Nodal Vs Infra-Nodal Block

 

Nodal vs Infra-Nodal Atrioventricular (AV) Block

Understanding the level of atrioventricular conduction block is clinically crucial because nodal and infra-nodal blocks differ significantly in mechanism, ECG appearance, prognosis, and management.

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1. Definition Based on Anatomical Level

Nodal (Supra-Hisian) Block

• Site of block: AV node

• Above the His bundle

• Usually functional and reversible

Infra-Nodal (Intra-/Infra-Hisian) Block

• Site of block: His bundle or bundle branches

• Below the AV node

• Structural disease of the conduction system

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2. Pathophysiology

Nodal Block

• Caused by increased vagal tone or AV-nodal suppressing drugs

• AV node has slow conduction and decremental properties

• Often transient

Common causes: • Beta-blockers

• Calcium channel blockers (verapamil, diltiazem)

• Digoxin

• Inferior wall myocardial infarction

• High vagal tone (sleep, athletes)

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Infra-Nodal Block

• Caused by degenerative or ischemic disease of His-Purkinje system

• Lacks decremental conduction

• High risk of sudden progression to complete heart block

Common causes: • Anterior wall myocardial infarction

• Fibrosis (Lev–Lenègre disease)

• Cardiomyopathies

• Post-cardiac surgery

• Infiltrative diseases (amyloidosis, sarcoidosis)

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3. ECG Characteristics

Nodal Block – ECG Features

• Narrow QRS complexes

• Mobitz type I (Wenckebach) most common

• Progressive PR prolongation → dropped beat

• Escape rhythm: junctional, stable (40–60 bpm)

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Infra-Nodal Block – ECG Features

• Wide QRS complexes (often pre-existing BBB)

• Mobitz type II common

• Fixed PR intervals with sudden dropped beats

• Escape rhythm: ventricular, slow and unstable (20–40 bpm)

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4. Response to Autonomic Maneuvers

Maneuver Nodal Block Infra-Nodal Block

Atropine Improves conduction Worsens or no response

Exercise Improves block Worsens block

Carotid massage Worsens block Minimal effect

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5. Prognosis

Nodal Block

• Usually benign

• Rarely progresses to complete heart block

• Often reversible

Infra-Nodal Block

• Poor prognosis

• High risk of sudden complete heart block and syncope

• Frequently requires permanent pacing

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6. Management Approach

Nodal Block

• Treat underlying cause

• Withdraw AV-nodal blocking drugs

• Observation if asymptomatic

• Temporary pacing rarely needed

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Infra-Nodal Block

• Permanent pacemaker indicated, even if asymptomatic

• Temporary pacing in acute settings (e.g., MI)

• Close monitoring for progression

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7. Key Clinical Pearls

• Narrow QRS + Wenckebach → think nodal

• Wide QRS + Mobitz II → think infra-nodal

• Atropine responsiveness suggests nodal disease

• Infra-nodal block is a pacing disease until proven otherwise

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8. Summary Table

Feature Nodal Block Infra-Nodal Block

Level AV node His-Purkinje

QRS Narrow Wide

Common Type Mobitz I Mobitz II

Escape Rhythm Junctional Ventricular

Prognosis Benign High risk

Pacemaker Rare Usually required

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