All Class I antiarrhythmic agents share a common mechanism: blockade of voltage-gated fast sodium channels (Nav1.5, encoded by SCN5A) in cardiac myocytes. Nav1.5 cycles through three functional states, resting (closed), open, and inactivated, and Class I drugs bind preferentially to open or inactivated channels, exhibiting what is termed use-dependent (or frequency-dependent) blockade.1,2 Use-dependence means that channel blockade accumulates with increasing heart rate: at higher rates, more channels cycle through the open/inactivated states per unit time, increasing drug binding and amplifying the reduction in phase 0 upstroke velocity (dV/dtmax) and conduction velocity.1 This property underlies both the therapeutic efficacy of these agents during tachyarrhythmias and their proarrhythmic danger during rapid, disorganized ventricular arrhythmias in diseased myocardium.
The Ia/Ib/Ic subclassification introduced by Harrison (1985) is based on the time constant of recovery from channel block: specifically, the rate at which drug-bound channels recover to the resting (drug-free) state during the diastolic interval.2,3 Class Ia agents (quinidine, procainamide, disopyramide) have intermediate recovery kinetics of 1 to 10 seconds, prolong action potential duration and the QT interval via concurrent rapid delayed rectifier potassium current (IKr) blockade, and produce moderate QRS widening; their principal proarrhythmic risks are torsades de pointes and ventricular tachycardia in structural heart disease. Class Ib agents (lidocaine, mexiletine) have fast recovery kinetics of less than 1 second, shorten action potential duration, cause minimal QRS widening, and carry low proarrhythmic risk at therapeutic levels. Class Ic agents (flecainide, propafenone) have slow recovery kinetics of greater than 10 seconds, cause minimal change in action potential duration, produce marked QRS widening, and carry the highest proarrhythmic risk: monomorphic ventricular tachycardia and ventricular fibrillation in structurally diseased myocardium.
The clinical implication of slow recovery kinetics (Class Ic) is profound: at physiologic heart rates (60–100 bpm), the diastolic interval (~600–800 ms) is insufficient for complete channel recovery, so block accumulates with each beat. During tachycardia or in regions of slow conduction (as in peri-infarct scar), this effect is dramatically amplified, converting heterogeneous conduction delay into sustained re-entrant circuits.2
Class Ia agents block INa with intermediate kinetics and additionally block IKr, producing QT prolongation that carries torsades de pointes (TdP) risk. They also possess anticholinergic properties of varying degrees, which can accelerate AV nodal conduction and partially offset their direct effects on the AV node.3
Quinidine is the prototypical Class Ia agent and the oldest antiarrhythmic drug in clinical use, derived from the bark of the cinchona tree. Beyond INa blockade, quinidine blocks IKr, slow delayed rectifier potassium current (IKs), Ito, and L-type calcium current (ICaL), and has significant α1-adrenergic and muscarinic receptor blocking (anticholinergic) activity. The net ECG effects are: modest QRS widening, QT prolongation (IKr block), and PR interval that may shorten due to anticholinergic AV nodal acceleration.3
Bioavailability: ~70–80% (sulfate salt); first-pass metabolism is moderate. Protein binding: ~80–90% (alpha-1 acid glycoprotein and albumin). Metabolism: Hepatic via CYP3A4 (cytochrome P450 3A4); active metabolite 3-hydroxyquinidine also has antiarrhythmic activity. Half-life: 6–8 hours; prolonged in hepatic impairment and heart failure. Elimination: Renal (20% unchanged); dose reduction in renal impairment.
Maintenance of sinus rhythm in atrial fibrillation/flutter (largely supplanted by safer agents). Brugada syndrome: quinidine's Ito blockade normalizes the phase 1 notch and is used for VF storm and as an adjunct to ICD therapy in symptomatic patients where other options are unavailable. Short QT syndrome: quinidine prolongs QT and reduces VF inducibility.
