Antiarrhythmics Part 1: The Vaughan-Williams Classes & Sodium-Channel Blockers
There's a dark irony at the heart of rhythm drugs: the very medicines meant to fix a dangerous heartbeat can CAUSE a fatal one. A famous trial gave patients a drug that beautifully suppressed their extra beats — and quietly killed more of them than placebo. That lesson haunts every antiarrhythmic. Start with the map that organizes them all, then meet the sodium-channel blockers where the story begins.
In the late 1980s, doctors reasoned that since extra ventricular beats after a heart attack predicted sudden death, suppressing them with a powerful sodium-channel drug should save lives. The trial (called CAST) was stopped early — not because it worked, but because the drug was killing patients: it silenced the extra beats yet TRIGGERED lethal rhythms in scarred hearts. It's the defining cautionary tale of antiarrhythmic drugs: they walk a razor's edge between fixing a rhythm and causing a worse one. To understand why, we start with how these drugs are classified.
The Vaughan-Williams map
Antiarrhythmics are grouped by the channel they block. The classic Vaughan-Williams system sorts these drugs by which part of the cardiac action potential (from the foundations article) they target. Class I blocks the sodium channel (the phase-0 upstroke), slowing conduction. Class II are the beta blockers, which calm the pacemaker and AV node. Class III blocks the potassium channel (phase-3 repolarization), prolonging the action potential and the QT. Class IV blocks the calcium channel, slowing the AV node. Four classes, four channels — the same wiring diagram, now used as a drug map. This article is Class I.
Class I: three flavours of sodium block
Class I drugs block the sodium channel, but they split into three subgroups by how strongly and how they also affect repolarization. Class IA (quinidine, procainamide, disopyramide) block sodium moderately AND block potassium, so they also prolong the action potential and QT; they treat both atrial and ventricular arrhythmias but carry the risk of QT-related torsades, and each has quirks — quinidine's 'cinchonism' (headache, tinnitus, GI upset) and procainamide's drug-induced lupus. Class IB (lidocaine, mexiletine) block sodium weakly and SHORTEN the action potential; they act preferentially on damaged, ischaemic ventricular tissue, making intravenous lidocaine useful for ventricular arrhythmias — its main side effects are on the brain (confusion, seizures). Class IC (flecainide, propafenone) block sodium STRONGLY, markedly slowing conduction; they're effective for atrial fibrillation and SVT in structurally normal hearts.
The CAST lesson lives in Class IC. Flecainide and propafenone are excellent for arrhythmias in a STRUCTURALLY NORMAL heart — but in a heart scarred by a previous heart attack or with structural disease, their strong sodium block creates zones of slow conduction that can spark lethal ventricular arrhythmias, raising death rates (exactly what CAST showed). The rule: never give a Class IC drug to a patient with ischaemic or structural heart disease. It's the sharpest example that an antiarrhythmic can be proarrhythmic — the cure becoming the killer.
- Vaughan-Williams: I = Na⁺ block, II = beta blockers, III = K⁺ block, IV = Ca²⁺ block.
- IA (quinidine, procainamide): Na⁺ + K⁺ block, prolong QT; procainamide → drug-induced lupus.
- IB (lidocaine): weak Na⁺ block, acts on ischemic ventricle; IV for ventricular arrhythmias; CNS effects.
- IC (flecainide, propafenone): strong Na⁺ block; for AF/SVT in normal hearts only.
- Never give Class IC in ischemic/structural heart disease — proarrhythmic (the CAST lesson).
- Giving flecainide/propafenone to a scarred or ischemic heart. It can trigger fatal arrhythmias.
- Forgetting IA drugs and many others prolong the QT → torsades risk.
- Assuming suppressing extra beats always saves lives. CAST proved the opposite.
- Ignoring lidocaine's CNS toxicity (confusion, seizures) at high blood levels.
Why must Class IC drugs (flecainide) be avoided in a patient with a prior heart attack?
- Vaughan-Williams: I Na⁺, II beta, III K⁺, IV Ca²⁺ — mapped to the action-potential phases.
- IA (quinidine/procainamide) also prolong QT; IB (lidocaine) targets ischemic ventricle.
- IC (flecainide/propafenone) strongly block Na⁺ — for AF/SVT in structurally normal hearts only.
- Antiarrhythmics can be proarrhythmic (the CAST lesson) — match the drug to the heart.
- Katzung BG. Basic & Clinical Pharmacology — Agents Used in Cardiac Arrhythmias (Class I).
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antiarrhythmic drugs.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antidysrhythmic drugs.
- CAST Investigators — Cardiac Arrhythmia Suppression Trial (proarrhythmia).
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antiarrhythmic drugs.
