AV Block Spectrum

1

Recognition at a Glance

Type	PR Interval	P:QRS Ratio	Regularity	QRS Width	Key Feature
Normal	120–200 ms	1:1	Regular	Narrow	All P waves conducted with normal PR
1st Degree	>200 ms	1:1	Regular	Usually narrow	Prolonged but constant PR; all beats conducted
Wenckebach	Progressive ↑	Variable (e.g., 4:3)	Group beating	Narrow	PR lengthens then dropped beat; grouped cycles
Mobitz II	Constant	Variable	Irregular	Usually wide (BBB)	Fixed PR with sudden non-conduction
2:1 Block	Constant (conducted)	2:1	Regular	Variable	Every other P blocked; cannot classify I vs II from this alone
High-Grade	Constant (conducted)	≥3:1	Irregular	Variable	≥2 consecutive blocked P waves with some conduction
Complete	None (AV dissociation)	No relation	Regular P, Regular QRS	Narrow or wide	Independent atrial and ventricular rates; no AV relationship

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AV Conduction Anatomy

The AV conduction axis comprises four sequential structures: the AV node (compact node within Koch's triangle), the His bundle (penetrating the central fibrous body), the right and left bundle branches, and the distal Purkinje network. Each level has distinct physiological properties that determine the type of block produced when disease or enhanced vagal tone impairs conduction.

The AV node demonstrates decremental conduction — it conducts more slowly at faster rates. This property underlies first-degree AV block and Wenckebach periodicity. Nodal block is typically associated with narrow-QRS escape rhythms at 40–60 bpm and a favorable prognosis. The AV node has a rich autonomic innervation: vagal tone slows conduction (prolonging PR), while sympathetic tone accelerates it.

Infra-Hisian block (within or below the His bundle) produces Mobitz Type II block and is associated with wide-QRS escape rhythms at 20–40 bpm. These escape rhythms are unreliable, and infra-Hisian block carries significant risk of syncope and sudden cardiac death. The His-Purkinje system conducts in an all-or-none fashion without decremental properties, which is why Mobitz II shows a constant PR before sudden failure.

3

Clinical Decision Points

When is AV block benign?

First-degree AV block and Mobitz Type I (Wenckebach) are generally benign when they occur in young, healthy individuals, trained athletes, or during sleep. These patterns reflect enhanced vagal tone acting on the AV node and do not indicate structural conduction system disease. They resolve with exercise or atropine and do not require pacing.

When does block require pacing?

Pacing is indicated for symptomatic Mobitz Type II, high-grade AV block, and complete heart block regardless of symptoms. Symptomatic first-degree block (PR >300 ms causing hemodynamic compromise) and symptomatic Wenckebach not attributable to reversible causes also warrant pacing consideration.

2:1 Block: Type I or Type II?

When 2:1 block is present, classification requires additional clues. A narrow QRS with moderate PR prolongation favors Type I (nodal). A wide QRS with bundle branch block pattern favors Type II (infra-nodal). Atropine or exercise can help: improvement in conduction suggests nodal block (Type I), while worsening suggests infra-nodal block (Type II). An EP study with His bundle recording provides definitive localization.

ACC/AHA Pacing Indications Summary

Class I: Third-degree or high-grade AV block at any anatomic level with bradycardia-related symptoms, pauses ≥3 seconds, or escape rate <40 bpm in awake patients
Class I: Symptomatic Mobitz Type II second-degree AV block, regardless of QRS width
Class IIa: Asymptomatic Mobitz Type II with wide QRS (≥120 ms)
Class IIb: First-degree AV block with PR >300 ms causing symptoms attributable to AV dyssynchrony
Class III (no benefit): Asymptomatic first-degree AV block or Mobitz Type I without hemodynamic compromise

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Key References

Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140(13):e382–e482. doi:10.1161/CIR.0000000000000628
Barold SS, Hayes DL. Second-degree atrioventricular block: a reappraisal. Mayo Clin Proc. 2001;76(1):44–57. doi:10.4065/76.1.44
Vijayaraman P, Bordachar P, Ellenbogen KA. The Continued Search for Physiological Pacing: Where Are We Now? J Am Coll Cardiol. 2017;69(25):3099–3114. doi:10.1016/j.jacc.2017.05.005
Strauss DG, Selvester RH, Wagner GS. Defining left bundle branch block in the era of cardiac resynchronization therapy. Am J Cardiol. 2011;107(6):927–934. doi:10.1016/j.amjcard.2010.11.010

Interactive ECG & Ladder Diagram

AV Node Conduction