Overview: Singh-Vaughan Williams Classification
Anti-arrhythmic drugs are classified by the Singh-Vaughan Williams system based on their primary electrophysiological mechanism. Understanding this framework is essential for the ICU, where arrhythmias are common and drug selection must account for haemodynamic compromise, organ failure, and drug interactions.
| Class | Primary Mechanism | Ion Channel/Receptor | Key Drugs |
|---|---|---|---|
| Ia | Na⁺ channel block (intermediate kinetics) | $Na_V$ (use-dependent) + K⁺ | Quinidine, Procainamide, Disopyramide |
| Ib | Na⁺ channel block (fast kinetics) | $Na_V$ (fast off-rate) | Lignocaine, Mexiletine |
| Ic | Na⁺ channel block (slow kinetics) | $Na_V$ (slow off-rate) | Flecainide, Propafenone |
| II | β-adrenergic blockade | β₁/β₂ receptors | Metoprolol, Esmolol, Propranolol |
| III | K⁺ channel block → AP prolongation | $I_{Kr}$, $I_{Ks}$ (hERG) | Amiodarone, Sotalol, Dofetilide, Ibutilide |
| IV | Ca²⁺ channel block | $I_{CaL}$ (L-type) | Verapamil, Diltiazem |
| Misc | Various | IK, $I_{Ca}$, Na⁺/K⁺ ATPase | Adenosine, Magnesium, Digoxin |
Mechanisms of Arrhythmogenesis
Before discussing drugs, understanding the three arrhythmogenic mechanisms is critical:
Automaticity
Enhanced automaticity occurs when spontaneous phase 4 depolarisation is accelerated, seen in ischaemia, hypokalaemia, catecholamine excess (all common in ICU). Relevant to: ventricular ectopy, accelerated junctional rhythms.
Triggered Activity
Abnormal depolarisations arising during or after a preceding action potential:
- Early afterdepolarisations (EADs): during phase 2/3, promoted by bradycardia and QT prolongation → mechanism of torsades de pointes
- Delayed afterdepolarisations (DADs): during phase 4, promoted by intracellular Ca²⁺ overload → digitalis toxicity, catecholaminergic VT
Re-entry
The most common mechanism for sustained tachyarrhythmias (AF, flutter, VT). Requires a unidirectional block and differential conduction velocities in adjacent tissue, common in ischaemic and fibrotic myocardium.
Class I, Sodium Channel Blockers
Mechanism
All Class I drugs produce use-dependent block of voltage-gated $Na_V$ channels, the drug preferentially binds the open or inactivated state. This slows phase 0 depolarisation, reduces conduction velocity, and elevates the excitability threshold. Subclass differences relate to kinetics of channel recovery (off-rate):
$$\text{Recovery kinetics: Ib (fast) < Ia (intermediate) < Ic (slow)}$$
Class Ia
- Drugs: Quinidine, Procainamide, Disopyramide
- Additional K⁺ channel block → prolongs action potential duration and QT interval
- Procainamide IV: used for VT and AF cardioversion; active metabolite NAPA (also Class III activity) accumulates in renal failure
- ICU concern: Negative inotropy, hypotension, lupus-like syndrome (procainamide with long-term use)
Class Ib
- Drugs: Lignocaine (Lidocaine), Mexiletine
- Fast off-rate → preferentially suppresses depolarised, ischaemic tissue (ideal for ischaemia-related VT)
- Lignocaine IV: Loading dose 1-1.5 mg/kg, infusion 1-4 mg/min; hepatic first-pass eliminates oral bioavailability
- Toxicity: CNS (seizures, confusion) before cardiac toxicity, monitor in hepatic failure (reduced clearance) and in low cardiac output states (reduced hepatic blood flow)
Class Ic
- Drugs: Flecainide, Propafenone
- Slowest off-rate → marked QRS widening, profound conduction slowing
- Contraindicated post-MI and in structural heart disease (CAST trial: increased mortality)
- Propafenone also has β-blocking activity
Class II, β-Adrenergic Receptor Blockers
Mechanism
Block β₁ (and β₂) receptors → reduce sympathetically-driven automaticity, slow SA node rate (negative chronotropy), slow AV nodal conduction (increase PR interval, negative dromotropy), reduce contractility (negative inotropy).
