5.1 Vaughan Williams Antiarrhythmic Classification & EP Electrophysiology

5.1 Vaughan Williams Antiarrhythmic Classification & EP Electrophysiology

Antiarrhythmic drugs (AADs) represent both therapeutic tools and critical variables during diagnostic electrophysiology (EP) studies and catheter ablation procedures. A comprehensive understanding of cardiac cellular electrophysiology, ion channel gating kinetics, and the classical Vaughan Williams classification system is essential for the Registered Cardiac Electrophysiology Specialist (RCES). Antiarrhythmic medications alter myocardial conduction velocity, refractoriness, and automaticity, directly modifying baseline intracardiac intervals, arrhythmia inducibility, and patient hemodynamics.

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1. Biophysics of the Cardiac Action Potential

The electrical activity of myocardial tissue is broadly divided into two physiological categories:

1. Fast-Response Action Potentials: Found in working atrial myocytes, ventricular myocytes, and the specialized His-Purkinje conduction system. These cells exhibit a highly negative resting membrane potential (approximately -85 to -90 mV) and a rapid upstroke velocity.
2. Slow-Response Action Potentials: Found in pacemaker tissues of the sinoatrial (SA) node and atrioventricular (AV) node. These cells exhibit a less negative resting membrane potential (-55 to -65 mV), spontaneous Phase 4 diastolic depolarization, and a slow upstroke velocity mediated primarily by calcium influx.

Fast-Response Myocyte Action Potential:
  Voltage (mV)
   +20 ├──────┐ (Phase 1: Ito)
       │      │
     0 ├─     └──────────────┐ (Phase 2 Plateau: ICa-L vs IKr/IKs)
       │                     │
   -40 ├                      └────────┐ (Phase 3: IKr, IKs)
       │ (Phase 0: INa)                │
   -80 ├───────────────────────────────┴──────── (Phase 4: IK1 resting)
       └────────────────────────────────────────

Phases of the Fast-Response Action Potential

• Phase 0 (Rapid Depolarization): Triggered when an incoming electrical impulse depolarizes the cell membrane to the threshold potential (approximately -65 to -70 mV). This opens voltage-gated, fast sodium channels ($Nav1.5$), generating an explosive inward sodium current ($I_{Na}$). The maximum rate of voltage change over time, known as $dV/dt_{max}$ (or phase 0 upstroke velocity), determines the conduction velocity of the electrical wavefront through the myocardium. Intracellular potential shifts rapidly from -90 mV to approximately +20 to +30 mV.
• Phase 1 (Early Rapid Repolarization): Rapid inactivation of fast $Na^+$ channels combined with the brief activation of the transient outward potassium current ($I_{to}$). Potassium ions rapidly exit the cell, producing the characteristic "notch" on the action potential waveform.
• Phase 2 (Plateau Phase): A delicate equilibrium between depolarizing inward currents and repolarizing outward currents. Inward calcium flux through voltage-gated L-type calcium channels ($I_{Ca-L}$) maintains cellular depolarization, triggering calcium-induced calcium release (CICR) from the sarcoplasmic reticulum for excitation-contraction coupling. This inward current is precisely counterbalanced by outward potassium currents ($I_{Kr}$ and $I_{Ks}$).
• Phase 3 (Rapid Repolarization): Closure and inactivation of L-type calcium channels while outward delayed rectifier potassium currents dominate. Phase 3 repolarization is mediated by two primary potassium components:
  • $I_{Kr}$ (Rapid delayed rectifier): Rapidly activating and inactivating outward potassium current; highly sensitive to pharmacological blockade by Class III antiarrhythmics.
  • $I_{Ks}$ (Slow delayed rectifier): Slowly activating outward potassium current; critical for repolarization reserve, especially during adrenergic stimulation. Efflux of $K^+$ repolarizes the membrane potential back toward the negative resting baseline, terminating the effective refractory period (ERP).
• Phase 4 (Resting Membrane Potential): In working myocytes, Phase 4 maintains a stable electrical baseline at approximately -90 mV, governed by the inward rectifier potassium current ($I_{K1}$). High resting $K^+$ conductance keeps the resting potential close to the Nernst equilibrium potential for potassium ($E_K$).

