ICU antiarrhythmic drugs — comprehensive (Vaughan-Williams classification)

Overview & definition

Antiarrhythmic drugs are agents that modify the generation or propagation of cardiac action potentials to suppress arrhythmias. In the ICU they are deployed across the full spectrum — atrial fibrillation (the commonest ICU arrhythmia), narrow-complex SVT, ventricular tachycardia, and the peri-arrest arrhythmias — and the decision of which drug (or cardioversion) hinges on three questions: (1) is the patient haemodynamically stable or unstable? (unstable → electricity, not drugs); (2) what is the rhythm (atrial vs ventricular, narrow vs broad, regular vs irregular)? and (3) is there structural heart disease / LV dysfunction (this bars the use of flecainide and several Class I agents)? [1]

The pharmacological framework is the Vaughan-Williams classification — a four-class scheme (plus a residual "other" group) based on the dominant ion channel or receptor the drug acts upon.[1] It is imperfect (many drugs span several classes — amiodarone has Class I, II, III and IV actions — and it does not capture clinical context), yet it remains the lingua franca of the exam and the bedside. The Sicilian Gambit is an alternative mechanistic framework that classifies drugs by their precise molecular target (channel, receptor, pump) and the resulting cellular effect, and is useful for deeper mechanistic reasoning.[1]

A cardinal principle of ICU antiarrhythmic therapeutics: every antiarrhythmic is also potentially pro-arrhythmic. The very drugs used to suppress arrhythmias can prolong QT (→ torsades), depress conduction (→ heart block), or depress contractility (→ hypotension/heart failure).[6] Hence the standing rule — treat the patient, not the ECG: if the patient is shocked, ischaemic, or in pulmonary oedema, go straight to synchronised electrical cardioversion and reserve pharmacological management for the stable.

The Vaughan-Williams classification

Class I — sodium channel blockers (phase 0)

Class I drugs bind the fast voltage-gated Na⁺ channel and slow phase 0 (depolarisation) of the cardiac action potential, thereby depressing conduction velocity. They are sub-classified by their kinetics of binding/unbinding and their effect on action-potential duration (APD).[1]

Ia — quinidine, procainamide, disopyramide

• Mechanism: intermediate Na⁺ channel kinetics; moderate phase-0 depression AND K⁺ channel block → prolong APD and QT. They are the only Class I subgroup that lengthens repolarisation.
• Procainamide is the most relevant in modern ICU practice: an IV agent historically used for haemodynamically tolerated monomorphic VT and for AF rhythm control (e.g., pre-excited AF in WPW). It is hepatically acetylated to the active metabolite N-acetylprocainamide (NAPA), which has Class III activity. Adverse effects include hypotension (negative inotropy + vasodilation on IV push — give slowly), QT prolongation/torsades, and a lupus-like syndrome (arthralgia, rash, ANA positive) with chronic oral use. It has been withdrawn from many markets, limiting contemporary availability.
• Quinidine: Class Ia + antimalarial; causes cinchonism (tinnitus, headache, nausea), thrombocytopenia, QT prolongation/torsades ("quinidine syncope"), and digoxin interaction (displaces digoxin from tissue binding → doubles digoxin levels — halve the digoxin dose). Also widens the QRS and can unmask a Brugada pattern.
• Disopyramide: potent negative inotrope (used historically in HOCM to reduce outflow gradient) with marked anticholinergic side effects (urinary retention, dry mouth, glaucoma). [1]

Ib — lidocaine, phenytoin, mexiletine

• Mechanism: fast Na⁺ channel kinetics (bind/recover rapidly) and they shorten APD. Crucially, they preferentially act on ischaemic/depolarised tissue (where Na⁺ channels spend more time in the inactivated state that Ib drugs bind) — hence their niche in ischaemic VT.
• Lidocaine is the prototype: first-line for monomorphic VT in the setting of acute ischaemia/infarction. Dose: 1–1.5 mg/kg IV bolus (repeat to 3 mg/kg), then infusion 1–4 mg/min. It is ineffective for SVT/AF. Toxicity is primarily CNS — perioral tingling, tinnitus, slurred speech, agitation progressing to seizures and coma — and relates to serum level; avoid in high-degree AV block. Hepatic clearance (CYP) means dose-adjust in severe liver disease or heart failure (reduced hepatic blood flow).
• Phenytoin: uniquely useful for digoxin-toxic arrhythmias (the antiarrhythmic of choice for digitalis-induced VT) and for long-QT-related ventricular arrhythmias in some contexts.
• Mexiletine: the oral analogue of lidocaine (for chronic ventricular ectopy, long-QT type 3). [1]