ICU-Relevant Agents
| Drug | Selectivity | Route | Half-life | ICU Use |
|---|---|---|---|---|
| Esmolol | β₁ selective | IV infusion | ~9 min | Acute rate control: AF, SVT, post-op hypertension |
| Metoprolol | β₁ selective | IV/PO | 3-7 h | Rate control, post-ACS |
| Propranolol | Non-selective | IV/PO | 4-6 h | Thyroid storm, phaeochromocytoma |
| Labetalol | β + α₁ | IV | 5-8 h | Hypertensive emergency |
- Esmolol is the preferred ICU agent due to ultra-short action (hydrolysed by red cell esterases); loading dose 500 mcg/kg over 1 min, infusion 50-200 mcg/kg/min
- Caution in acute decompensated heart failure (may precipitate cardiogenic shock), reactive airway disease, and significant bradycardia/heart block
- β-blockers reduce mortality post-MI through anti-arrhythmic and anti-remodelling effects
Class III, Potassium Channel Blockers
Mechanism
Block repolarising K⁺ currents (primarily $I_{Kr}$ via hERG channels, and $I_{Ks}$) → prolonged action potential duration → prolonged QT interval → increased refractory period. This widens the therapeutic window against re-entry but carries the risk of EADs and torsades de pointes (TdP).
Amiodarone, The Most Important ICU Anti-Arrhythmic
Amiodarone is a class III agent but has multi-class effects, it also blocks $Na_V$ channels (Class I), has β-blocking activity (Class II), and $I_{CaL}$ blockade (Class IV). This "paradoxical" profile makes it broadly effective but pharmacologically complex.
Pharmacokinetics:
| Parameter | Value |
|---|---|
| Oral bioavailability | 20-55% (variable) |
| Volume of distribution | 40-70 L/kg (highly lipophilic) |
| Protein binding | >96% |
| Half-life | 40-55 days (chronic) |
| Metabolism | Hepatic; active metabolite desethylamiodarone |
| Elimination | Biliary (not renally cleared, safe in renal failure) |
Dosing (ICU, IV):
- Loading: 150-300 mg IV over 10-20 min (or 5 mg/kg over 30-60 min)
- Infusion: 900 mg over 24 h (1 mg/min for 6 h, then 0.5 mg/min for 18 h)
- Further loading may be required for recurrent arrhythmias
Indications: AF with haemodynamic compromise, VF/pulseless VT (300 mg IV in ACLS), VT with preserved haemodynamics, rate control in AF when other agents contraindicated
Toxicity (chronic):
| Organ System | Toxicity |
|---|---|
| Pulmonary | Pneumonitis, fibrosis (cumulative dose-related) |
| Thyroid | Hypothyroidism or hyperthyroidism (contains ~37% iodine by weight) |
| Hepatic | Elevated transaminases, hepatitis |
| Ocular | Corneal microdeposits (nearly universal), optic neuropathy (rare) |
| Cardiac | Bradycardia, QT prolongation, TdP (rare due to multi-class effects) |
| Neurological | Peripheral neuropathy, tremor, ataxia |
Drug interactions: Potentiates warfarin, digoxin toxicity (inhibits P-glycoprotein), increases levels of many QT-prolonging drugs; inhibits CYP2D6 and CYP3A4.
Sotalol
- D-sotalol: pure Class III; DL-sotalol (racemic): Class II + III
- Renally cleared, dose reduce in renal failure (high risk of TdP accumulation)
- Significant risk of TdP (2-4%); avoid with QTc >470 ms
Dofetilide and Ibutilide
- Pure $I_{Kr}$ blockers; primarily for AF cardioversion
- Dofetilide: Renally cleared; QTc monitoring mandatory; TdP risk ~1-3%
- Ibutilide: IV only; ~4% TdP incidence; used for AF/flutter cardioversion in hospital
Class IV, Calcium Channel Blockers
Mechanism
Block L-type ($I_{CaL}$) voltage-gated calcium channels. Since SA node automaticity and AV node conduction depend on calcium-dependent ("slow response") action potentials, Class IV drugs reduce automaticity and slow AV conduction, rate control without significant action potential prolongation.