Pacemaker Tissue & The Funny Current ($I_f$)

In the SA node and AV node, myocytes lack $I_{K1}$, preventing a fixed, stable resting potential. Instead, pacemaker cells exhibit spontaneous Phase 4 diastolic depolarization. As the membrane repolarizes to -40 to -50 mV at the end of Phase 3, specialized hyperpolarization-activated cyclic nucleotide-gated (HCN) channels open, generating the funny current ($I_f$)—a mixed inward sodium/potassium current. Inward calcium current through transient T-type ($I_{Ca-T}$) and subsequent L-type ($I_{Ca-L}$) calcium channels completes diastolic depolarization up to threshold (-40 mV), initiating Phase 0 depolarization. Phase 0 in nodal tissue is driven entirely by $I_{Ca-L}$, which explains why conduction velocity in the AV node is markedly slower than in His-Purkinje tissue.

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2. Vaughan Williams Class I: Sodium Channel Blockers

Class I antiarrhythmic agents bind to voltage-gated fast sodium channels ($Nav1.5$), suppressing the inward $I_{Na}$ current and reducing $dV/dt_{max}$. This slows intra-atrial, His-Purkinje, and intraventricular conduction velocities, manifesting electrographically as widening of the QRS complex. Class I agents are subclassified based on their binding/dissociation kinetics and their secondary effects on action potential duration (APD) and repolarization.

Class I Subclass Comparison:
  Subclass   INa Blockade     Binding Kinetics   Effect on APD / ERP   ECG Manifestation
  ────────   ────────────     ────────────────   ───────────────────   ─────────────────
  Class IA   Moderate         Intermediate       Prolongs              ↑ QRS, ↑ QT
  Class IB   Mild / Fast      Rapid (Fast on/off) Shortens             Normal QRS, ↓ QT
  Class IC   Marked / Potent  Slow (Slow on/off)  No change / Minimal   ↑↑ QRS, Normal QT

Class IA: Procainamide, Quinidine, Disopyramide

Class IA agents exhibit intermediate dissociation kinetics from sodium channels and simultaneously inhibit repolarizing outward potassium currents ($I_{Kr}$). Consequently, they slow conduction velocity (prolonging QRS) while simultaneously prolonging action potential duration and refractoriness (prolonging QT interval).

Procainamide (Pronestyl)

• Electrophysiological Effects: Slows conduction across working atrial and ventricular myocardium, prolongs the His-ventricular (HV) interval, and significantly prolongs the effective refractory period (ERP) of accessory atrioventricular pathways (Kent bundles) in both anterograde and retrograde directions.
• Primary EP Indications:
  1. Pre-Excited Atrial Fibrillation (WPW with AFib): Procainamide is the intravenous drug of choice in hemodynamically stable pre-excited AFib. By preferentially blocking conduction and prolonging refractoriness in the accessory pathway, it slows the rapid, irregular ventricular response and terminates the arrhythmia.
  2. Hemodynamically Stable Ventricular Tachycardia (VT): Intravenous termination of monomorphic VT.
  3. Diagnostic Provocation during EP Study: Infused during diagnostic EP testing to evaluate infranodal conduction reserve. If procainamide induces spontaneous infranodal block or prolongs the HV interval to $\ge 100\text{ ms}$, infranodal conduction disease is confirmed, establishing an indication for permanent pacing.
• IV Dosing Protocol: Administered as a weight-based loading infusion of 10 to 17 mg/kg IV, infused slowly at a rate of 20 to 50 mg/min (maximum cumulative loading dose: 1,000 mg [1.0 g]). Once the load is complete, a continuous maintenance infusion of 1 to 4 mg/min may be maintained.
• Mandatory Endpoints to Terminate Procainamide Infusion:
  1. QRS Widening >50% over baseline (reflects excessive intraventricular sodium channel blockade and severe conduction toxicity).
  2. Development of Significant Systemic Hypotension (procainamide exerts ganglionic blocking and mild peripheral vasodilatory properties).
  3. Arrhythmia Termination (successful termination of VT or pre-excited AFib).
  4. Attainment of Maximum Dose (1,000 mg) without arrhythmia termination.
• Chronic Adverse Effects: Long-term oral administration induces a drug-induced systemic lupus erythematosus (SLE) syndrome in 20% to 30% of patients, characterized by arthralgias, fever, serositis, and circulating antinuclear antibodies (ANA) targeting histones. Hepatic metabolism via $N$-acetyltransferase-2 (NAT2) produces $N$-acetylprocainamide (NAPA), an active metabolite that possesses pure Class III potassium-blocking properties and is cleared renally. In slow acetylators or renal dysfunction, NAPA accumulates, dramatically increasing the risk of Torsades de Pointes.

