The atrioventricular conduction system in congenital heart disease. A concept relevant for cardiac surgeons

Cardiovascular Pathology, Academic Medical Center, University of Amsterdam, Netherlands
Anton E. Becker

Some insight into developmental aspects of the atrioventricular (AV) conduction system is essential to understand the position of the AV node and connecting bundle in congenitally malformed hearts.
The AV node and conduction tissues develop from the embryonic myocardium, which envelops the primitive heart tube prior to chamber formation. After completion of chamber formation embryonic myocardium remains in the dorsal wall of the atria, extending from the site of the sinus node to the AV canal. In fact, it remains as a zone around the AV canal and it is from this “ring” of embryonic myocardium that the AV node develops, at the site where it meets the ventricular septum. At this point the AV conduction bundle develops.
For normal development of the AV conduction system it is essential that the AV canal expands to the right and for the ventricular septum to develop normally. In other words, in case of a congenital heart malformation in which one of these normal developmental aspects is at fault, one may anticipate an abnormal disposition of the AV node and bundle. Examples are:
・Tricuspid atresia (absent rightward outgrowth of the AV junction, leading to “absent connection”)
・Atrioventricular septal defect (common AV junction, also in case of the so-called incomplete variety)
・Straddling AV valve (tricuspid valve straddles inferior part of VS; incomplete rightward outgrowth of AV junction)
・Double inlet LV (abnormal ventricular outgrowth which results in a VS anterior to AV junction)
・Congenitally corrected transposition (malalignment between AS and VS) The key to understanding the concept is to realise that the AV node and bundle are derived from the embryonic myocardium which in the early stages of development encircles the AV junction. Once there is abnormal rightward outgrowth of the AV junction or abnormal septation one should anticipate an abnormal disposition of the AV node and bundle. The site to look for is the site where the AV junction meets the ventricular septum.