Ic — flecainide, propafenone

• Mechanism: slow Na⁺ channel kinetics → marked depression of conduction velocity (prominent QRS widening), minimal effect on APD.
• Flecainide is powerfully effective at cardioverting recent-onset AF ("pill-in-the-pocket" strategy in selected outpatients) and is used pharmacologically for AF rhythm control in patients with a structurally normal heart. The cardinal caveat: AVOID in any structural heart disease or LV dysfunction. The CAST trial (Cardiac Arrhythmia Suppression Trial, 1989) randomised post-MI patients with asymptomatic ventricular ectopy to flecainide/encainide vs placebo — the drug arm had excess mortality (suppression of ectopy but pro-arrhythmic death in damaged myocardium). This landmark result transformed antiarrhythmic prescribing: Class Ic drugs are forbidden in ischaemic/cardiomyopathic hearts. Flecainide also causes a flecainide challenge (Brugada diagnostic test — unmask ST elevation in V1–V3) and is contraindicated in established Brugada syndrome.
• Propafenone: Class Ic + weak beta-blockade (Class II) — similar AF role and the same structural-heart-disease prohibition. [1]

Class II — beta-blockers

• Mechanism: competitive blockade of cardiac β1-adrenoceptors → ↓ phase-4 depolarisation (automaticity), slow SA node rate, slow AV nodal conduction, and reduce the catecholamine-driven triggered activity that drives many ICU arrhythmias. They reduce myocardial oxygen demand and, in ischaemia, reduce infarct size and arrhythmic death.[1]
• Indications: first-line rate control for AF (and SVT termination); ischaemia-driven ventricular ectopy; long-QT syndrome; perioperative/critical-illness adrenergic surge.
• Agent selection in ICU:
  • Metoprolol: cardioselective beta-1; 5 mg IV slow push (repeat up to 3 doses) or oral. The workhorse ICU beta-blocker.
  • Esmolol: ultra-short-acting (half-life ~9 minutes) — metabolised by plasma esterases (independent of liver/kidney) — ideal for titration and for testing beta-blocker tolerance (if it causes hypotension/bradycardia, it wears off in minutes). Useful for AF rate control in the unstable-but-not-yet-shocked patient.
  • Bisoprolol/atenolol/carvedilol: oral, chronic.
  • Propranolol: non-selective (beta-1 + beta-2) — avoid in asthma/COPD; used for thyroid storm and beta-blocker overdose modelling.
  • Landiolol: ultra-short-acting, highly beta-1 selective (half-life ~4 min); used (especially in Japan/Europe) for AF rate control in the acutely ill/septic patient where cardioselectivity and rapid offset matter.
• Cautions: avoid/decrease in acute decompensated heart failure until euvolaemic (negative inotropy), severe asthma/COPD (bronchospasm — use cardioselective cautiously), bradycardia and high-degree AV block, and acute RV infarct. Never stop chronic beta-blockers abruptly in ICU (rebound ischaemia/tachyarrhythmia — continue or switch to IV).

Class III — potassium channel blockers

Class III drugs block the delayed-rectifier potassium current (IKr), prolonging phase-3 repolarisation → prolonged APD and refractory period → broadened QT, which suppresses re-entry. They are the most-used antiarrhythmics in modern ICU practice.[1]

Amiodarone — the workhorse ICU antiarrhythmic

Amiodarone is the single most effective and broadly applicable antiarrhythmic in the ICU. It is genuinely multichannel: predominant Class III (K⁺ block) but also Class I (Na⁺), Class II (non-competitive beta-blockade), and Class IV (Ca²⁺ channel) activity. This breadth explains its efficacy across AF, VT, and VF and the fact that it is safe in heart failure (minimal negative inotropy). [1]

