Antiarrhythmics — Vaughan-Williams Classification

Overview & definition

The antiarrhythmics — the Vaughan-Williams classification (4 classes by the ion channel / receptor target). The Sicilian gambit (the alternative — by the mechanism). The clinical use in the ICU (the AF, the VT, the SVT).[1]

The Vaughan-Williams classification groups antiarrhythmics by the dominant ion channel or receptor the drug acts upon — proposed by EM Vaughan Williams in 1970 and refined in 1984.[1] It is imperfect (many drugs span several classes — amiodarone has Class I, II, III and IV actions — and it ignores clinical context such as structural heart disease), yet it remains the lingua franca of the exam and the bedside. The Sicilian Gambit (1991) is the alternative mechanistic framework: it classifies drugs by their precise molecular target (channel, receptor, pump), the resulting cellular effect (e.g. ↓ automaticity, ↑ refractoriness), and the clinical goal — 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).[12] 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.

Three questions decide which drug (or electricity): (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 flecainide and several Class I agents — CAST).[2]

The Vaughan-Williams classification

The clinical use

• The amiodarone (class III + I + II + IV — the multichannel): the VT, the AF. The half-life (the weeks). The adverse: the pulmonary fibrosis, the thyroid (both hypo and hyper), the hepatitis, the corneal microdeposits, the photosensitivity, the QT prolongation + the torsades.[1]
• The lidocaine (class Ib): the VT (the post-MI; the ischemic). The - the - the.[1]
• The beta-blockers (class II): the rate control (the AF). The esmolol (the short half-life — the ICU titration).[1]
• The adenosine: the SVT termination (the AV node block). The 6 mg then 12 mg rapid IV push. The - the - the (the — the asthma, the heart transplant).[1]
• The digoxin: the AF rate control (the vagomimetic + the - the - the). The toxicity (the - the - the).[1]
• The magnesium: the torsades (the - the - the).[1]

Red flags

The Singh–Vaughan Williams mechanism & the Sicilian Gambit

The original Vaughan-Williams scheme classifies by the dominant ion-channel effect, but the Singh–Vaughan Williams refinement and the Sicilian Gambit extend it by mapping each drug to a precise molecular target and the resulting electrophysiological consequence. The cellular effects that antiarrhythmics exploit are:[1]

• Reduce automaticity (phase-4 depolarisation) — by blocking the pacemaker current (I_f) or β-receptors (Class II), or Ca²⁺ channels (Class IV) at the SA/AV node. This slows a rapidly firing focus.
• Reduce conduction velocity (phase-0 depolarisation) — by blocking the fast Na⁺ channel (Class I). This abolishes re-entry by converting unidirectional block into bidirectional block.
• Prolong the refractory period / action-potential duration — by blocking the delayed-rectifier K⁺ current I_Kr (Class III). This lengthens the time a re-entrant wavefront encounters refractory tissue, abolishing the circuit.
• Reduce triggered activity (early and delayed afterdepolarisations) — magnesium suppresses EADs (torsades); calcium blockers and beta-blockers suppress DADs (digoxin toxicity, catecholaminergic VT). [1]

The Sicilian Gambit arranges drugs on a grid of targets (Na⁺, Ca²⁺, K⁺ channels; β-adrenoceptors; muscarinic receptors; the Na⁺/K⁺ pump) crossed with effects (membrane stabilisation, automaticity, conduction, refractoriness, contractility). Its virtue is that it explains why a particular drug suits a particular arrhythmia — and predicts the pro-arrhythmic and toxic profile. Its weakness is complexity at the bedside, which is why the 4-class shorthand endures.[1]

Class-specific ECG signatures: Class Ia prolongs QRS and QT; Class Ib shortens QT (the only group that does); Class Ic widens QRS (marked conduction slowing) with little QT effect; Class II and IV prolong the PR interval (AV-nodal effect); Class III prolongs the QT interval. [1]

