Distinguishing Infra-nodal from Nodal Atrioventricular Block
Infra-nodal AVB is distinguished from nodal AVB primarily by QRS morphology, PR interval characteristics, and response to autonomic changes: infra-nodal block typically presents with a wide QRS complex (≥120 ms), normal or minimally prolonged PR interval, and worsens with atropine, while nodal block characteristically shows a narrow QRS complex, prolonged PR interval, and improves with atropine 1, 2, 3.
Key Electrocardiographic Distinguishing Features
QRS Duration and Morphology
- Infra-nodal block: Wide QRS complex (≥120 ms), often with bundle branch block pattern, particularly LBBB morphology 2, 3
- Nodal block: Narrow QRS complex (<120 ms) in the vast majority of cases 3, 4
PR Interval Characteristics
- Infra-nodal block: Normal or only slightly prolonged PR interval, suggesting the delay occurs below the AV node 2, 5
- Nodal block: Prolonged PR interval (>200 ms), reflecting delayed conduction through the AV node itself 2, 3
Pattern-Specific Clues
In 2:1 AV Block
When examining 2:1 AV block (where only one PR interval is visible before the blocked P wave):
- Infra-nodal: Normal PR interval with wide QRS suggests His-Purkinje disease 2, 4
- Nodal: Prolonged PR interval with narrow QRS suggests AV nodal disease 2, 4
- Critical caveat: Outside acute MI, sustained 2:1 or 3:1 block with wide QRS occurs in the His-Purkinje system in 80% of cases 4
In Mobitz Type I vs Type II
- Mobitz Type I with narrow QRS: Almost always AV nodal block 3
- Mobitz Type I with wide QRS: Infra-nodal in 60-70% of cases (barring acute MI) 3
- Mobitz Type II (correctly defined with constant PR intervals): Always infra-nodal 1, 3, 4
Electrophysiologic Study Findings
When invasive EPS is performed 1, 5:
- Infra-nodal block:
- Normal AH interval (55-125 ms)
- Prolonged HV interval (>55 ms) or block occurring after His deflection
- HV interval ≥70 ms indicates significant infra-nodal disease requiring pacing 1
- Nodal block:
- Prolonged AH interval (>125 ms)
- Normal HV interval (35-55 ms)
Response to Autonomic Maneuvers
- Infra-nodal block: Worsens with atropine administration (increased atrial rate overwhelms diseased His-Purkinje system) 4
- Nodal block: Improves with atropine (reduces vagal tone on AV node) 1
Clinical Significance and Pitfalls
The distinction is critical because all infra-nodal blocks require permanent pacing regardless of symptoms 3, while nodal blocks may be managed conservatively if asymptomatic and not associated with progressive disease 1.
Common Diagnostic Pitfalls:
- Vagal surges can cause simultaneous sinus slowing and AV nodal block that superficially resembles Type II block—absence of sinus slowing is crucial for diagnosing true Type II block 3
- Concealed His bundle extrasystoles can produce pseudo-AV block patterns mimicking both Type I and Type II block 3
- 2:1 block cannot be classified as Type I or Type II based on ECG alone—look for accompanying rhythms or perform EPS 4
- Alternating bundle branch block (QRS alternating between LBBB and RBBB morphologies) is pathognomonic for severe infra-nodal disease and mandates permanent pacing (Class I indication) 1
Special Populations:
In neuromuscular diseases (Kearns-Sayre syndrome, Emery-Dreifuss muscular dystrophy), progressive infra-nodal disease is common and pacing is indicated even with preserved 1:1 conduction 1.