• Pharmacokinetics — the problem of the long half-life: amiodarone is enormously lipophilic (accumulates in fat, liver, lung, skin), is highly protein-bound, and has a terminal half-life of ~60 days (range 25–100) after chronic dosing. Effects therefore accumulate and persist for months after cessation. IV onset is faster (minutes) but full tissue loading takes weeks. This long half-life underpins both its smooth efficacy and its toxicity profile.
• Dosing:
  • VT/VF arrest: 300 mg IV bolus (after the 3rd shock in cardiac arrest), then 150 mg if needed.
  • Stable VT / AF chemical cardioversion: 300 mg IV over 1 hour (or 5 mg/kg), then 900 mg over 24 hours via infusion (≈1 g/day), then oral maintenance 100–200 mg daily.
  • Oral loading: 600 mg/day for 1–2 weeks, then 200–400 mg/day for several weeks, then 100–200 mg maintenance.
• Toxicity — multi-organ (the price of breadth):
  • Pulmonary: interstitial pneumonitis / pulmonary fibrosis — the most feared, potentially fatal complication; baseline + surveillance chest imaging and TLCO (DLCO). Present with new dyspnoea/cough/interstitial infiltrates — stop immediately.
  • Thyroid: the drug is ~40% iodine by weight → both hypothyroidism AND hyperthyroidism (Wolff-Chaikoff effect vs Jod-Basedow phenomenon). Check TFTs at baseline and 6-monthly.
  • Hepatic: hepatitis / transaminitis (baseline + periodic LFTs); rare cirrhosis.
  • Cardiac: QT prolongation (universal but torsades is paradoxically uncommon because amiodarone also homogenises repolarisation and has beta/Ca-blocking effects) — nonetheless monitor; bradycardia and AV block.
  • Skin/eye: corneal microdeposits (virtually universal, usually asymptomatic — refer if visual change), blue-grey skin discolouration and photosensitivity (sun protection), optic neuropathy (rare).
  • Neuro: peripheral neuropathy, tremor, ataxia.
  • Drug interactions: potent CYP3A4 and CYP2C9 inhibitor + P-glycoprotein inhibitor → raises levels of warfarin (halve warfarin dose), digoxin (halve digoxin dose), statins, and many others. [1]

Sotalol

• Class III (K⁺ block, IKr) plus Class II (non-selective beta-blockade — the racemic drug; the d-isomer is pure Class III but is pro-arrhythmic without the protective beta-block, so racemic sotalol is used). Renal excretion (dose-adjust in renal impairment). Used for AF rhythm control and ventricular ectopy. Reverse-use dependence means its QT-prolonging effect is greatest at slow heart rates → torsades risk, particularly in bradycardia, hypokalaemia, and renal failure; initiate in-hospital with QT monitoring. [1]

Ibutilide

• IV Class III for pharmacological cardioversion of recent-onset AF/flutter, particularly flutter. Carries the highest torsades risk of any antiarrhythmic (≈4–8%) — give with magnesium pre-treatment and continuous ECG monitoring for at least 4 hours after the dose. [1]

Dofetilide

• Pure IKr blocker; oral, for AF rhythm control. Marked QT/torsades risk → mandatory in-hospital initiation with creatinine clearance-based dosing and continuous QT monitoring (mandatory 3-day monitored admission in some jurisdictions). [1]

Dronedarone

• A benzofuran derivative of amiodarone WITHOUT the iodine moiety → less pulmonary/thyroid toxicity but also less effective. Used for paroxysmal AF rhythm control. CONTRAINDICATED in heart failure — the PALLAS trial showed increased mortality/cardiovascular events in permanent AF, and ANDROMEDA showed harm in decompensated HF. Avoid in NYHA IV or recent decompensated HF, and with strong CYP3A4 inhibitors (severe interaction). [1]

Class IV — calcium channel blockers (non-dihydropyridine)

• Mechanism: block the L-type Ca²⁺ channel, principally at the SA and AV nodes (which are Ca²⁺-dependent), slowing conduction and increasing refractoriness. Only the non-dihydropyridines (verapamil, diltiazem) have AV-nodal antiarrhythmic activity; the dihydropyridines (nifedipine, amlodipine, felodipine) are vascular-selective vasodilators with no clinically useful AV-nodal effect.
• Indications: rate control of AF (when beta-blockers fail or are contraindicated); termination of AV-nodal re-entrant tachycardia (AVNRT) and AV-re-entrant tachycardia (AVRT) (adenosine is usually first-line).
• Verapamil: 5–10 mg IV slow push (repeat after 15–30 min); potent AV-nodal blockade and a negative inotrope.
• Diltiazem: 0.25 mg/kg IV (≈20 mg) over 2 min, repeat 0.35 mg/kg after 15 min, then infusion 5–15 mg/h; less negative inotropy than verapamil, often preferred in ICU for AF rate control.
• CRITICAL contraindications: (1) broad-complex tachycardia of uncertain origin — if it is actually VT, verapamil can cause cardiovascular collapse and death (negative inotropy + vasodilation in a poorly tolerating ventricle; the "verapamil death" scenario); (2) HOCM (negative inotropy worsens the dynamic outflow gradient); (3) WPW with AF (blocks the AV node → preferential conduction down the accessory pathway → VF); (4) heart failure/LV dysfunction (negative inotropy); (5) beta-blocker co-administration (severe bradycardia/asystole — never give IV verapamil and IV beta-blocker together). [1]