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, depressing conduction velocity. They are sub-classified by their kinetics of binding/unbinding and their effect on action-potential duration (APD). The state-dependent binding concept is exam gold: Class Ib drugs bind preferentially to inactivated channels (which predominate in ischaemic/depolarised tissue), explaining their niche in ischaemic VT.[1]

Class Ia — quinidine, procainamide, disopyramide

• Mechanism: intermediate Na⁺ channel kinetics; moderate phase-0 depression AND K⁺ channel block → prolong APD and QT. The only Class I subgroup that lengthens repolarisation.
• Indications: historically atrial and ventricular arrhythmias; procainamide IV still used for tolerated VT and to terminate WPW+AF; disopyramide for HOCM (negative inotropy reduces outflow gradient) and vagally-mediated AF.
• Doses: procainamide 20–50 mg/min IV until response, hypotension, QRS widens >50%, or 17 mg/kg; then 1–4 mg/min infusion. Quinidine largely historic. Disopyramide 100–200 mg PO.
• Adverse effects: quinidine — cinchonism (tinnitus, headache), thrombocytopenia, torsades ("quinidine syncope"), doubles digoxin levels (displace from tissue binding); procainamide → active metabolite NAPA (N-acetylprocainamide, a Class III), drug-induced lupus (anti-histone antibodies, ~30% chronic) — more common in slow acetylators; disopyramide — potent anticholinergic (urinary retention, glaucoma) and negative inotropy.
• Contraindications: long QT, torsades, digoxin toxicity; disopyramide in HFrEF/AV block.
• QT risk: HIGH. Largely superseded by amiodarone; availability is increasingly limited. [1]

Class Ib — lidocaine, phenytoin, mexiletine

• Mechanism: fast Na⁺ channel kinetics (bind/recover rapidly each cycle) and they shorten APD. Crucially, they preferentially act on ischaemic/depolarised tissue (where Na⁺ channels dwell in the inactivated state that Ib drugs bind) — hence their niche in ischaemic VT.[1]
• Indications: monomorphic VT in acute ischaemia/infarction (first-line); digoxin-toxic arrhythmias (phenytoin); neuropathic pain (mexiletine, oral lidocaine analogue). Ineffective for AF/SVT — do not reach for them in atrial arrhythmia.
• Doses: lidocaine 1–1.5 mg/kg IV bolus (→ 3 mg/kg total, may repeat), then 1–4 mg/min infusion (reduce in HF/hepatitis/age >70). Phenytoin loading 10–15 mg/kg IV slowly.
• Adverse effects: predominantly CNS — perioral tingling, metallic taste, tinnitus, light-headedness, seizures, coma (level-related); hypotension/bradycardia at high dose; phenytoin — gingival hyperplasia, nystagmus, ataxia, infusion-site reactions (purple-glove), hypotension (propylene glycol vehicle).
• Contraindications: atrial arrhythmias (won't work); severe AV block; prior idiosyncrasy. Reduce dose in liver failure/low cardiac output (lidocaine is hepatic-flow dependent).
• QT risk: NONE (shortens QT). This is the safest Class I group for the QT interval.

Class Ic — flecainide, propafenone

• Mechanism: slow Na⁺ channel kinetics → marked depression of conduction velocity (prominent QRS widening), minimal effect on APD/QT. Strong use-dependence (more block at faster rates).
• Indications: pharmacological cardioversion of recent-onset AF ("pill-in-the-pocket" in selected patients); AVNRT/AVRT; ventricular ectopy suppression (historically — see CAST). Propafenone has weak β-blocking activity.
• Doses: flecainide 2 mg/kg IV over 10 min (or 200–300 mg PO); propafenone 2 mg/kg IV over 10 min (or 450–600 mg PO).
• Adverse effects: pro-arrhythmia (incessant VT, organise AF into atrial flutter with 1:1 conduction — so ALWAYS co-prescribe an AV-nodal blocker), negative inotropy, QRS widening.
• Contraindications: any structural heart disease / LV dysfunction / ischaemia — CAST (1989/1991) showed increased mortality; established Brugada; HFrEF; second-/third-degree AV block. Flecainide challenge (provocative test) diagnoses Brugada but is contraindicated once established.[2]
• QT risk: low-moderate (mainly conduction/QRS, not QT) — but the pro-arrhythmic risk is lethal in damaged hearts.

