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.