"Other" — adenosine, digoxin, magnesium

Adenosine

• Mechanism: agonist at the A1 adenosine receptor → opens inward-rectifier K⁺ channels + blocks Ca²⁺ entry → transient AV-nodal block. Endogenous adenosine underlies the brief asystole of the diving reflex and of reperfusion.
• Use: first-line for termination of AV-nodal re-entrant tachycardia (AVNRT) and AVRT (the regular narrow-complex SVTs). It will NOT cardiovert AF (it only transiently slows AV conduction, briefly revealing the irregularly irregular baseline) but is diagnostic — by unmasking the underlying atrial rhythm during the brief AV block.
• Dose: 6 mg rapid IV push followed by a brisk flush (it is destroyed in seconds — half-life ~10 seconds — so a central/large-bore line and a fast two-syringe technique are essential). If no response within 1–2 minutes, 12 mg rapid push, then a further 12 mg if needed. Warn the patient about the obligate, brief unpleasant sensation — flushing, chest tightness, sense of doom, and a few seconds of asystole.
• Contraindications/caveats: asthma (A2b/A3 receptor-mediated bronchospasm — avoid); denervated (transplanted) hearts (denervation hypersensitivity → prolonged asystole — halve the dose or use alternatives); 2nd/3rd-degree AV block (without pacing); WPW with AF (may preferentially conduct down the accessory pathway). It is safe in pregnancy. Always have resuscitation equipment to hand — the brief asystole occasionally persists. [1]

Digoxin

• Mechanism: inhibits the Na⁺/K⁺-ATPase → ↑ intracellular Na⁺ → reduced Na⁺/Ca²⁺ exchange → ↑ intracellular Ca²⁺ → positive inotropy; and vagomimetic (parasympathomimetic) action → slowed AV-nodal conduction (the basis of its rate-control use in AF). It has a narrow therapeutic index and accumulates in renal failure.
• Use: rate control of AF, especially when beta-blockers/non-DHP CCBs are contraindicated — notably in heart failure with reduced EF (it adds inotropy without further depression) and in HOCM (where beta-blockers are first but digoxin does not worsen the gradient, unlike CCBs in some contexts). Onset is slow (hours) — not a first-line agent for acute rate control in the haemodynamically compromised.
• Toxicity: arrhythmias (atrial tachycardia with block, bidirectional VT — virtually pathognomonic, premature ventricular complexes), bradycardia/AV block; gastrointestinal (anorexia, nausea, vomiting); neurological (confusion, weakness); visual disturbance — xanthopsia (yellow-green vision), blurred vision, photophobia. Predisposed by hypokalaemia (K⁺ and digoxin compete for the same ATPase — hypokalaemia potentiated toxicity — always correct K⁺), hypomagnesaemia, hypercalcaemia, hypothyroidism, renal failure.
• Antidote: digoxin-specific Fab antibody fragments (Digibind/DigiFab) — for life-threatening toxicity (arrhythmias, hyperkalaemia, haemodynamic instability, ingestion >10 mg in adults). Treat hyperkalaemia with Fab, NOT calcium — calcium can precipitate irreversible ventricular arrhythmia in digoxin toxicity ("stone heart" — though this is debated, the safer default is to use Fab and to be cautious with calcium). Magnesium is adjunctive. [1]

Magnesium

• Mechanism: cofactor for the Na⁺/K⁺-ATPase; suppresses early afterdepolarisations (the trigger for torsades) and L-type Ca²⁺ channels; membrane stabiliser.
• Use: first-line for torsades de pointes (2 g [10 mL of 20% solution] IV bolus over 1–2 minutes, then infusion) — effective even when serum magnesium is "normal"; also digoxin toxicity arrhythmias, polymorphic VT, and repletion of hypomagnesaemia (which drives both torsades and refractory hypokalaemia — you cannot correct K⁺ if Mg²⁺ is low). [1]

Compare: Class I sub-groups

[1]

Compare: AF rate-control vs rhythm-control agents

[4]

Compare: amiodarone vs lidocaine for VT

[3]

