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Topic

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Left branch of atrioventricular bundle

The left branch of the atrioventricular (AV) bundle, commonly called the left bundle branch (LBB), is a key component of the cardiac conduction system. It originates from the bundle of His at the upper part of the interventricular septum and divides rapidly into anterior and posterior fascicles.

The LBB runs subendocardially along the left side of the interventricular septum. Its anterior fascicle extends toward the anterior papillary muscle and anterosuperior wall of the left ventricle, while the posterior fascicle descends toward the posteroinferior wall and posterior papillary muscle. Both fascicles further branch into Purkinje fibers, which distribute electrical impulses throughout the left ventricular myocardium.

The LBB is vital for synchronous contraction of the left ventricle. Blockage or disruption of this pathway produces left bundle branch block (LBBB), an important ECG finding associated with conduction delay, cardiomyopathy, or ischemic heart disease.

Synonyms

  • Left bundle branch (LBB)

  • Left branch of AV bundle

  • Ramus sinister fasciculi atrioventricularis

Function

  • Conducts impulses from the bundle of His to the Purkinje fibers of the left ventricle

  • Ensures synchronous activation of the left ventricular myocardium

  • Divides into anterior and posterior fascicles for coordinated contraction

  • Supports cardiac output efficiency by maintaining conduction timing

  • Abnormalities cause conduction delay and ventricular dyssynchrony

MRI Appearance

T1-weighted images:

  • Direct visualization of the LBB is limited due to its microscopic size

  • Appears indirectly as part of the subendocardial conduction system along the interventricular septum

T2 Cine (Cardiac-gated MRI):

  • LBB not directly visualized, but functional effects (wall motion abnormalities in LBBB) are evident

  • Cine MRI shows asynchronous septal motion (“septal flash”) in conduction delay

  • Useful for assessing ventricular dyssynchrony before CRT (cardiac resynchronization therapy)

CT Appearance

CT Coronary Angiography (CCTA):

  • LBB not directly visible, but CT depicts septal myocardium and coronary arterial supply (LAD branches)

  • Can identify calcification, ischemia-related fibrosis, or structural septal abnormalities affecting conduction

CT Delayed Enhancement (CTDE):

  • Demonstrates subendocardial or midwall hypo-/hyperattenuation in septum due to infarction or fibrosis

  • Correlates with areas that may involve the LBB conduction pathway

CT image

Left branch of atrioventricular bundle anatomy CT axial image -img-00000-00000