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