Left Bundle Branch
The structure indicated is the left bundle branch
of the cardiac conducting system.
The conduction system of the heart is responsible for the synchronised contraction of the atria
and is comprised of the following main structures:
- Sinoatrial node
- Atrioventricular node
- Atrioventricular bundle of His
- Right and left bundle branches
- Purkinje fibres
The electrical activity of the heart begins in the sinoatrial node
and travels to the atrioventricular node
. From the atrioventricular node the impulses pass down the Bundle of His
, whereupon the conducting fibres divide into the right and left bundle branches
The right bundle branch
is comprised of a single fascicle
, whereas the left bundle branch
is comprised of an anterior fascicle
and posterior fascicle
. The bundle branches terminate by dividing into Purkinje fibres
which pierce the inner ventricular walls to maintain synchronised contraction.
The conduction system can be interrupted if the bundle branches are injured through underlying heart disease, acute ischaemia and tissue necrosis (myocardial infarct), fibrosis or other tissue injury. Injury to the bundle branches causes delay in ventricular depolarisation, and the electrical impulse travels through alternative pathways to bring about ventricular contraction. This is known as bundle branch block
and is visualised on the ECG as abnormal, widened QRS complexes in the anterior chest leads.