Quinidine & Disopyramide

• Quinidine: Historical Class IA agent. Possesses potent vagolytic (anticholinergic) and alpha-adrenergic blocking properties. It causes "quinidine syncope" secondary to drug-induced Torsades de Pointes, even at subtherapeutic serum concentrations.
• Disopyramide (Norpace): Displays marked anticholinergic (urinary retention, severe xerostomia, closed-angle glaucoma exacerbation) and profound negative inotropic effects. Clinically utilized in obstructive hypertrophic cardiomyopathy (HCM) to reduce left ventricular outflow tract (LVOT) gradients and suppress atrial arrhythmias.

Class IB: Lidocaine, Mexiletine

Class IB agents have the fastest on/off binding kinetics of all sodium channel blockers (dissociation time constant $\tau < 0.5\text{ seconds}$). They bind preferentially to open and inactivated sodium channels and rapidly dissociate during diastole. Crucially, Class IB agents shorten action potential duration and refractoriness, primarily by blocking the late inward sodium current ($I_{Na-late}$).

• Tissue Selectivity: Class IB agents are virtually ineffective in normal atrial tissue because atrial action potentials are brief and resting potentials are highly negative, allowing rapid drug dissociation. In contrast, they exhibit extreme selectivity for depolarized, ischemic, acidotic, or rapidly firing ventricular myocardium. Ischemia keeps sodium channels in the inactivated state, allowing lidocaine to bind tightly and suppress abnormal automaticity and triggered activity.
• Primary Indications: Acute suppression of ventricular tachycardia and ventricular fibrillation (VT/VF) occurring during acute myocardial ischemia or infarction; suppression of digitalis-induced ventricular ectopy.
• Lidocaine Dosing: IV bolus of 1.0 to 1.5 mg/kg over 2 to 3 minutes; supplemental boluses of 0.5 to 0.75 mg/kg every 5 to 10 minutes (maximum total: 3 mg/kg), followed by a continuous infusion of 1 to 4 mg/min.
• Central Nervous System (CNS) Toxicity: Because lidocaine crosses the blood-brain barrier rapidly, toxic serum levels (>5 mcg/mL) produce progressive neurotoxicity: circumoral paresthesias, metallic taste, drowsiness, confusion, dysarthria, visual disturbances, muscle twitching, generalized tonic-clonic seizures, and respiratory arrest.
• Mexiletine: An orally bioavailable Class IB analog of lidocaine utilized for long-term suppression of refractory ventricular arrhythmias and as an adjuvant in Long QT Syndrome Type 3 (LQT3) to block pathological gain-of-function late sodium currents ($I_{Na-late}$). Common toxicities include severe gastrointestinal distress, nausea, tremor, and ataxia.

Class IC: Flecainide, Propafenone

Class IC agents exhibit the slowest binding and dissociation kinetics (dissociation time constant $\tau > 10\text{ to }30\text{ seconds}$). They cause intense, continuous block of fast sodium channels throughout the entire cardiac cycle, producing profound reduction in $dV/dt_{max}$ and pronounced slowing of conduction across the atria, AV node, His-Purkinje system, and ventricles. However, they have virtually no effect on action potential duration or the QT interval.

• Use-Dependence: Because dissociation is exceptionally slow, sodium channel blockade accumulates at faster heart rates (use-dependence or frequency-dependence). As tachycardia accelerates, the QRS complex widens progressively, worsening conduction slowing.
• The 1:1 AV Conduction Hazard in Atrial Flutter:

  When flecainide or propafenone is administered to treat atrial flutter, the drug's potent sodium channel blockade slows the atrial flutter cycle length (e.g., slowing atrial rate from 300 bpm down to 200–220 bpm). At 300 bpm, the AV node conducts with a 2:1 ratio (ventricular rate 150 bpm). When the atrial rate slows to 200 bpm, the longer cycle length allows the AV node to recover excitability and conduct every single flutter wave down to the ventricles (1:1 AV conduction). The ventricular rate abruptly jumps from 150 bpm to 220 bpm, accompanied by severe rate-dependent QRS widening (mimicking ventricular tachycardia) and catastrophic hemodynamic collapse. Mandatory Rule: A Class IC antiarrhythmic must NEVER be prescribed for atrial flutter or atrial fibrillation without a co-administered AV nodal blocking agent (such as a beta-blocker or calcium channel blocker)!