Class II — beta-blockers (β-adrenoceptor antagonists)

• 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: rate control of AF/flutter/SVT (first-line); termination of AVNRT (with vagal manoeuvres); SVT prophylaxis; rate control in thyrotoxicosis/aerobic-exercise arrhythmia; reduces mortality post-MI and in HFrEF (the only antiarrhythmic class proven to improve survival); beta-blocker overdose (treated — see toxicology); congenital long-QT syndrome.
• Agents & doses:
  • Metoprolol 5 mg IV slow push, repeat every 5 min up to 15 mg; PO 25–100 mg.
  • Esmolol — loading 500 mcg/kg/min over 1 min then 50–300 mcg/kg/min infusion. Half-life ~9 minutes, cleared by plasma esterases (independent of liver/kidney) → ideal for ICU titration; if it causes hypotension/bradycardia it wears off in minutes.
  • Landiolol — ultra-short-acting β1-selective (half-life ~4 min), used for critically ill/septic AF in some regions.
  • Bisoprolol/carvedilol/nebivolol — oral chronic therapy (HFrEF mortality benefit).
• Adverse effects: bradycardia, AV block, hypotension, negative inotropy (worsens decompensated HF), bronchospasm (non-selective), masking of hypoglycaemia, fatigue, erectile dysfunction, hyperkalaemia (β2 blockade impairs cellular K⁺ uptake).
• Contraindications: decompensated heart failure (acute), severe asthma/bronchospasm (non-selective), advanced AV block (without pacemaker), severe peripheral vascular disease, cocaine intoxication (unopposed α → hypertension — use benzodiazepines first; consider labetalol/carvedilol which block α too).
• QT risk: low (and chronic use SHORTENS QT — used to treat congenital long-QT). Class II is among the safest antiarrhythmics for QT.

Class III — potassium channel blockers (prolong repolarisation)

Class III drugs block the delayed-rectifier potassium current (I_Kr), 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. A unifying caveat: reverse-use dependence — the QT-prolonging effect is greatest at slow heart rates, so bradycardia, hypokalaemia and hypomagnesaemia amplify torsades risk.[1]

Amiodarone — the workhorse ICU antiarrhythmic (full deep dive)

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 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.[1]

Dosing:

• Cardiac arrest (VF/pulseless VT): 300 mg IV bolus after the 3rd shock; 150 mg at the 5th shock; then 900 mg infusion over 24 h.
• Stable VT / pharmacological cardioversion of AF: 300 mg IV over 20–60 min (or 5 mg/kg), then 900 mg over 24 h (1 g/day).
• Oral loading: 600–1600 mg/day for 1–3 weeks → maintenance 100–400 mg/day. [1]

The four-pillar toxicity profile — know all four: [1]

1. PULMONARY toxicity (amiodarone-induced pulmonary toxicity, APT). The most feared — interstitial pneumonitis / pulmonary fibrosis, occasionally organising pneumonia or ARDS. Risk relates to cumulative dose and duration; onset months-years into therapy (but acute pulmonary toxicity after IV bolus is described). Presents with dry cough, dyspnoea, new infiltrates on imaging. Mortality 10–20% if unrecognised. Diagnosis is one of exclusion; baseline + surveillance chest imaging and DLCO are mandatory. Treatment = stop amiodarone ± corticosteroids.[1]
2. THYROID toxicity (both directions). Amiodarone is ~40% iodine by weight — a single 200 mg tablet delivers ~75 mg of organic iodine (50× the daily requirement). This produces both amiodarone-induced thyrotoxicosis (AIT, type 1 iodine-induced and type 2 destructive thyroiditis) and amiodarone-induced hypothyroidism. AIT is harder to treat and carries excess mortality in the ICU. Check TSH at baseline and every 6 months.
3. HEPATIC toxicity. Transaminitis (common, often subclinical, up to ~25%), clinically significant hepatitis, and rarely cirrhosis/acute liver failure. The IV formulation (polysorbate 80 vehicle) can cause acute hepatotoxicity. Check LFTs at baseline and every 6 months.
4. SKIN & ocular toxicity. Blue-grey (slate-grey) discolouration of sun-exposed skin + photosensitivity (cosmetically distressing, slowly reversible on cessation); corneal microdeposits (virtually universal — vortex keratopathy, usually asymptomatic but rarely cause halos/photophobia — reversible). Less heralded: bluish nail/skin deposits, and chronic peripheral neuropathy, tremor, ataxia, optic neuropathy (rare, serious).[1]