Quinidine has one of the most complex toxicity profiles in pharmacology:3,4 Quinidine syncope (TdP): Occurs in 1–3% of patients. Paradoxically more common at low serum concentrations and slow heart rates (reverse use-dependence). Requires immediate discontinuation, IV magnesium, and temporary pacing if recurrent. Cinchonism: A dose-related syndrome of tinnitus, hearing loss, visual disturbance, headache, and confusion. Resembles salicylate toxicity and reflects CNS effects. GI toxicity: Nausea, vomiting, and diarrhea in up to 30–40% of patients; a leading cause of drug discontinuation. Immune-mediated: Thrombocytopenia (drug-dependent antibodies), lupus-like syndrome, and hepatotoxicity. Drug interactions: Inhibits CYP2D6 (raising levels of metoprolol, codeine) and P-glycoprotein (doubling digoxin levels). Requires digoxin dose reduction when co-prescribed.
Procainamide shares quinidine's Class Ia sodium channel blockade but has weaker anticholinergic activity and weaker IKr block. Its principal antiarrhythmic importance in contemporary practice lies with its active metabolite, N-acetylprocainamide (NAPA), which is a pure Class III agent (IKr blocker) with no Na+ channel activity. The NAPA:procainamide ratio varies significantly by acetylator phenotype, creating patient-to-patient variability in QT effects.3
Bioavailability: ~75–95% orally; IV formulation available for acute use. Acetylation: Hepatic N-acetyltransferase 2 (NAT2). Slow acetylators accumulate procainamide and have lower NAPA levels; fast acetylators have higher NAPA levels and greater QT risk. Half-life: Procainamide 3–4 hours; NAPA 6–8 hours. Both accumulate significantly in renal impairment (NAPA is renally cleared). Therapeutic range: Procainamide 4–10 µg/mL; combined (procainamide + NAPA) 10–30 µg/mL. Monitor both in renal impairment.
IV procainamide: Guideline-supported (Class IIa) for hemodynamically stable wide-complex tachycardia when VT is confirmed or strongly suspected. Also used for Wolff-Parkinson-White syndrome (WPW)-associated AF with rapid ventricular response via the accessory pathway (where AV nodal agents are contraindicated). Oral procainamide: Largely abandoned due to the need for 4-hourly dosing, mandatory monitoring, and lupus risk. Sustained-release formulations (procainamide SR) partially mitigated compliance issues but have not restored its use.
Drug-induced lupus erythematosus (DILE): Occurs in 15–40% of patients on long-term therapy; more common in slow acetylators. Characterized by arthralgias, pleuritis, pericarditis, and elevated ANA (particularly anti-histone antibodies). Renal involvement is rare (unlike idiopathic SLE). Resolves on drug discontinuation. TdP: Principally mediated by NAPA's IKr blockade. Risk increases in renal impairment and with concomitant QT-prolonging drugs. Hypotension (IV): Due to peripheral vasodilation (α1-blockade) and modest negative inotropy. IV infusion rate must not exceed 50 mg/min; stop if BP falls >15 mmHg or QRS widens >50% from baseline. Hematologic: Agranulocytosis (rare, ~0.5%; monitor complete blood count (CBC) in first 3 months of therapy).
Disopyramide is the most potent anticholinergic agent among the Class Ia drugs and has the most pronounced negative inotropic effect, a combination that limits its use but also confers a unique therapeutic niche.3
Oral bioavailability ~80%; half-life 6–8 hours; renal and hepatic elimination (dose-adjust in both).
Hypertrophic obstructive cardiomyopathy (HOCM): the negative inotropic effect reduces left ventricular outflow tract (LVOT) gradient. Also used in neurally-mediated (vasovagal) syncope, where vagolytic effects may reduce reflex bradycardia.
Anticholinergic toxicity: Urinary retention (particularly in men with BPH), dry mouth, constipation, blurred vision, and precipitation of narrow-angle glaucoma. Cardiac: QT prolongation and TdP; heart failure exacerbation (contraindicated in EF <40% outside the HOCM indication); AV block.
