Left Bundle Branch Block (LBBB)

Table of Contents

Table of Contents

Brandon Fainstad, MD
Expert review pending


  1. Identify the broad monophasic R wave characteristic of left bundle branch block (LBBB) morphology

Teaching Instructions

Plan to spend 5 minutes familiarizing yourself ECG.  Have the image pulled up on the presenting screen or monitor.

Have one learner provide a systematic interpretation of the ECG. If they do not do so on their own, ask them to commit to if the QRS is wide or narrow and what the QRS morphology is.

Diagnosis: Atrial fibrillation at a rate of 115bpm with left bundle branch block

Teaching: A wide QRS (>120ms) is typically due to either a RBBB or LBBB, resulting in a predictable depolarization pattern or ‘morphology'. LBBB morphology is defined by a broad, monophasic R wave in leads I and V6.  This is differentiated from a RBBB, classically a rSR' or “bunny ears” in lead V1 and a wide and slurred S in V6.  


Take Home Point

A wide QRS is defined as >120ms and commonly fits either a left bundle branch block (LBBB) or right bundle branch block (RBBB) morphology.  This ECG demonstrates the broad, monophasic R wave in lead I and V6 typical of a LBBB.


Goldberger, A. L., Shvilkin, A., Goldberger, Z. D. (2017). Clinical Electrocardiography: A Simplified Approach E-Book. United States: Elsevier Health Sciences.

Brandon Fainstad


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