Clinical applications by rhythm

Atrial fibrillation (the commonest ICU arrhythmia)

ICU patients are primed for AF: sympathetic surge, electrolyte disturbance (K⁺/Mg²⁺), sepsis/inflammation, fluid overload/atrial stretch, catecholamines, and pre-existing structural disease. The rate-vs-rhythm decision and anticoagulation drive management.[4]

• Unstable AF (hypotension/shock, ischaemic chest pain, acute pulmonary oedema, altered conscious state) → synchronised DC cardioversion (120–200 J biphasic) — do not wait for drugs.
• Stable AF — RATE CONTROL is first-line for most (especially chronic, elderly, asymptomatic): beta-blocker (metoprolol IV 5 mg, esmolol infusion) or non-DHP CCB (diltiazem) or digoxin (if HF). Target HR <110 (lenient — AFFIRM/RACE 2 showed non-inferiority and fewer hospitalisations) or <80–90 if symptomatic.
• Stable AF — RHYTHM CONTROL if new-onset (<48 h), symptomatic despite rate control, younger, or first episode: amiodarone IV (300 mg over 1 h then infusion — the ICU default, especially with structural heart disease) or flecainide (only if structurally normal heart).
• AF-CHF trial (Roy 2008): in AF + heart failure, a rhythm-control strategy (amiodarone-based) was NOT superior to rate control for mortality — supporting rate control as the default in AF + HFrEF, with rhythm control reserved for symptoms/failed rate control.[2]
• Anticoagulation: assess stroke risk (CHA₂DS₂-VASc) and bleeding risk (HAS-BLED). For cardioversion, ensure therapeutic anticoagulation for ≥3 weeks before (or exclude left atrial thrombus by TOE) if AF >48 h/unknown duration.

Ventricular tachycardia

• Pulseless VT/VF → defibrillation (unsynchronised) + CPR + amiodarone 300 mg IV after the 3rd shock (then 150 mg for the 5th).
• Stable monomorphic VT (broad complex, regular, no pulse compromise) → amiodarone 300 mg IV over 20–60 min then infusion, OR procainamide (where available). Lidocaine for ischaemia-driven VT.
• Torsades de pointes (polymorphic VT in the setting of long QT) → magnesium 2 g IV bolus, stop the offending QT-prolonging drug, correct K⁺ (>4.5) and Mg²⁺ (>1.0), and if bradycardia-paced → overdrive pacing or isoprenaline to increase the heart rate and shorten QT.[5]

Supraventricular tachycardia (AVNRT / AVRT)

• Vagal manoeuvres first (modified Valsalva — REVERT trial improves success).
• Adenosine 6 mg → 12 mg rapid IV push — terminates most AVNRT/AVRT by transient AV block.
• Verapamil/diltiazem or beta-blocker if adenosine fails/contraindicated. [1]

Special: WPW (accessory pathway) with AF

• AVOID AV-nodal blockers (adenosine, verapamil, diltiazem, digoxin, beta-blockers) — they block the AV node and preferentially drive conduction down the accessory pathway → rapid anterograde conduction → degeneration to VF.
• Use amiodarone (or procainamide/flecainide — blocks the accessory pathway) or synchronised DC cardioversion if unstable. Recognise WPW + AF by the irregularly irregular broad-complex tachycardia with pre-excited (delta-wave) QRS. [1]

[1] [1]

Clinical pearls

Red flags

Evidence and trials

[2] [3]

Prognosis

Exam SAQ

[1]

References

1. [1]Vaughan Williams EM A classification of antiarrhythmic actions reassessed after a decade of new drugs J Clin Pharmacol, 1984.PMID 6144698
2. [2]Roy D, Talajic M, Nattel S, et al. Rhythm control versus rate control for atrial fibrillation and heart failure N Engl J Med, 2008.PMID 18565859
3. [3]Kudenchuk PJ, Brown SP, Daya M, et al. Amiodarone, Lidocaine, or Placebo in Out-of-Hospital Cardiac Arrest N Engl J Med, 2016.PMID 27043165
4. [4]Zimetbaum P Antiarrhythmic drug therapy for atrial fibrillation Circulation, 2012.PMID 22249528
5. [5]Gowda RM, Khan IA, Wilbur SL Torsade de pointes: the clinical considerations Int J Cardiol, 2004.PMID 15203254
6. [6]Heist EK, Ruskin JN Drug-induced arrhythmia Circulation, 2010.PMID 20921449