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
ventricles 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.