• The CAST Trial & Structural Heart Disease Contraindication: The landmark Cardiac Arrhythmia Suppression Trial (CAST, 1989) evaluated whether suppressing asymptomatic ventricular ectopy with Class IC drugs (encainide, flecainide) in post-myocardial infarction patients with reduced ejection fraction would reduce arrhythmic death. The trial was halted prematurely because patients randomized to Class IC agents suffered a statistically significant 2.5- to 3-fold increase in lethal proarrhythmic mortality and cardiac arrest compared to placebo. Conduction slowing across border-zone ischemic scar tissue created ideal substrates for lethal, unceasing reentrant ventricular tachycardia.
  • Black Box Contraindication: Class IC agents are strictly contraindicated in patients with coronary artery disease, prior myocardial infarction, ischemic heart disease, left ventricular systolic dysfunction ($LVEF < 50% $), or significant structural heart disease (e.g., severe left ventricular hypertrophy).
• Propafenone (Rythmol): In addition to potent Class IC sodium channel blockade, propafenone exhibits mild, non-selective beta-adrenergic receptor blocking properties (approximately 1/40th the potency of propranolol), which can cause bronchospasm in asthmatic patients and resting sinus bradycardia.

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3. Vaughan Williams Class II: Beta-Adrenergic Receptor Blockers

Class II agents competitively antagonize endogenous catecholamines (epinephrine and norepinephrine) at cardiac $\beta_1$ and $\beta_2$ adrenergic receptors. Beta-adrenergic stimulation normally activates adenylyl cyclase, increasing intracellular cyclic adenosine monophosphate (cAMP) and activating protein kinase A (PKA), which phosphorylates L-type calcium channels and phospholamban.

Sympathetic Tone:  Catecholamines ──> β1 Receptor ──> ↑ cAMP ──> ↑ ICa-L / ↑ If ──> Faster SA/AV Conduction
Class II Blockade: Beta-Blocker   ──┤ β1 Receptor ──> ↓ cAMP ──> ↓ ICa-L / ↓ If ──> Slower SA/AV Conduction (↑ AH)

Cellular & Electrophysiological Mechanics

• Depression of Pacemaker Automaticity: Class II blockers reduce cAMP, decreasing $I_f$ and $I_{Ca-L}$ in the SA and AV nodes. This reduces the slope of Phase 4 diastolic depolarization, lowering the resting and intrinsic discharge rate of pacemaker cells.
• Slowing of AV Nodal Conduction: By suppressing $I_{Ca-L}$, beta-blockers slow Phase 0 upstroke velocity in AV nodal tissue. On intracardiac electrograms, this manifests as a prolongation of the AH interval (atrial-to-His bundle transit time) and a marked increase in the AV nodal effective refractory period (AVN ERP). The HV interval (His-to-ventricle conduction) is unaffected because His-Purkinje conduction is sodium-dependent.
• Anti-arrhythmic Protection: Beta-blockers suppress delayed afterdepolarizations (DADs) triggered by intracellular calcium overload, dampening triggers for focal atrial tachycardia, AVNRT, catecholaminergic polymorphic ventricular tachycardia (CPVT), and ischemic ventricular fibrillation.

