Mobitz type I 2nd degree AV block

Table of Contents

Table of Contents

Cullen Buchanan, MD


  1. Identify second degree, type 1 AV block (aka Mobitz 1 or Wenckebach)

Teaching Instructions

Plan to spend ~ 5-10 min reviewing the teaching materials. 

Have your readers develop a systematic approach to reading ECGs. Refer to How to Read an ECG for additional details. 

Rate: Ventricular rate is ~ 60 bpm. 

Axis: Normal (QRS is upright in both leads I and II)

Rhythm: The rhythm at a glance is irregular with “clusters” of beats followed by a pause. This should raise suspicion for dropped beats, or non-conducted P waves. There are P waves that precede QRS complexes before a non-conducted P wave. The PR intervals progressively lengthen prior to the dropped beat. The discrepancy is most notable in the beats immediately preceding and immediately following the dropped beat. This is consistent with 2nd degree type 1 (or Mobitz 1) AV block. 

Of note, this is in contrast to Mobitz type 2 2nd degree AV block in that there is no progressive PR prolongation. 

Intervals: QRS interval is widened in a rsR' morphology in V1 and large slurred S wave in V6. This is consistent with a RBBB. QTc is also slightly prolonged and visually about 1/2 the RR interval. 

Final diagnosis: Second degree type 1 AV block with pre-existing RBBB

Additional learning: 

  • Asymptomatic second degree type 1 AV block is a benign rhythm with low risk to progress to third degree or complete heart block. 
  • Symptomatic second degree type 1 AV block should undergo thorough investigation into structural heart disease and systemic illness as possible etiologies.

Presentation Board

Take Home Points

  1. Mobitz 1 can be differentiated from Mobitz 2 by gradual prolongation of the PR interval prior to non-conducting P waves and a dropped QRS.

  2. Asymptomatic second degree type 1 AV block is a benign rhythm with low risk to progress to third degree or complete heart block.


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: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines and the Heart Rhythm Society [published correction appears in Circulation. 2019 Aug 20;140(8):e506-e508]. Circulation. 2019;140(8):e382-e482. doi:10.1161/CIR.0000000000000628

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