Class Ib agents have fast dissociation kinetics from Nav1.5, meaning channel blockade is minimal at normal heart rates but increases substantially in ischemic tissue (where resting membrane potential is less negative, keeping more channels in the inactivated state) and at rapid rates. This selective action on abnormal tissue is the pharmacodynamic basis for their use in ventricular, rather than supraventricular, arrhythmias.1,3
Lidocaine blocks INa with marked selectivity for inactivated channels, shortens the action potential duration (particularly in Purkinje fibers), and has minimal effect on atrial tissue or the AV node. It does not prolong the QT interval and has negligible effect on QRS duration at therapeutic concentrations. Its antiarrhythmic action is most pronounced in ischemic, depolarized (less negative Vm) myocardium, the exact conditions that favor inactivated channel binding.5
Route: IV only for antiarrhythmic use (oral bioavailability <35% due to extensive first-pass hepatic extraction). Distribution: Rapid two-compartment distribution; initial half-life ~8 minutes (distribution); terminal half-life ~1.5–2 hours. Metabolism: Hepatic CYP1A2 and CYP3A4 to active (monoethylglycinexylidide, monoethylglycinexylidide (MEGX)) and inactive metabolites. Hepatic clearance is flow-dependent; significantly reduced in heart failure, hepatic disease, and with concomitant beta-blockers (which reduce hepatic blood flow). Dosing: IV bolus 1–1.5 mg/kg, may repeat 0.5–0.75 mg/kg at 5–10 min intervals; maintenance infusion 1–4 mg/min. Reduce maintenance infusion by 50% in heart failure or hepatic impairment. Therapeutic range: 1.5–5 µg/mL.
Acute management of hemodynamically stable ventricular tachycardia or ventricular fibrillation when amiodarone is unavailable or contraindicated (ACLS secondary agent). Post-cardiac arrest (pulseless VT/VF): second-line after amiodarone per AHA guidelines, supported by the ALPS trial data. Historically used for prophylaxis of ventricular arrhythmias post-MI; this indication has been abandoned following evidence of increased mortality from bradycardia and asystole.
Lidocaine toxicity is primarily neurologic and concentration-dependent:5 Mild (<5 µg/mL): Perioral numbness, lightheadedness, tinnitus, drowsiness. Moderate (5–9 µg/mL): Dysarthria, nystagmus, confusion, disorientation. Severe (>9 µg/mL): Seizures (treat with benzodiazepines), respiratory arrest. Cardiac depression (bradycardia, hypotension, AV block) is rare at therapeutic levels but occurs with massive overdose. Accumulation risk: In heart failure and hepatic disease, MEGX accumulates and contributes to CNS toxicity. Infusion duration beyond 24 hours increases toxicity risk.
Mexiletine is the oral analogue of lidocaine, with an identical electrophysiologic mechanism but adequate oral bioavailability (~90%) and a half-life of 10–12 hours, permitting twice- or three-times daily dosing. It is metabolized by CYP2D6 (significant pharmacogenomic variability) with renal excretion of a minority of unchanged drug.3 Indications: Chronic suppression of ventricular arrhythmias; adjunctive therapy in Long QT Syndrome Type 3 (LQT3) where persistent late INa (INaL) blockade shortens QTc; combination with amiodarone for refractory ventricular arrhythmias to allow amiodarone dose reduction. GI toxicity: Nausea, vomiting, and abdominal discomfort in up to 40% of patients; take with food to reduce. The most common reason for discontinuation. Neurologic: Tremor, dizziness, ataxia, and diplopia at higher concentrations. Therapeutic range: 0.5–2.0 µg/mL. Narrow therapeutic window.
Class Ic agents produce the most potent sodium channel blockade of any antiarrhythmic drugs. Their slow dissociation kinetics mean that channel block is not relieved during normal diastolic intervals, resulting in tonic (rate-independent) depression of conduction under all conditions, with further amplification during tachycardia. This makes them highly effective at converting and preventing atrial arrhythmias in structurally normal hearts, but profoundly dangerous in structurally diseased myocardium.6,7
Flecainide blocks INa with marked use-dependence, producing QRS widening proportional to both drug concentration and heart rate. It also blocks IKr (weakly, usually insufficient to prolong QT) and ryanodine receptor 2 (RyR2) at low concentrations, the latter conferring a unique role in catecholaminergic polymorphic ventricular tachycardia (CPVT) management. Its effect on AV nodal conduction is modest; it primarily slows atrial and ventricular myocardial conduction and His-Purkinje conduction.6
Bioavailability: ~90%; not significantly affected by food. Half-life: 12–27 hours; allows twice-daily dosing. Metabolism: Hepatic CYP2D6 (significant polymorphism). Poor metabolizers achieve higher plasma levels; adjust dose in CYP2D6 poor metabolizers. Elimination: ~30% renal unchanged; reduce dose in significant renal impairment (creatinine clearance (CrCl) <35 mL/min). Therapeutic range: 0.2–1.0 µg/mL.
