Chapter 8 · Module 4

Class III Agents: Potassium Channel Blockers

Action potential duration prolongation, reverse use-dependence, and the five Class III agents

Mechanism and the Reverse Use-Dependence Paradox

Shared Mechanism

Potassium Channel Blockade

Block repolarizing potassium channels → prolong phase 3
Action potential duration and effective refractory period extend
QT interval prolongs on electrocardiogram
Antiarrhythmic effect: eliminate excitable gap in re-entrant circuits
Do NOT slow conduction velocity (unlike Class I)

Central Safety Paradox

Reverse Use-Dependence

Action potential duration prolongation is GREATEST at slow heart rates
Slow rate = most favorable condition for early afterdepolarizations and torsades de pointes
Action potential duration prolongation is LEAST during tachycardia when protection is most needed
Amiodarone exception: multi-channel profile produces more uniform prolongation across rates
Torsades de pointes risk: hypokalemia, hypomagnesemia, bradycardia, female sex compound the risk

Amiodarone — Multi-Channel Mechanism and Toxicity

Multi-Class Mechanism

All Four Vaughan Williams Classes

Class I: sodium channel block → slows conduction in fast-response tissue
Class II: beta-adrenergic block → slows sinoatrial rate, atrioventricular node
Class III: potassium channel block → prolongs action potential duration throughout heart
Class IV: calcium channel block → reduces atrioventricular nodal conduction
Calcium channel block counteracts early afterdepolarization formation → torsades de pointes below 1%
Loading required: half-life 40–55 days; effects persist weeks after stopping

Organ Toxicity — Monitor All

Six Systems Affected

Thyroid: hypothyroidism (most common) or hyperthyroidism; check every 6 months
Pulmonary: interstitial pneumonitis; annual chest radiograph; potentially fatal
Hepatic: transaminase elevation; liver function tests every 6 months
Corneal: microdeposits (nearly universal, not a reason to stop)
Dermatologic: photosensitivity; blue-gray discoloration (irreversible)
Drug interactions: warfarin (reduce dose 33%), digoxin (halve dose), statins (myopathy risk)

Comparative Overview — Five Class III Agents

Agent	Torsades de Pointes Risk	Route	Safe in Heart Failure with Reduced Ejection Fraction	Key Points / Contraindications
Amiodarone	Below 1% (lowest)	Oral + intravenous	YES	Multi-organ toxicity requires monitoring; long half-life 40–55 days; loading required; first-line for ventricular tachycardia/ventricular fibrillation; most effective for atrial fibrillation
Sotalol	2–4%	Oral	NO (ejection fraction below 40%)	Dual Class II + III; renally eliminated — strict renal dose adjustment mandatory; contraindicated in asthma; in-hospital initiation required (3 days telemetry)
Dofetilide	1–3%	Oral	YES (trial evidence)	Pure potassium channel blocker; strict renal dosing; contraindicated if creatinine clearance below 20; in-hospital initiation required; avoid drugs that reduce renal cation transport
Ibutilide	4–8% (highest)	Intravenous only	Caution (severe dysfunction)	Acute cardioversion of atrial fibrillation/flutter only; 4-hour post-dose monitoring required; resuscitation equipment mandatory; contraindicated in prolonged corrected QT interval at baseline
Dronedarone	Low	Oral	NO — contraindicated	Non-iodinated amiodarone analogue; no thyroid/pulmonary toxicity; contraindicated in permanent atrial fibrillation and heart failure with reduced ejection fraction; paroxysmal/persistent atrial fibrillation only with preserved ejection fraction

The High-Yield Clinical Rule

Among Class III agents, only amiodarone and dofetilide are safe for rhythm control in heart failure with reduced ejection fraction. Sotalol, ibutilide (caution), and dronedarone should be avoided. Dronedarone has two absolute contraindications established by trial evidence: heart failure with reduced ejection fraction (increased mortality) and permanent atrial fibrillation (increased mortality and stroke). All Class III agents prolong QT — monitor and correct electrolytes before and during therapy.