Esmolol (Brevibloc): The Intra-Procedural Standard

• Pharmacokinetics: Esmolol is an ultra-short-acting, cardioselective $\beta_1$ receptor antagonist. It undergoes rapid enzymatic hydrolysis by cytosolic esterases located inside red blood cells (erythrocytes), independent of renal or hepatic function.
• Elimination Half-Life: Approximately 9 minutes, with full offset of clinical beta-blockade achieved within 20 to 30 minutes following cessation of infusion.
• EP Laboratory Applications:
  1. Acute Arrhythmia Rate Control: Rapid slowing of ventricular response in atrial fibrillation or atrial flutter during invasive catheter procedures.
  2. Termination of Tachycardias: Terminating AV nodal-dependent reentrant tachycardias (AVNRT) or catecholamine-dependent idiopathic RVOT ventricular tachycardias.
  3. Testing Pacemaker Independence: Safely evaluating patient hemodynamics prior to long-acting AV nodal ablation.
• IV Dosing: Loading bolus of 500 mcg/kg IV over 1 minute, followed by a maintenance infusion of 50 mcg/kg/min. If response is inadequate, repeat 500 mcg/kg boluses are administered every 5 minutes while stepping up the maintenance infusion in 50 mcg/kg/min increments (up to a maximum of 200 to 300 mcg/kg/min).

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4. Vaughan Williams Class III: Potassium Channel Blockers & Repolarization Modifiers

Class III antiarrhythmic agents prolong the cardiac action potential duration (APD) without altering the Phase 0 upstroke velocity or tissue conduction velocity. Their primary cellular mechanism is the inhibition of outward delayed rectifier potassium currents, predominantly the rapid component ($I_{Kr}$). By delaying Phase 3 repolarization, Class III drugs lengthen the effective refractory period (ERP) across atrial myocytes, ventricular myocytes, and accessory pathways, rendering myocardium refractory to reentrant wavefronts.

• Electrographic Signature: Class III blockade prolongs cellular repolarization, which manifests on the surface ECG as prolongation of the QT and QTc intervals.

Amiodarone (Cordarone, Pacerone)

Amiodarone is a complex benzofuran derivative containing 37% iodine by weight. Although classified as a Class III agent, amiodarone exhibits electrophysiological properties spanning all four Vaughan Williams classes:

1. Class I Action: Blocks inactivated and open fast sodium channels, exhibiting frequency-dependent conduction slowing at rapid heart rates.
2. Class II Action: Exerts non-competitive alpha- and beta-adrenergic receptor antagonism, lowering heart rate and blunting catecholamine surges.
3. Class III Action: Blocks multiple outward potassium currents ($I_{Kr}, I_{Ks}, I_{to}, I_{K1}$), substantially prolonging action potential duration and refractoriness across all cardiac tissues.
4. Class IV Action: Weakly blocks L-type calcium channels ($I_{Ca-L}$), contributing to AV nodal conduction slowing.

Pharmacokinetics and Dosing

Amiodarone has an enormous volume of distribution (>60 L/kg) due to extensive accumulation in adipose tissue, liver, and lungs. Its elimination half-life ranges from 40 to 60 days.

• Acute IV Loading (ACLS / EP Lab): Rapid bolus of 150 mg IV over 10 minutes (diluted in 100 mL D5W). Followed by a continuous infusion of 1.0 mg/min for 6 hours (360 mg), then 0.5 mg/min for the subsequent 18 hours (540 mg), delivering a total 24-hour baseline load of approximately 1,000 mg.
• Oral Loading: 800 to 1,600 mg daily in divided doses until a cumulative total body load of 8 to 10 grams is attained, followed by maintenance dosing of 100 to 400 mg daily.

The Amiodarone "Torsades Paradox"

Despite causing profound QTc prolongation (frequently >500 ms), amiodarone is associated with an exceptionally low incidence of Torsades de Pointes (<1%), unlike pure Class III agents. Biophysically, this occurs because amiodarone prolongs action potential duration homogeneously across all myocardial layers (epicardium, endocardium, and mid-myocardial M-cells). By repolarizing the tissue uniformly, it does not increase transmural dispersion of repolarization (TDR). Furthermore, its concurrent beta-blocking and calcium-channel-blocking properties suppress the early afterdepolarizations (EADs) that trigger torsadogenic runs.