Paroxysmal AF/AFL: Highly effective for rhythm control in patients without structural heart disease, left ventricular hypertrophy (LVH), or ischemic disease. "Pill-in-the-pocket" strategy: single oral dose (200–300 mg) taken by the patient at onset of palpitations for self-cardioversion of paroxysmal AF. atrioventricular nodal reentrant tachycardia (AVNRT) and atrioventricular reentrant tachycardia (AVRT): Effective for both acute termination and prophylaxis of AV nodal-dependent re-entrant tachycardias. CPVT: Low-dose flecainide reduces RyR2-mediated DAD triggered activity in patients refractory to maximally tolerated beta-blockade.
Propafenone is a Class Ic agent with additional weak beta-adrenergic blocking activity (β1-antagonism approximately 1/40th the potency of propranolol) and very weak L-type calcium channel blockade. Its metabolite 5-hydroxypropafenone (produced by CYP2D6) retains Class Ic activity but lacks beta-blocking effects. In CYP2D6 poor metabolizers, the parent compound accumulates and beta-blocking effects are more pronounced, increasing the risk of bradycardia and bronchospasm.6 Bioavailability: Highly variable (~5–50%) due to saturable first-pass metabolism; increases non-linearly with dose. Half-life: 2–10 hours (extensive metabolizers); 10–32 hours (poor metabolizers).
Indications: Identical to flecainide for AF/SVT; "pill-in-the-pocket" strategy also applicable (450–600 mg single oral dose). Must pre-treat with AV nodal agent to prevent 1:1 AFL conduction. Additional cautions: Beta-blocking activity may cause bronchospasm; use with caution in reactive airway disease. CYP2D6, CYP3A4, and CYP1A2 substrate with multiple interaction potential.
STRUCTURAL HEART DISEASE
The Cardiac Arrhythmia Suppression Trial (CAST) is among the most consequential clinical trials in cardiovascular pharmacology. Its findings permanently altered antiarrhythmic prescribing practice and remain the primary evidence base for the structural heart disease contraindication to Class Ic (and by extension, Class Ia) agents.7
The CAST hypothesis derived from the observation that frequent ventricular premature beats (VPBs) after myocardial infarction were associated with increased mortality. The prevailing assumption was that suppressing VPBs would reduce sudden cardiac death. CAST was a randomized, placebo-controlled trial enrolling patients with post-MI LV dysfunction (EF ≤45%) and asymptomatic or mildly symptomatic VPBs. Participants were randomized to encainide, flecainide, moricizine, or placebo after demonstrating VPB suppression on each active drug.7
CAST was stopped early due to significantly increased mortality in the encainide and flecainide arms. Despite effectively suppressing VPBs, both drugs increased the rate of arrhythmic death and cardiac arrest compared to placebo (encainide/flecainide: 4.5% vs. placebo: 1.2%; relative risk 2.5).7 CAST II subsequently demonstrated that moricizine also increased early mortality, though the late effect was neutral.
CAST Trial: The Core Clinical Message
Suppression of ventricular ectopy is a surrogate endpoint, not a therapeutic goal. Drugs that suppress VPBs can simultaneously increase arrhythmic mortality. The mechanism: use-dependent Na+ channel blockade in peri-infarct scar converts benign micro-re-entrant circuits into sustained, fatal ventricular tachycardia or fibrillation. Class Ic (and Class Ia) agents are CONTRAINDICATED in structural heart disease, including: prior MI, LV dysfunction (EF <40%), LVH >1.4 cm, and significant coronary artery disease. This contraindication applies regardless of arrhythmia type. Even using flecainide for AF in a patient with prior MI is contraindicated.
The CAST findings drive a critical decision branch in antiarrhythmic drug selection for atrial fibrillation rhythm control:8 In patients with no structural heart disease and no significant left ventricular hypertrophy, flecainide, propafenone, sotalol, and dronedarone are all acceptable for rhythm control. In patients with significant left ventricular hypertrophy (wall thickness greater than 1.4 cm), flecainide and propafenone are contraindicated; amiodarone, dofetilide, or sotalol with preserved ejection fraction are preferred. In patients with coronary artery disease or prior myocardial infarction, flecainide and propafenone are contraindicated; sotalol, dofetilide, or amiodarone are appropriate. In patients with heart failure with reduced ejection fraction below 40%, only amiodarone and dofetilide are acceptable; flecainide, propafenone, sotalol, and dronedarone are all contraindicated.
