Is Amiodarone an AV Nodal Blocker?
Yes, amiodarone functions as an AV nodal blocker through its calcium channel antagonism and beta-receptor blockade properties, slowing AV nodal conduction and prolonging AV nodal refractoriness. 1, 2
Mechanism of AV Nodal Blockade
Amiodarone slows heart rate and atrioventricular nodal conduction via calcium channel and beta-receptor blockade, which are class IV and class II antiarrhythmic properties respectively. 1 This is distinct from its better-known class III effects (potassium channel blockade that prolongs repolarization). 1
The drug prolongs intranodal conduction (Atrial-His interval) and refractoriness of the atrioventricular node (ERP AVN) after intravenous administration. 3 These effects on the AV node are particularly prominent with IV administration, where the initial acute effects are predominantly focused on the AV node, causing intranodal conduction delay and increased nodal refractoriness. 3
Classification and Multi-Class Properties
While amiodarone is traditionally classified as a class III antiarrhythmic agent, it possesses electrophysiologic characteristics of all four Vaughan Williams classes. 1, 3 The class II (antisympathetic) and class IV (calcium channel blockade) properties are specifically responsible for the negative dromotropic effects on the sinus node and for the slowing of conduction and prolongation of refractoriness in the AV node. 3
Class 4 properties contribute to amiodarone's ability to slow the ventricular response in atrial fibrillation and to prevent AV node reentrant arrhythmias. 4 This makes amiodarone effective for rate control in atrial fibrillation through its AV nodal blocking effects. 5
Clinical Recognition in Guidelines
The ACC/AHA/HRS guidelines classify amiodarone alongside other drugs with "SA and/or AV nodal-blocking properties" and recommend precautions in patients with AV block greater than first degree or SA node dysfunction. 2 This official classification confirms amiodarone's status as an AV nodal blocker from a regulatory and clinical practice standpoint.
The European Society of Cardiology Heart Failure guidelines recommend amiodarone as an alternative agent for rate control in atrial fibrillation when beta-blockers or digoxin cannot be tolerated, specifically for its ability to control ventricular rate through AV nodal effects. 2
Important Clinical Caveats
Combination therapy with multiple AV nodal blocking agents, such as amiodarone plus beta-blockers or digoxin, carries a high risk of severe bradycardia, third-degree AV block, and asystole. 2 This additive effect underscores amiodarone's genuine AV nodal blocking properties.
Drug-related bradycardia occurs in 4.9% of patients receiving IV amiodarone, regardless of dose. 6 This bradycardia results directly from the AV nodal suppression and sinus node depression caused by the drug's class II and IV effects. 1, 4
Amiodarone depresses sinus node automaticity and atrioventricular nodal conduction through its sympatholytic properties and calcium channel effects. 5 Clinicians must monitor for second- or third-degree heart block, which represents an absolute contraindication to continued therapy without pacemaker support. 6
Comparison to Traditional AV Nodal Blockers
Unlike pure AV nodal blockers (beta-blockers, calcium channel blockers, digoxin), amiodarone is considered a second-line agent for rate control due to its complex pharmacology and significant toxicity profile. 2 However, it may be considered for rate control when other measures are unsuccessful or contraindicated, particularly in patients with systolic heart failure where traditional AV nodal blockers may worsen hemodynamics. 2
The ACC/AHA guidelines give a Class IIa recommendation to IV amiodarone for acute rate control when beta-blockers are ineffective or contraindicated. 1 This positioning reflects both its efficacy as an AV nodal blocker and the preference for agents with more predictable pharmacokinetics as first-line therapy.