Left bundle branch block (LBBB) is a cardiac conduction abnormality seen on the electrocardiogram (ECG). 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:
1)The heart rhythm must be supraventricular in origin
2)The QRS duration must be ≥ 120 ms
3)There should be a QS or rS complex in lead V1
4)There should be a RsR' wave in lead V6.
5)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.

Among the causes of LBBB are:
Aortic stenosis
Dilated cardiomyopathy
Acute myocardial infarction
Extensive coronary artery disease
Primary disease of the cardiac electrical conduction system
Long standing hypertension leading to aortic root dilatation and subsequent aortic regurgitation.


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) and a "left posterior fascicular block" (LPFB).This refers to the bifurcation of the left bundle branch.