Drug interactions (the exam favourite): amiodarone inhibits CYP3A4, CYP2C9, and P-glycoprotein → halve the dose of warfarin (marked INR rise) and halve the dose of digoxin (P-gp), and increase levels of statins (simvastatin — rhabdomyolysis risk), ciclosporin, flecainide, phenytoin. Additive QT prolongation with all Class Ia/III drugs, macrolides, fluoroquinolones, antipsychotics, methadone. [1]

QT prolongation: amiodarone reliably prolongs the QT — but torsades is uncommon (it homogeneously prolongs repolarisation across the myocardial wall, reducing transmural dispersion — unlike ibutilide/sotalol). Nonetheless, correct K⁺/Mg²⁺ and avoid co-prescription with other QT-prolongers where possible. [1]

Sotalol

• Class III (K⁺ block, I_Kr) 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.
• QT risk: HIGH. [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.
• Dose: 1 mg IV over 10 min (0.01 mg/kg <60 kg), repeat once if needed.
• QT risk: HIGHEST. [1]

Dofetilide

• Pure I_Kr 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, with specialist prescriber qualification required). [1]

Dronedarone

• A benzofuran derivative of amiodarone WITHOUT the iodine moiety → less pulmonary/thyroid/skin toxicity but also less effective than amiodarone. Used for paroxysmal/persistent AF rhythm control. ATHENA (2009) showed reduced CV hospitalisation/death in stable AF — the one positive dronedarone trial.[7]
• CONTRAINDICATED in heart failure — ANDROMEDA (Køber 2008) showed increased mortality in decompensated HFrEF (early excess deaths).[9] PALLAS (Connolly 2011) showed increased mortality/stroke/CV hospitalisation in permanent (long-standing persistent) AF → dronedarone is now restricted to paroxysmal/persistent AF that can be cardioverted to sinus rhythm.[8]
• AVOID with strong CYP3A4 inhibitors (severe interaction) and in severe hepatic impairment. QT risk moderate.

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.[1]
• Indications: rate control of AF/flutter (when beta-blocker contraindicated); termination of AVNRT (IV verapamil/diltiazem); idiopathic left ventricular fascicular VT (verapamil-sensitive) — the one VT that verapamil treats.
• Doses: diltiazem 0.25 mg/kg IV (~20 mg) over 2 min, repeat 0.35 mg/kg at 15 min, then 5–15 mg/h infusion; verapamil 5–10 mg IV over 2 min (repeat 5–10 mg at 15–30 min if needed). Oral forms for chronic rate control.
• Adverse effects: bradycardia, AV block, hypotension, negative inotropy (worsens HFrEF), constipation (verapamil).
• Contraindications: broad-complex tachycardia of uncertain origin (if it is VT → cardiovascular collapse/death — the classic "verapamil death"); HFrEF (negative inotropy); HOCM (worsens outflow gradient); WPW + AF (→ VF); second-/third-degree AV block; concomitant IV beta-blocker (severe bradycardia/asystole).
• QT risk: none. (These drugs slow the AV node and shorten the ventricular response, not QT.)

