The intrinsic conduction system of the heart is demonstrated in the diagram below.

1 – Conduction pathways

  • The conduction pathways of the heart consist of:
  • The sinoatrial node
  • The Atrioventricular node
  • The bundle of His
  • The left and right bundles
  • The Pukinje Fibres

2 – The SA node

Signals arising from the SA node stimulate the atria to contract and are conducted to the AV node (the P wave on the ECG).

Normally, electrical activity is spontaneously generated by the SA node (the ‘physiological pacemaker’).

The impulse is propagated from the right atrium to the left atrium vid Backmann’s bundle.

Conduction to the AV node occurs in specialised tissue called internodal tracts.

3 – The AV node

The AV node introduces a critical delay (the PR interval)into the conduction system allowing blood to flow from the atria into the ventricles.

The distal part of the AV node is called the Bundle of His which splits into two bundle branches in the inter-ventricular septum (left and right).

4 – The left bundle branch

The left bundle branch activates the left ventricle, whilst the right bundle activates the right ventricle.

The left bundle splits further into the left anterior fascicle and left posterior fascicle. The two bundle branches taper out to produce numerous Purkinje fibres which stimulate individual groups of myocardial cells to contact.

The spread of electrical activity through the ventricular myocardium produces the QRS complex on the ECG.

5 – Disruption to the conduction system

Disruption to parts of this conduction system will result in failure to generate impulses (e.g. sick sinus syndrome, sinus arrest) or in failure to propagate impulses at various levels of the system (e.g. AV nodal block, complete heart block, etc.).

Disease or damage to any part of this conduction pathway may cause abnormalities on the ECG manifesting as bradycardia. It is important to recognise that not all impaired conduction cause symptoms. Conventional pacing is a very effective treatment for significant bradycardias.