CorpusHuman Anatomy
CardiovascularOrgan part

Left bundle branch

Crus sinistrum fasciculi atrioventricularis

Also known as: LBB, left anterior fascicle, left posterior fascicle

The left bundle branch is a broad, sheet-like branch of the bundle of His. It runs down the left side of the septum and divides into an anterior (superior) fascicle and a posterior (inferior) fascicle, with additional septal fibres, to activate the left ventricle.

Show on body map Cardiac Conduction System & ECG

Overview

The left bundle branch fans out under the endocardium of the left side of the septum soon after it leaves the bundle of His.

The left anterior fascicle is long and thin. It runs toward the anterolateral papillary muscle, is supplied by the LAD and is often blocked. The left posterior fascicle is short and broad. It runs toward the posteromedial papillary muscle, has a dual blood supply and is rarely blocked on its own. Septal fibres start the normal left-to-right activation of the septum.

Location

Left side of the interventricular septum, just below the endocardium.

Function

  • Activates the left ventricle and the septum.
  • Starts the normal left-to-right depolarisation of the septum.

Clinical significance

Left bundle branch block

In LBBB the QRS is 120 ms or wider, with broad, notched R waves in I, aVL, V5 and V6, a deep S wave in V1 and loss of the septal q waves. S2 may split paradoxically (reversed splitting). Causes include ischaemic heart disease, hypertension, aortic stenosis and cardiomyopathy. Because the ventricles contract out of step, patients with heart failure and LBBB benefit from cardiac resynchronisation therapy. Sgarbossa criteria help detect an acute MI when LBBB is present.

Fascicular blocks

Left anterior fascicular block causes left axis deviation of about −45° or more, with qR in aVL and rS in II, III and aVF. Left posterior fascicular block causes right axis deviation. RBBB plus a fascicular block is called bifascicular block and can progress to complete heart block.

Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.