Anatomy of the Atrioventricular Junctions with Regard to Ventricular Preexcitation

In the normal heart, the atrioventricular junctions surround the orifices of the mitral and tricuspid valves. The septal area of the junctions is much smaller than generally thought, being made up of the fibrous membranous septum and the muscular atrioventricular septum. The left atrioventricular junction gives the potential for muscular atrioventricular contiguities only in relation to the mural leaflet of the mitral valve. The right junction extends from the area posterior to the muscular atrioventricular septum to the supraventricular crest of the right ventricle. Anomalous pathways for conduction, which produce pre-excitation, can be found anywhere within these atrioventricular junctions. The pathways usually are the muscular accessory connections responsible for the Wolff-Parkinson-White syndrome, which can exist in the left, septal, or right junctions. Specific muscular connections are found in the presence of Purkinje cell tumors, diverticulums of the coronary sinus, or when taking origin from nodes of Kent at the acute margin of the ventricular mass. The latter connections are responsible for most examples of so-called Mahaim conduction, and are also described as atriofascicular tracts. The true Mahaim fibers are best described as nodoventricular or fasciculo-ventricular connections, while the pathway previously labelled as atriofascicular by the European Study Group is now best distinguished as an atrio-Hisian tract. The slow and fast pathways into the atrioventricular node are composed or ordinary atrial myocardium, the orientation of the fibers probably producing preferential conduction.