Extracardiac Toxicities

Because of its prolonged half-life and tissue deposition, chronic amiodarone therapy carries severe multi-organ toxicities:

• Pulmonary Toxicity: Chronic interstitial pneumonitis, organizing pneumonia, or acute pulmonary fibrosis (incidence: 2% to 5%; fatal in up to 10% of affected patients). Characterized by non-productive cough, progressive dyspnea, bilateral infiltrates, and reduced DLCO on pulmonary function testing.
• Thyroid Dysfunction: Amiodarone's heavy iodine content alters thyroid biochemistry:
  • Hypothyroidism (up to 15% to 20% of patients): Driven by the Wolff-Chaikoff effect (excess inorganic iodine shuts down thyroid hormone synthesis).
  • Hyperthyroidism (Amiodarone-Induced Thyrotoxicosis, AIT): Either Type 1 (Jod-Basedow phenomenon in autonomous nodular goiters) or Type 2 (destructive chemical thyroiditis).
• Ophthalmologic: Corneal microdeposits (vortex keratopathy / cornea verticillata) appear in nearly 100% of patients on chronic therapy; usually asymptomatic and reversible. Optic neuropathy or optic neuritis occurs rarely and can lead to permanent blindness.
• Hepatic: Elevation of serum transaminases (AST/ALT). Chronic accumulation can lead to non-alcoholic steatohepatitis and cirrhosis.
• Dermatologic: Photosensitivity (severe solar erythema) and cosmetic slate-gray to bluish-purple facial skin hyperpigmentation.
• Vascular Phlebitis: Peripheral IV infusion of concentrated amiodarone causes intense chemical thrombophlebitis; peripheral infusions should never exceed 2 mg/mL and ideally require central venous catheter access with in-line filtration.

Sotalol (Betapace): Reverse Use-Dependence

Sotalol is a racemic mixture of $D$-sotalol and $L$-sotalol. It combines non-selective beta-blockade ($L$-isomer) with Class III $I_{Kr}$ potassium channel blockade ($D$- and $L$-isomers in a 1:3 ratio of Class III to beta-blocking activity).

• Reverse Use-Dependence: Pure $I_{Kr}$ blockers display reverse use-dependence (reverse frequency-dependence). The drug binds with higher affinity and dissociates more slowly from the $I_{Kr}$ channel when the channel is at rest or cycling slowly.

  Clinical Consequence: The degree of potassium channel blockade and resulting action potential prolongation is greatest at slow heart rates (bradycardia) and following long compensatory pauses. Conversely, at rapid heart rates, the Class III effect diminishes.

• Torsades de Pointes Risk: Because bradycardia amplifies QT prolongation and promotes early afterdepolarizations (EADs), sotalol carries a high incidence of pause-dependent Torsades de Pointes (1.5% to 4%), particularly in females, patients with hypokalemia/hypomagnesemia, and those with underlying renal impairment. Sotalol is eliminated entirely unchanged by the kidneys ($t_{1/2} \approx 12\text{ hours}$); dosing must be strictly adjusted according to creatinine clearance ($CrCl$).

Dofetilide (Tikosyn): Pure $I_{Kr}$ Blockade

Dofetilide is a potent, pure $I_{Kr}$ blocker that lacks any beta-blocking, sodium-channel-blocking, or calcium-blocking activity.

• Strict Inpatient Initiation Protocol: FDA mandates a minimum 72-hour continuous ECG telemetry monitoring protocol (or a minimum of 5 doses) during dofetilide initiation or dose titration.
• Creatinine Clearance-Based Dosing:
  • $CrCl > 60\text{ mL/min}$: 500 mcg PO twice daily.
  • $CrCl\ 40\text{ to }60\text{ mL/min}$: 250 mcg PO twice daily.
  • $CrCl\ 20\text{ to }39\text{ mL/min}$: 125 mcg PO twice daily.
  • $CrCl < 20\text{ mL/min}$: Strictly contraindicated.
• Stopping / Dose-Adjustment Criteria: 12-lead ECG is recorded 2 to 3 hours post each dose. If baseline QTc exceeds 440 ms (or 500 ms in bundle branch block), dofetilide is contraindicated. If the post-dose QTc increases by >15% over baseline or exceeds 500 ms, the dose must be reduced immediately; if subsequent doses still exceed 500 ms, dofetilide must be permanently discontinued.

Ibutilide (Corvert): Acute Cardioversion

Ibutilide is an intravenous Class III agent that prolongs repolarization by blocking $I_{Kr}$ while concurrently activating a slow, inward sodium current ($I_{Na-slow}$).

