control in AF/flutter and SVT. Oral metoprolol succinate (25–200 mg daily) provides chronic rate control and is first-line in HFrEF.

Atenolol (selective beta-1) offers once-daily dosing and renal elimination (dose-adjust for CKD). Less lipophilic than metoprolol with fewer CNS side effects. No mortality benefit demonstrated in HFrEF.

Nadolol (non-selective) is the preferred beta-blocker for long QT syndrome (LQTS) and catecholaminergic polymorphic VT (CPVT) due to its long half-life providing consistent 24-hour coverage and evidence of superior arrhythmia suppression compared to selective beta-blockers. Renally eliminated; dose-adjust for CKD.

Propranolol (non-selective) is lipophilic with extensive hepatic metabolism. Used for thyrotoxicosis-associated arrhythmias (also blocks peripheral T4→T3 conversion), infantile LQTS, and anxiety-related sinus tachycardia. Available in IV form for acute use.

Esmolol (selective beta-1, ultra-short-acting) has a half-life of only 9 minutes, making it ideal for acute rate control when hemodynamic stability is uncertain. Administered as IV infusion (loading dose 500 mcg/kg over 1 min, then 50–200 mcg/kg/min). Metabolized by red blood cell esterases, independent of hepatic/renal function.

Class IV — Non-Dihydropyridine Calcium Channel Blockers

Verapamil is a potent AV nodal blocker used for acute SVT termination (2.5–5 mg IV over 2 min, may repeat 5–10 mg), rate control in AF/flutter, and the treatment of fascicular VT (verapamil-sensitive, left posterior fascicular VT). Verapamil is strictly contraindicated in WPW with AF (pre-excited AF) as AV nodal blockade can divert conduction exclusively down the accessory pathway, precipitating VF. Also contraindicated in HFrEF (negative inotropy) and in wide-complex tachycardia of uncertain mechanism.

Diltiazem provides similar AV nodal blockade with somewhat less negative inotropy than verapamil. IV diltiazem (0.25 mg/kg bolus over 2 min, repeat 0.35 mg/kg if needed, then infusion 5–15 mg/hr) is a mainstay for acute rate control in AF/flutter. Oral diltiazem (120–360 mg/day extended-release) for chronic rate control. Same contraindications as verapamil regarding pre-excited AF and HFrEF.

Class III antiarrhythmics prolong repolarization by blocking potassium channels (primarily I_Kr), extending the effective refractory period and action potential duration. This class includes the most potent and most toxic antiarrhythmic drugs. All carry risk of QTc prolongation and torsades de pointes (TdP), requiring careful monitoring.

Amiodarone

Amiodarone is the most effective and most widely used antiarrhythmic drug, possessing properties of all four Vaughan-Williams classes: sodium channel blockade (Class I), beta-blockade (Class II), potassium channel blockade (Class III), and calcium channel blockade (Class IV). It also has anti-thyroid properties (contains 37% iodine by weight). Despite its extraordinary efficacy, amiodarone carries a substantial toxicity burden that requires systematic surveillance.

IV loading: For VT/VF arrest: 300 mg IV bolus (may repeat 150 mg). For stable VT or AF: 150 mg over 10 min, then 1 mg/min for 6 h, then 0.5 mg/min for 18 h (total ~1050 mg/24 h). Oral loading: 400–800 mg/day for 2–4 weeks (total loading dose 8–10 g), then maintenance 100–200 mg daily. The extraordinarily long half-life (40–55 days) means steady state is not reached for months, and effects persist for weeks to months after discontinuation.

Drug interactions: Amiodarone inhibits CYP2C9, CYP2D6, CYP3A4, and P-glycoprotein. It doubles digoxin levels (reduce digoxin dose by 50%) and increases warfarin effect by 30–50% (reduce warfarin dose by one-third to one-half and monitor INR closely). It can potentiate the bradycardic effects of beta-blockers and calcium channel blockers.