"Class V" — 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. Half-life ~10 seconds (rapidly deaminated/uptaken by RBCs) → effects vanish within one circulation time.
• Indication: termination of regular narrow-complex SVT (AVNRT/AVRT — re-entry involving the AV node). Diagnostic use in stable irregular/broad tachycardia (transient AV block reveals underlying atrial activity). Will NOT cardiovert AF (only transiently slows it).
• Dose: 6 mg rapid IV push followed immediately by a flush, then 12 mg if no response; can repeat 12 mg once. Must warn the patient of the brief unpleasant "doom"/asystole/sense of impending death. Lower dose (3 mg) in denervated (transplanted) hearts, central line administration, or dipyridamole pretreatment.
• Adverse effects: flushing, chest tightness, dyspnoea, brief asystole (obligate — the mechanism); usually resolves within seconds.
• Contraindications: asthma/COPD with bronchospasm (adenosine is a bronchoconstrictor); denervated (transplanted) hearts (denervation hypersensitivity → prolonged asystole — halve the dose); second-/third-degree AV block; sick sinus syndrome. Always have resuscitation gear ready.[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.
• Indications: rate control of AF (especially in HF/HOCM where beta-blocker/CCB are problematic — it is the one rate-control drug safe in HFrEF); adjunct in HFrEF (mild symptomatic benefit; neutral mortality). Onset slow (hours) — not for acute rate control alone.
• Dose: 500 mcg IV (repeated every 6–8 h to loading ~1 mg), then 62.5–250 mcg/day (renal-adjusted). Therapeutic level 0.5–0.9 ng/mL (modern low targets).
• Toxicity: atrial tachycardia with block, bidirectional VT (pathognomonic), bradycardia/AV block, premature ventricular complexes, nausea/vomiting, xanthopsia (yellow-green vision), confusion, hyperkalaemia (the ATPase is poisoned → K⁺ leaks out of cells — a high K⁺ suggests severe toxicity). Predisposed by hypokalaemia (digoxin and K⁺ compete for the ATPase — always correct K⁺), renal failure, hypomagnesaemia, hypercalcaemia, hypothyroidism.
• Antidote: digoxin-specific Fab antibody fragments (Digibind/DigiFab) for life-threatening arrhythmia, hyperkalaemia, massive overdose. Do NOT give calcium (theoretical "stone heart" — controversial, but Fab is definitive).[12]
• QT risk: low (digoxin does not prolong QT — at toxic levels it shortens QT and causes the characteristic "digoxin effect" sagging ST segments).

Magnesium

• Mechanism: cofactor for the Na⁺/K⁺-ATPase; suppresses early afterdepolarisations (the trigger for torsades) and L-type Ca²⁺ channels; membrane stabiliser. Effective even when serum Mg²⁺ is "normal" (intracellular depletion matters more than serum).
• Indications: torsades de pointes (first-line); digoxin toxicity adjunct; hypomagnesaemia; polymorphic VT; AF rate control (refractory); eclampsia prophylaxis/treatment.
• Dose: 2 g (8 mmol) IV over 1–2 min (torsades), then 1–2 g/h infusion; 1–2 g IV in cardiac arrest with suspected hypomagnesaemia/torsades.
• Adverse effects: flushing, hypotension, bradycardia, areflexia (dose-related; sign of toxicity — check reflexes), respiratory depression at high levels; potentiates neuromuscular blockers.
• Contraindications: significant renal failure (risk of hypermagnesaemia — reduce dose, monitor levels/reflexes), heart block.
• QT risk: SHORTENS QT (anti-torsadogenic). [1]

QT prolongation risk — ranked

Beyond antiarrhythmics, common ICU QT-prolonging culprits (the "hidden killer"): macrolides (azithromycin, clarithromycin), fluoroquinolones (ciprofloxacin, moxifloxacin), antipsychotics (haloperidol — common in ICU delirium), methadone, ondansetron, fluconazole. Risk is additive and amplified by bradycardia, hypokalaemia, hypomagnesaemia. QTc >500 ms or a >60 ms rise from baseline → stop/reduce the culprit and recheck.[12]

