2:1 AV block

Table of Contents

Table of Contents

Yilin Zhang, MD


  1. Identify second degree AV block

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: Traditionally leads II and V1 are the best leads to evaluate the P wave morphology and are typically used for the rhythm strip. Looking solely at lead two, you may mistakenly believe there is a 1:1 ratio between the P and QRS. Leads V1 and III reveal an unconducted P wave. This becomes even more apparent when comparing his current ECG to a prior ECG from 3 months ago (slide 2). The T wave morphology is different in both leads V1 and III. The rhythm here is second degree AV block with 2:1 conduction. With 2:1 conduction, it is impossible to distinguish between Mobitz type II or Mobitz type II. 

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: 2:1 second degree AV block with pre-existing RBBB

Presentation Board

Take Home Points

  1. Second degree AV block with 2:1 conduction can be mistaken for sinus rhythm. Though leads II and V1 are classically the best leads to evaluate P wave morphology, carefully evaluate all leads for unconducted P waves. 
  2. Second degree AV block with 2:1 conduction cannot distinguish between Mobitz type I (aka Wenkebach), which is benign, and Mobitz type II, which can degenerate into complete (3rd degree) heart block. 


Goldberger AL, Goldberger ZD, and Shvilkin A. Atrioventricular Conduction Abnormalities, Part 1: Delays, Blocks, and Dissociation Syndromes. In: Goldberger AL, ed. Goldberger's Clinical Electrocardiography A Simplified Approach9th edition. Philadelphia, PA: Elsevier; 2018.

Yilin Zhang


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