Sotalol

Sotalol combines non-selective beta-blockade (Class II, both enantiomers) with potassium channel blockade (Class III, primarily the d-enantiomer). It is the racemic (d,l) mixture. Class III effects are dose-dependent: meaningful QTc prolongation typically occurs at doses ≥160 mg/day. Sotalol exhibits reverse use-dependence — greater QTc prolongation at slower heart rates — increasing TdP risk during bradycardia.

Dosing by CrCl: CrCl >60 mL/min: 80–160 mg PO q12h; CrCl 40–60: q24h dosing; CrCl <40: contraindicated or use with extreme caution (extended intervals). Maximum dose 320 mg/day for ventricular arrhythmias, 240 mg/day for AF. Must be initiated in a monitored setting (inpatient telemetry for ≥3 days or 5 doses) with QTc monitoring: hold if QTc >500 ms (or >520 ms if QRS >120 ms).

Dofetilide

Dofetilide is a pure Class III agent (selective I_Kr blocker) approved for AF/flutter cardioversion and maintenance of sinus rhythm. It is the most tightly regulated antiarrhythmic: mandatory 3-day inpatient initiation with continuous telemetry, serial QTc monitoring, and strict renal dose adjustment. Prescribing is restricted to certified physicians who have completed the manufacturer’s education program (REMS).

Renal dosing (strict CrCl-based):

QTc limits for dose adjustment: Measure QTc 2–3 hours after each dose. If QTc increases by >15% from baseline or exceeds 500 ms (or >550 ms if QRS >120 ms), the dose must be reduced or the drug discontinued. If the starting dose is 500 mcg BID, reduce to 250 mcg BID; if 250 mcg BID, reduce to 125 mcg BID; if 125 mcg BID, discontinue.

Absolute drug interaction contraindications (these drugs inhibit renal cation transport and increase dofetilide levels, risking TdP):

Ibutilide

Ibutilide is an IV-only Class III agent that activates slow inward sodium current and blocks I_Kr, prolonging repolarization. Used for acute pharmacologic cardioversion of AF/flutter (more effective for flutter than AF). Dose: 1 mg IV over 10 min; may repeat once after 10 min if ineffective. Conversion rate: ~50% for atrial flutter, ~30% for AF. QTc monitoring for at least 4–6 hours post-infusion is mandatory. TdP risk is 2–4%, higher in patients with hypokalemia, hypomagnesemia, or pre-existing QTc prolongation. Pre-treat with IV magnesium (1–2 g) to reduce TdP risk. Often used to facilitate DC cardioversion (“ibutilide-facilitated cardioversion”).

Dronedarone

Dronedarone is a non-iodinated benzofuran derivative structurally related to amiodarone but lacking the iodine moiety, resulting in a shorter half-life (~13–19 hours) and significantly reduced thyroid and pulmonary toxicity. It has multichannel blocking properties (I_Kr, I_Ks, I_Na, I_Ca,L) with antiadrenergic effects. Indicated for maintenance of sinus rhythm in patients with non-permanent AF/flutter without severe HF.

The ATHENA trial (2009) demonstrated that dronedarone reduced the composite of cardiovascular hospitalization or death in patients with paroxysmal or persistent AF. However, the ANDROMEDA trial showed increased mortality in patients with NYHA Class III–IV HF (recently decompensated), and the PALLAS trial showed harm in permanent AF. Therefore, dronedarone is contraindicated in NYHA Class III–IV HF and in permanent AF. Dose: 400 mg PO BID with meals. Causes a benign increase in serum creatinine (inhibits tubular secretion, not true GFR change).

Several important antiarrhythmic agents do not fit neatly into the Vaughan-Williams classification. These drugs act through distinct mechanisms including purinergic receptor activation, cardiac glycoside effects, funny current (I_f) inhibition, and late sodium current blockade.