Amiodarone toxicity — monitoring schedule

Compare: Class I sub-groups

[1]

Compare: AF rate-control vs rhythm-control agents

[13]

Compare: amiodarone vs lidocaine for VT

[6]

Compare: amiodarone vs dronedarone

[7]

FlowSteps: management of new-onset atrial fibrillation in the ICU

[13]

FlowSteps: management of ventricular tachycardia in the ICU

[6]

FlowSteps: termination of regular narrow-complex SVT in the ICU

[10] [1] [1]

Clinical pearls

Red flags

Evidence and trials

[2] [3] [5] [6] [4] [8] [9] [7] [10]

Prognosis & exam framing

• AF in ICU: commonest ICU arrhythmia (10–30% of critically ill, higher post-cardiac surgery); independently associated with longer ICU stay, stroke, and mortality — but mostly a marker of illness severity. Most resolves with correction of the trigger.
• Rate vs rhythm (AFFIRM, AF-CHF, RACE II): rate control is non-inferior to rhythm control for mortality in stable AF and in AF + HFrEF; lenient rate control (HR <110) is non-inferior to strict — rate control is the default.[3][5][4]
• ALPS (2016): amiodarone/lidocaine no overall OHCA survival benefit; benefit in witnessed/bystander-CPR subgroup.[6]
• Amiodarone toxicity: pulmonary fibrosis mortality 10–20% if unrecognised; thyroid dysfunction in ~10–20% on chronic therapy; routine surveillance (TFTs, LFTs, chest imaging, DLCO) is mandatory — the toxicity is the reason amiodarone is not first-line for long-term outpatient AF despite being most effective.[1]
• Torsades: mortality high if untreated; magnesium terminates most episodes; prognosis driven by the underlying QT-prolonging cause and substrate.[11]
• Dronedarone: useful ONLY in stable paroxysmal/persistent AF (ATHENA); harmful in HFrEF (ANDROMEDA) and permanent AF (PALLAS).[7][8][9]
• Drug-induced arrhythmia: pro-arrhythmia from QT-prolonging drugs (antiarrhythmics, antibiotics, antipsychotics, methadone) is a leading iatrogenic cause of sudden death — vigilance for QTc >500 ms and additive risk is essential.[12]

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]Echt 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.PMID 1900101
3. [3]Wyse DG, Waldo AL, DiMarco JP, et al. A comparison of rate control and rhythm control in patients with atrial fibrillation N Engl J Med, 2002.PMID 12466506
4. [4]Van Gelder IC, Groenveld HF, Crijns HJ, et al. Lenient versus strict rate control in patients with atrial fibrillation N Engl J Med, 2010.PMID 20231232
5. [5]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
6. [6]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
7. [7]Hohnloser SH, Crijns HJ, van Eickels M, et al. Effect of dronedarone on cardiovascular events in atrial fibrillation N Engl J Med, 2009.PMID 19213680
8. [8]Connolly SJ, Camm AJ, Halperin JL, et al. Dronedarone in high-risk permanent atrial fibrillation N Engl J Med, 2011.PMID 22082198
9. [9]Køber L, Torp-Pedersen C, McMurray JJ, et al. Increased mortality after dronedarone therapy for severe heart failure N Engl J Med, 2008.PMID 18565860
10. [10]Appelboam A, Reuben A, Mann C, et al. Postural modification to the standard Valsalva manoeuvre for emergency treatment of supraventricular tachycardias (REVERT): a randomised controlled trial Lancet, 2015.PMID 26314489
11. [11]Gowda RM, Khan IA, Wilbur SL Torsade de pointes: the clinical considerations Int J Cardiol, 2004.PMID 15203254
12. [12]Heist EK, Ruskin JN Drug-induced arrhythmia Circulation, 2010.PMID 20921449
13. [13]Zimetbaum P Antiarrhythmic drug therapy for atrial fibrillation Circulation, 2012.PMID 22249528