• Clinical Use: Rapid chemical cardioversion of acute atrial fibrillation and atrial flutter. Conversion rates are significantly higher for atrial flutter (60% to 70%) than for atrial fibrillation (40% to 50%), with conversion typically occurring within 20 to 30 minutes of infusion.
• Dosing: 1.0 mg IV infused over 10 minutes in patients weighing $\ge 60\text{ kg}$ (0.01 mg/kg if $<60\text{ kg}$). If the arrhythmia does not terminate within 10 minutes post-infusion, a second identical dose is administered.
• Torsades Hazard: Sustained or non-sustained polymorphic VT / Torsades de Pointes develops in 4% to 5% of patients, often within 1 to 3 hours of administration. Continuous cardiac telemetry and immediate defibrillator availability are mandatory for at least 4 hours post-infusion.

Dronedarone (Multaq)

Dronedarone is a non-iodinated synthetic amiodarone derivative designed to avoid thyroid and pulmonary toxicities. While effective for maintaining sinus rhythm in paroxysmal AFib, it carries critical black box contraindications:

• The ANDROMEDA & PALLAS Trials: In the ANDROMEDA trial (patients with symptomatic decompensated heart failure) and the PALLAS trial (patients with permanent atrial fibrillation), dronedarone was associated with a two-fold increase in mortality, heart failure hospitalizations, and stroke. Dronedarone is strictly contraindicated in NYHA Class III–IV heart failure, recently decompensated Class II heart failure, or permanent atrial fibrillation.

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5. Vaughan Williams Class IV: Non-Dihydropyridine Calcium Channel Blockers

Class IV antiarrhythmics comprise the non-dihydropyridine calcium channel blockers, primarily verapamil (a phenylalkylamine) and diltiazem (a benzothiazepine). These agents selectively inhibit voltage-sensitive L-type calcium channels ($I_{Ca-L}$).

Class IV Target: L-Type Calcium Channel (ICa-L) Blockade
Atrial Myocytes:       Mild shortening of plateau (minimal APD effect)
AV Nodal Tissue:       ↓ Upstroke velocity (Phase 0) ──> Slows Conduction (↑ AH Interval)
                       ↑ Refractoriness (Phase 2)    ──> Prolongs AV Nodal ERP

Cellular & Electrophysiological Effects

Because the SA and AV nodal action potentials depend entirely on inward calcium currents for Phase 0 upstroke and cell-to-cell propagation, Class IV agents exert profound depressive effects on nodal tissue:

• AV Nodal Conduction Delay: Suppresses AV nodal conduction velocity, directly prolonging the AH interval on His bundle electrograms and prolonging the surface PR interval.
• Prolongation of AV Nodal Refractoriness: Markedly increases the effective refractory period of the AV node (AVN ERP). This makes verapamil and diltiazem highly effective for terminating reentrant arrhythmias that incorporate the AV node as an essential limb of the circuit (e.g., AVNRT and orthodromic AVRT) and for ventricular rate control in atrial fibrillation.
• Negative Inotropy: By reducing intracellular calcium entry during Phase 2, Class IV agents reduce myocardial contractility and are contraindicated in acute decompensated heart failure or severe systolic dysfunction ($LVEF < 40% $).

The Lethal WPW / Pre-Excited AFib Contraindication

[!CAUTION] Fatal Paradox: Verapamil/Diltiazem in Pre-Excited Atrial Fibrillation When a patient with Wolff-Parkinson-White (WPW) syndrome develops atrial fibrillation, electrical impulses can propagate to the ventricles down two parallel pathways: the normal AV node and the anomalous accessory pathway.

If verapamil or diltiazem is inadvertently administered:

1. Selective AV Nodal Block: The drug blocks conduction across the slow, decremental AV node without slowing conduction across the accessory pathway (which has non-decremental, sodium-channel-dependent conduction).
2. Reflex Adrenergic Surge: Peripheral arterial vasodilation induced by calcium blockade causes systemic hypotension, triggering a baroreceptor-mediated massive sympathetic discharge of endogenous catecholamines.
3. Accelerated Pathway Conduction: High adrenergic tone directly shortens the anterograde refractory period of the accessory pathway.

Unchecked fibrillatory impulses conduct 1:1 across the accessory pathway directly into the ventricular myocardium at rates exceeding 300 to 350 bpm. This triggers ventricular fibrillation (VF) and immediate hemodynamic collapse! Class IV calcium channel blockers (and digoxin) are strictly contraindicated in pre-excited atrial fibrillation.

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6. Summary Comparison: Vaughan Williams Classification