Excessive sodium channel blockade produces progressive QRS widening. A QRS duration >25% wider than baseline warrants dose reduction or discontinuation. At toxic concentrations, the QRS widens to a sinusoidal morphology, a pattern pathognomonic of severe Na+ channel toxicity. Management includes:9 Sodium bicarbonate (IV): 1–2 mEq/kg bolus; alkalinization and increased extracellular Na+ competitively displace drug from the channel. First-line for Class Ic and tricyclic antidepressant Na+ channel toxicity. Hypertonic saline: Alternative if bicarbonate is insufficient. Lipid emulsion therapy: IV 20% lipid emulsion ("lipid rescue") for refractory cardiovascular toxicity from lipid-soluble Na+ channel blockers. ECMO: Extracorporeal support as a bridge in refractory cases while drug clears.
Flecainide and propafenone slow atrial flutter rate (from ~300 bpm to ~200–220 bpm) by depressing atrial conduction. At these slower atrial rates, AV nodal Wenckebach periodicity may resolve, permitting 1:1 AV conduction and paradoxical ventricular rate acceleration to 200+ bpm. This is a class-specific proarrhythmic complication and is the reason that an AV nodal blocking agent (beta-blocker or non-DHP CCB) must always be co-prescribed whenever Class Ic drugs are used for atrial arrhythmias.6
QT prolongation from IKr blockade predisposes to early afterdepolarization (EAD)-mediated triggered activity and TdP. Risk factors include: bradycardia, hypokalemia, hypomagnesemia, female sex, baseline QT prolongation, and combination with other QT-prolonging drugs. Acute management: IV magnesium sulfate 2 g over 1–2 minutes; correct electrolytes; withdraw the offending agent; and increase heart rate (IV isoproterenol or temporary pacing at 90–110 bpm) if recurrent episodes are pause-dependent.9
Hille B. Local anesthetics: hydrophilic and hydrophobic pathways for the drug-receptor reaction. J Gen Physiol. 1977;69(4):497–515
doi:10.1085/jgp.69.4.497Harrison DC. Antiarrhythmic drug classification: new science and practical applications. Am J Cardiol. 1985;56(4):185–187
doi:10.1016/0002-9149(85)90840-9Roden DM. Antiarrhythmic drugs. In: Brunton LL, Hilal-Dandan R, Knollmann BC, eds. Goodman & Gilman's: The Pharmacological Basis of Therapeutics. 13th ed. McGraw-Hill; 2018:547–571.
Roden DM. Risks and benefits of antiarrhythmic therapy. N Engl J Med. 1994;331(12):785–791
doi:10.1056/NEJM199409223311207Chinn K, Brown BS, Carlsson L. Lidocaine. In: Singh BN, Wellens HJJ, Hiraoka M, eds. Electropharmacological Control of Cardiac Arrhythmias. Futura; 1994:191–210.
Aliot E, Capucci A, Crijns HJ, Goette A, Tamargo J. Twenty-five years in the making: flecainide is safe and effective in the long-term treatment of atrial fibrillation. Europace. 2011;13(2):161–173
doi:10.1093/europace/euq382Echt DS, Liebson PR, Mitchell LB, et al. Mortality and morbidity in patients receiving encainide, flecainide, or placebo: the Cardiac Arrhythmia Suppression Trial. N Engl J Med. 1991;324(12):781–788
doi:10.1056/NEJM199103213241201January CT, Wann LS, Calkins H, et al. 2019 AHA/ACC/HRS focused update of the 2014 AHA/ACC/HRS guideline for the management of patients with atrial fibrillation. J Am Coll Cardiol. 2019;74(1):104–132
doi:10.1016/j.jacc.2019.01.011Brugada J, Katritsis DG, Arbelo E, et al. 2019 ESC Guidelines for the management of patients with supraventricular tachycardia. Eur Heart J. 2020;41(5):655–720
doi:10.1093/eurheartj/ehz467Connolly SJ, Dorian P, Roberts RS, et al. Comparison of beta-blockers, amiodarone plus beta-blockers, or sotalol for prevention of shocks from implantable cardioverter defibrillators: the OPTIC study. JAMA. 2006;295(2):165–171
doi:10.1001/jama.295.2.165