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Left bundle branch block
Classification and external resources

ECG characteristics of a typical LBBB showing wide QRS complexes with abnormal morphology in leads V1 and V6.
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 results in the left ventricle contracting later than the right ventricle.


ECG diagnosis

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 = or > 120 ms [2]
  • There should be a QS or rS complex in lead V1
  • There should be a monophasic R wave in leads I and V6.

It should be noted that LBBB renders the ST segment of chest leads unreliable.

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.

A mnemonic to remember the ECG changes is WiLLiaM MaRRoW, ie with LBBB there is an W in V1 and a M in V6 and with a RBBB there is a M in lead V1 and a W in lead V6


Among the causes of LBBB are:


  • Medical Care: Patients with LBBB require complete cardiac evaluation, and those with LBBB and syncope or near-syncope may require a pacemaker.
  • Surgical Care: Some patients with LBBB, a markedly prolonged QRS, and congestive heart failure may benefit from a pacemaker, which provides rapid left ventricular contractions.


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 bifurcation of the left bundle branch.

See also


External links



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