Verapamil vs Diltiazem
| Feature | Verapamil | Diltiazem |
|---|---|---|
| Cardiac effect | Greater negative inotropy | Moderate negative inotropy |
| Vascular selectivity | Lower (also blocks K⁺ channels) | Moderate |
| IV use | Yes, SVT, AF rate control | Yes, AF rate control |
| Oral bioavailability | ~20% | ~40% |
| Metabolism | Hepatic (CYP3A4) | Hepatic |
- Both prolong the PR interval and slow AV nodal conduction
- Contraindicated with concurrent IV β-blockade (risk of complete heart block and asystole)
- Contraindicated in WPW syndrome (can accelerate accessory pathway conduction)
- Contraindicated in pre-excited AF and significant LV dysfunction
- Verapamil/diltiazem + digoxin: increased digoxin levels and risk of heart block
Miscellaneous Anti-Arrhythmic Agents
Adenosine
- Mechanism: Activates inward rectifier $I_K$ and inhibits $I_{Ca}$ → profound but transient AV nodal blockade
- Duration of action: 10-15 seconds (inactivated by cellular uptake and deamination)
- Dose: 6 mg rapid IV push; if unsuccessful, 12 mg × 2; use central line for delivery if possible (faster transit time)
- Indications: Termination of paroxysmal SVT (re-entrant involving AV node); diagnostic use for broad complex tachycardia
- Toxicity: Flushing, chest tightness, bronchospasm, transient asystole (warn patient)
- Interactions: Theophylline is a competitive antagonist (higher doses required); dipyridamole potentiates effects
Magnesium
- Mechanism: Poorly understood; interacts with Na⁺/K⁺ ATPase, and modulates K⁺ and Ca²⁺ channels; stabilises membrane potential
- Indications: Torsades de pointes (first-line, regardless of serum Mg²⁺ level), digoxin-induced arrhythmias
- Dose: 2 g (8 mmol) MgSO₄ IV over 10-15 min, followed by infusion if required
- Toxicity: Muscle weakness, loss of reflexes, respiratory depression (at serum Mg²⁺ >4 mmol/L)
Proarrhythmic Risk and QT Prolongation
A major ICU concern is drug-induced QT prolongation and TdP. The hERG channel ($I_{Kr}$) is uniquely susceptible to block by structurally diverse drugs because of its unusually large inner vestibule. The critical concept is:
$$QTc = \frac{QT}{\sqrt{RR}} \quad \text{(Bazett's formula)}$$
Risk factors for TdP in ICU patients:
| Risk Factor | Mechanism |
|---|---|
| QTc >500 ms | Increased EAD risk |
| Hypokalaemia | Reduces repolarisation reserve |
| Hypomagnesaemia | Loss of membrane stabilisation |
| Bradycardia | Longer diastole → greater AP duration |
| Female sex | Longer baseline QTc |
| Polypharmacy | Additive QT prolongation |
| Renal/hepatic failure | Drug accumulation |
Common ICU QT-prolonging drugs beyond anti-arrhythmics include: haloperidol, ondansetron, azithromycin, fluconazole, methadone, hydroxychloroquine.
ICU Relevance
Arrhythmia Management Targets in Critical Illness
| Arrhythmia | First-line ICU Approach | Pharmacological Options |
|---|---|---|
| AF with rapid ventricular response (haemodynamically stable) | Rate control (HR <110 bpm) | Metoprolol IV, diltiazem IV, amiodarone IV |
| AF/SVT with haemodynamic instability | Synchronised DC cardioversion | Amiodarone for post-cardioversion maintenance |
| VF/pulseless VT (ACLS) | Single shock, then immediate CPR for 2 min | Amiodarone 300 mg IV after 3rd shock, then 150 mg |
| Torsades de pointes | Correct precipitants + Mg²⁺ | MgSO₄ 2 g IV; overdrive pacing if refractory |
| SVT (haemodynamically stable) | Vagal manoeuvres first | Adenosine 6-12 mg rapid IV; verapamil if adenosine fails |
Key Organ Failure Considerations
- Renal failure: Avoid/dose-reduce sotalol and dofetilide (TdP risk); procainamide NAPA accumulates; lignocaine relatively safe (hepatic); amiodarone safe (biliary elimination)
- Hepatic failure: Reduce lignocaine infusion rate (reduced clearance and reduced hepatic blood flow); reduce verapamil dose; amiodarone with caution (hepatotoxic)
- Low cardiac output: Lignocaine clearance reduced (reduced hepatic blood flow) → toxicity risk at standard infusion rates; reduce infusion to 1-2 mg/min
Monitoring in the ICU
- Continuous ECG monitoring: PR interval (Class II/IV), QRS duration (Class I), QTc (Class III)
- Daily 12-lead ECG when initiating or adjusting anti-arrhythmic therapy
- Electrolytes (K⁺, Mg²⁺) should be maintained in the normal-high range to reduce arrhythmia risk and TdP susceptibility
- Serum drug levels: lignocaine (therapeutic 1.5-5 µg/mL; toxic >6 µg/mL)
- Amiodarone: thyroid function, LFTs, and CXR at baseline and regularly during long-term therapy; pulmonary toxicity monitoring essential in long-term survivors
Drug Interactions Critical to ICU Practice
- Amiodarone + warfarin → significantly increased INR (may double); reduce warfarin dose by 30-50%
- Verapamil/diltiazem + β-blocker IV → risk of complete heart block and cardiovascular collapse; never combine IV
- Class III agents + other QT-prolonging drugs (antifungals, antipsychotics, antibiotics) → additive TdP risk; audit polypharmacy regularly
- Calcium channel blockers + dantrolene → haemodynamic instability and hyperkalaemia risk; invasive monitoring recommended if co-administration unavoidable