Pharmacology · Cardiovascular

Class I Agents: Sodium Channel Blockers

Use-dependence, three kinetic subclasses, and the structural heart disease contraindication

All Class I agents block cardiac sodium channels in a use-dependent manner — blockade intensifies at higher heart rates because more channels cycle through open and inactivated states; the three subclasses differ in how quickly they recover from blockade between beats, which determines their clinical profile and toxicity pattern.
Class Ia agents (quinidine, procainamide, disopyramide) have intermediate kinetics and also block potassium channels, prolonging the QT interval and carrying torsades de pointes risk; procainamide causes drug-induced lupus with chronic oral use; disopyramide's strong negative inotropy makes it useful for hypertrophic obstructive cardiomyopathy but contraindicated in reduced ejection fraction.
Class Ib agents (lidocaine, mexiletine) recover so rapidly that they have minimal effect on normal myocardium at rest but accumulate selectively in ischemic, depolarized tissue — lidocaine (IV only) is second-line for ventricular arrhythmias in ischemia; mexiletine (oral) also shortens QT in Long QT Syndrome Type 3 by blocking the persistent late sodium current.
Class Ic agents (flecainide, propafenone) have slow kinetics producing tonic conduction depression at all heart rates and marked QRS widening — highly effective for atrial fibrillation rhythm control in structurally normal hearts, including the pill-in-the-pocket strategy, but absolutely contraindicated in structural heart disease.
When using Class Ic agents for atrial fibrillation or flutter, an atrioventricular nodal blocking agent must always be co-prescribed — flecainide slows atrial flutter rate enough to enable 1:1 atrioventricular conduction, causing paradoxical ventricular rate acceleration to 200+ beats per minute.

Class I Subclasses — Kinetics and Electrocardiogram Effects

Class	Recovery Kinetics	Prototype Drugs	ECG Effect	Key Proarrhythmia Risk
Ia	Intermediate; also blocks potassium channels	Quinidine, procainamide, disopyramide	QRS widening + QT prolongation	Torsades de pointes
Ib	Rapid; fully relieves between beats at rest	Lidocaine, mexiletine	Minimal change at rest	Low — central nervous system toxicity at high levels
Ic	Slow; block not relieved between beats	Flecainide, propafenone	Marked QRS widening	Ventricular tachycardia/fibrillation in structural disease; atrial flutter with 1:1 conduction

Class Ia Agents

Class Ia

Quinidine

Sodium + potassium channel block + anticholinergic
Brugada syndrome; short QT syndrome
Torsades de pointes (reverse use-dependent)
Cinchonism: tinnitus, visual disturbance, confusion
Gastrointestinal toxicity — common cause of discontinuation

Class Ia

Procainamide

Sodium channel block; active metabolite has Class III activity
Intravenous: stable ventricular tachycardia; Wolff-Parkinson-White with atrial fibrillation
Drug-induced lupus (chronic oral use) — anti-histone antibodies
Torsades de pointes; hypotension with rapid intravenous infusion

Class Ia

Disopyramide

Sodium channel block + strong anticholinergic + strongest negative inotropy
Hypertrophic obstructive cardiomyopathy; vasovagal syncope
Urinary retention, dry mouth, constipation (anticholinergic)
Contraindicated in reduced ejection fraction

Class Ib vs Class Ic

Class Ib — Rapid Kinetics

Lidocaine & Mexiletine

Block relieves rapidly between beats — minimal effect in normal tissue at rest
Selective for ischemic, depolarized, or rapidly firing tissue
Shorten action potential duration; no QT prolongation
Lidocaine: intravenous only; second-line for ventricular tachycardia/ventricular fibrillation
Mexiletine: oral; chronic ventricular arrhythmias; adjunct in Long QT Syndrome Type 3
Toxicity: central nervous system (perioral numbness → seizures); gastrointestinal (mexiletine)

Safe in ischemia

Class Ic — Slow Kinetics

Flecainide & Propafenone

Block not relieved between beats — tonic conduction depression at all rates
Marked QRS widening; no significant QT change
Highly effective for atrial fibrillation/flutter in structurally normal hearts
Pill-in-the-pocket strategy for paroxysmal atrial fibrillation
Propafenone: additional weak beta-blocker activity — avoid in asthma
Always co-prescribe atrioventricular nodal blocker to prevent 1:1 atrial flutter conduction

Contraindicated in structural heart disease

The Cardiac Arrhythmia Suppression Trial — Core Lesson

Critical Safety Principle

The Cardiac Arrhythmia Suppression Trial (1991): encainide and flecainide suppressed ventricular premature beats after myocardial infarction but increased arrhythmic death 2.5-fold vs placebo. Mechanism: use-dependent sodium channel blockade converts non-sustained micro-re-entrant circuits in peri-infarct scar into sustained, fatal ventricular tachycardia or ventricular fibrillation. Ventricular premature beat suppression is a surrogate endpoint — not a clinical benefit. Class Ic (and Class Ia) agents are contraindicated in structural heart disease regardless of arrhythmia type.

Drug Selection for Atrial Fibrillation Rhythm Control by Substrate

Cardiac Substrate	Acceptable Agents	Contraindicated
No structural heart disease	Flecainide, propafenone, sotalol, dronedarone, amiodarone	None of the above
Left ventricular hypertrophy	Amiodarone, dofetilide	Flecainide, propafenone
Coronary artery disease / prior myocardial infarction	Sotalol, dofetilide, amiodarone	Flecainide, propafenone
Heart failure with reduced ejection fraction	Amiodarone, dofetilide	Flecainide, propafenone, sotalol, dronedarone

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