Adenosine

Adenosine activates the A1 purinergic receptor on AV nodal cells, opening I_K,Ado (acetylcholine-sensitive potassium channels) and inhibiting I_Ca,L via Gi protein signaling. This produces transient, near-complete AV block lasting 10–15 seconds. Half-life is <10 seconds (rapidly deaminated by red blood cell adenosine deaminase and endothelial cell uptake).

Dosing: 6 mg rapid IV push via large-bore peripheral or central line, followed by 20 mL saline flush → if no effect in 1–2 min, 12 mg → may repeat 12 mg once. Must be administered as rapid bolus (1–2 seconds). Therapeutic use: terminates SVTs with AV nodal-dependent circuits (AVNRT, AVRT). Diagnostic use: unmasks atrial activity in atrial tachycardia or flutter by transiently blocking AV conduction without terminating the tachycardia.

Key drug interactions:

• Dipyridamole, carbamazepine: block adenosine uptake and potentiate effect → reduce adenosine dose to 3 mg initial
• Theophylline, caffeine: competitive A1 receptor antagonists → may require higher doses (12–18 mg) or may render adenosine ineffective
• Heart transplant patients: denervated hearts are hypersensitive → start with 1–3 mg

Contraindications: second- or third-degree AV block (without pacemaker), sick sinus syndrome, severe asthma/COPD (can trigger bronchospasm via A2B receptors). Warn patients of transient side effects: flushing, chest tightness, dyspnea, and a sense of impending doom lasting <30 seconds.

Digoxin

Digoxin inhibits the Na⁺/K⁺-ATPase pump, increasing intracellular sodium, which in turn increases intracellular calcium via the Na⁺/Ca²⁺ exchanger, producing a positive inotropic effect. Its antiarrhythmic properties arise primarily from vagotonic effects that slow AV nodal conduction (increased ERP). Used for rate control in AF (especially in HFrEF, where beta-blockers and CCBs may be poorly tolerated) and HFrEF symptom management.

Narrow therapeutic index: therapeutic serum level 0.5–0.9 ng/mL (higher levels associated with increased mortality per DIG trial post-hoc analysis). Half-life ~36–48 hours (renally eliminated, dose-adjust for CKD). Toxicity manifests as GI symptoms (nausea, anorexia), visual disturbances (yellow-green halos, xanthopsia), and arrhythmias (virtually any rhythm disturbance, classically bidirectional VT, accelerated junctional rhythm, and atrial tachycardia with AV block).

Drug interactions: Amiodarone increases digoxin levels by ~100% (reduce digoxin dose by 50%). Verapamil increases digoxin levels by 50–75%. Quinidine doubles digoxin levels. Hypokalemia potentiates digoxin toxicity (both compete for Na⁺/K⁺-ATPase binding). Treat severe toxicity with digoxin-specific antibody fragments (Digibind/DigiFab).

Ivabradine

Ivabradine selectively inhibits the funny current (I_f) in sinoatrial node pacemaker cells, reducing heart rate without affecting blood pressure, contractility, or AV conduction. FDA-approved for HFrEF (SHIFT trial) and inappropriate sinus tachycardia (IST). Dose: 2.5–7.5 mg PO BID. Unique advantage in IST: reduces sinus rate without the hypotension, fatigue, and exercise intolerance associated with beta-blockers. Avoid in severe hepatic impairment. Visual phenomena (phosphenes/luminous flashes) occur in ~3% and are usually transient.

Ranolazine

Ranolazine primarily blocks the late sodium current (late I_Na), which reduces sodium-dependent calcium overload. FDA-approved as antianginal therapy. Off-label EP uses include AF suppression (particularly when combined with dronedarone — the HARMONY trial demonstrated synergistic AF reduction) and QTc shortening in LQT3. It also has modest I_Kr blocking properties (may slightly prolong QTc on ECG, but paradoxically suppresses TdP by reducing transmural dispersion of repolarization). Dose: 500–1000 mg PO BID. Metabolized by CYP3A4; avoid with strong CYP3A4 inhibitors. Contraindicated with Class Ia and Class III antiarrhythmics per labeling.

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