Left bundle branch block

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Left bundle branch block
Left bundle branch block ECG characteristics.png
ECG characteristics of a typical LBBB showing wide QRS complexes with abnormal morphology in leads V1 and V6.
Classification and external resources
ICD-10 I44.4 - I44.7
DiseasesDB 7352
eMedicine ped/2501

Left bundle branch block (LBBB) is a cardiac conduction abnormality seen on the electrocardiogram (ECG).[1] In this condition, activation of the left ventricle is delayed, which causes the left ventricle to contract later than the right ventricle.

ECG diagnosis[edit]

Electrocardiogram showing left bundle branch block and irregular rhythm due to supraventricular extrasystoles.
A left bundle branch block

The criteria to diagnose a left bundle branch block on the electrocardiogram:

  • The heart rhythm must be supraventricular in origin
  • The QRS duration must be ≥ 120 ms[2]
  • There should be a QS or rS complex in lead V1
  • There should be a notched ('M'-shaped) R wave in lead V6.

The T wave should be deflected opposite the terminal deflection of the QRS complex. This is known as appropriate T wave discordance with bundle branch block. A concordant T wave may suggest ischemia or myocardial infarction.


Amongst the causes of LBBB are:


  • Patients with LBBB require complete cardiac evaluation, and those with LBBB and syncope or near-syncope may require a pacemaker.
  • Some patients with LBBB, a markedly prolonged QRS (usually > 150 ms), and systolic heart failure may benefit from a biventricular pacemaker, which allows for better synchrony of heart contractions.[3]


There are also partial blocks of the left bundle branch: "left anterior fascicular block" (LAFB)[4] and a "left posterior fascicular block" (LPFB).[5] This refers to the bifurcation of the left bundle branch.

See also[edit]


  1. ^ "Conduction Blocks 2006 KCUMB". Retrieved 2009-01-20. 
  2. ^ "Lesson VI - EKG Conduction Abnormalities". Retrieved 2009-01-07. 
  3. ^ Stevenson WG, Hernaddez AF, Carson PE, et al. Indications for cardiac resynchronization therapy: 2011 update from the Heart Failure Society of America guideline committee. J Card Fail 2012; 18:94-106.
  4. ^ x20050921122910832459 at GPnotebook
  5. ^ x20050921123129832459 at GPnotebook

External links[edit]