Mobitz type I 2nd degree AV block

Table of Contents

Table of Contents

Cullen Buchanan, MD


  1. Identify second degree (Mobitz), type 1 (Wenckebach) AV block.
  2. Differentiate between Mobitz type I and type II AV block by physiology and ECG pattern.
  3. Describe the clinical significance of Mobitz type I.

Teaching Instructions

Preparation: Plan to spend 5-10 minutes familiarizing yourself with the animations of the PowerPoint and the key findings of this ECG.

Presentation Instructions: Present the image either by expanding the window (bottom right) in a browser or downloading the PowerPoint file (downloading is recommended).  Have the image pulled up in presenter mode before learners look at the screen to avoid revealing the diagnosis.  Ask a learner to provide an overall interpretation.  Then advance through the animations to prompt learners with key questions and reveal the associated graphics and teaching points. You can go back to prior graphics and questions by using the back arrow or scrolling back on the mouse wheel.

Official ECG Read: Mobitz, type I AV block with 5:4 conduction and right bundle branch block.

Diagnosis: Mobitz, type I in the setting of acute illness, likely to resolve and unlikely to progress to complete heart block.


  1. Second degree AV blocks are identified by a non-conducted P wave (‘dropped QRS’). Mobitz type I are defined by a increasingly prolonged PR interval preceding a dropped QRS.
  2. The increasingly prolonged PR interval in type I is due to a increasingly refractory AV node. Conversely, the dropped QRS in type II is due to intermittently refractory Purkinje system. Type II typically has a consistent single or bifascicular bundle branch block and only intermittently blocks the remaining bundle.  This is often due to underlying progressive disease and is therefore at risk of becoming complete heart block. First degree is more accurately described as prolonged PR interval because it is not actually
  3. Asymptomatic second-degree type 1 AV block is a benign rhythm with a low risk of progressing to third-degree or complete heart block.  Symptomatic second-degree type 1 AV block should undergo a thorough investigation into structural heart disease and systemic illness as possible etiologies.
  4. Of note, first degree AV block is more accurately described as a prolonged PR interval because it is not actually

Presentation Board

Take Home Points

  1. Mobitz type I can be differentiated from Mobitz type II by gradual prolongation of the PR interval prior to a dropped QRS.
  2. Asymptomatic second-degree (Mobitz), type 1 AV block is a benign rhythm with a low risk of progressing 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

Yilin Zhang


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