Left bundle branch block

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Left bundle branch block
Left bundle branch block ECG characteristics.svg
ECG characteristics of a typical LBBB showing wide QRS complexes with abnormal morphology in leads V1 and V6.
SpecialtyCardiology, Emergency Medicine

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


Among the causes of LBBB are:


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.

There are also partial blocks of the left bundle branch: "left anterior fascicular block" (LAFB)[3] and a "left posterior fascicular block" (LPFB).[4] This refers to the block after the bifurcation of the left bundle branch.

Diagnostic consequences[edit]

The presence of LBBB results in that electrocardiography (ECG) cannot be used to diagnose left ventricular hypertrophy or Q wave infarction, because LBBB in itself results in widened QRS complex, and changes in the ST segment consistent with ischemia or injury.[5]


  • 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.[6]

See also[edit]


  1. ^ "Conduction Blocks 2006 KCUMB". Retrieved 2009-01-20.
  2. ^ "Lesson VI - EKG Conduction Abnormalities". Retrieved 2009-01-07.
  3. ^ "more detailed information about left anterior fascicular block". GPnotebook.
  4. ^ "more detailed information about left posterior fascicular block". GPnotebook.
  5. ^ Emily Groepper; Nasar Nallamothu; Wilfred Lam; Frank Aguirre; Kristi Bergman; Patricia Good; Patricia Wright (May 13, 2014). "Electrocardiography > Left Bundle Branch Block (LBBB)". Southern Illinois University School of Medicine. Retrieved 2015-07-02.
  6. ^ 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.

External links[edit]

External resources