Saturday, 21 November 2009

Cardiac conduction system

To be able to pump blood and circulate it around the body, the heart will need to continuously generate electrical impulses. These impulses are normally transmitted through the heart via the cardiac conduction system, this causing the heart to contract a beat.


Sinoatrial (SA) node

This is located in the right atrium where the superior vena cava joins the atrial tissue mass, normally the SA node acts as the main pacemaker in the heart.

Under normal resting conditions, the SA node fires at 60 to 100 beats per minute.

Internodal tracts and Bachmann's bundle: 

From the SA node, the impulse travels through the right and left atrium, the impulse, may be transmitted along 3 internodal tracts:

  • Anterior
  • Wenckebach's
  • Thorel's
Atrioventricular (AV) node.

The atrioventricular (AV) node is positioned between the coronary sinus and the tricuspid valve's septal cusp, the AV node itself doesn't possess pacemaker cells, but the surrounding junctional tissue does.

The AV node conducts the atrial impulse to the ventricles, it causes a 0.04-second delay. This allows the ventricles to relax and fill with blood, while the atria contracts.

Bundle of His

Rapid conduction now resumes through the bundle of His, which divides into the right and left bundle branches and extends down either side of the interventricular septum. The left bundle branch then splits into 2 branches or fascicles: the left anterior fascicle, which extends through the anterior portion of the left ventricle, and the left posterior fascicle, which runs through the lateral and posterior portions of the ventricle. Impulses travel much faster down the left bundle branch that feeds the larger thicker-walled right ventricle.

The pacemaker site of the bundle of His, has a firing rate between 40 & 60 beats per minute. The bundle of His usually fires  when the SA node doesn't generate an impulse at a normal rate or when the impulse fails to reach the AV junction.

Purkinje fibers

This network of diffuse muscle fiber beneath the endocardium transmits impulses faster than any other part of the conduction system, this pacemaker site usually fires when the SA node and the AV junction fail to generate an impulse or when the normal impulse is blocked in both bundle branches. The purkinje fibers have a automatic firing rate of 15 to 40 beats per minute. The entire interventricular conduction system is also known as the His-Purkinje system.


Accessory pathways in some people, will become part of the intrinsic conduction system. These pathways permit impulse conduction between the atria and the ventricles in addition to normal AV conduction or in place of it. The accessory pathways, are thought to play a role in some arrhythmias, these include:

  • Bundle of Kent
  • Bundle of Mahaim
  • Bundle of James