The conduction system of the heart

The arrangement of the conduction tissues, being an integral part of the anatomy, varies according to the particular morphology of the normal or abnormal heart. The sinus node is a comma-shaped structure laying immediately subepicardially within the terminal groove. Usually it is found lateral to the crest of the atrial appendage and has tailed out towards the orifice of the inferior vena cava. The artery to the sinus node is a branch of the initial course of the right coronary artery in just over half of all cases and of the start of the left circumflex coronary artery in the majority of the remaining cases. The artery ascends through the interatrial groove and can enter the node either in pre-caval or retrocaval fashion. Sometimes it divides to enter from both ends, the two arteries then forming a circle around the superior cavoatrial junction. Rarely, but of major surgical significance when present, the artery can arise more distally from either the right or circumflex arteries to run across the right appendage or the roof of the left atrium to reach the node.

The specialized tissues which connect the atrial and ventricular muscle masses are arranged as a continuous axis which extends from the atrioventricular muscular septum, penetrates the atrioventricular membranous septum and branches on the crest of the muscular interventricular septum. The atrial component of the atrioventricular axis is contained exclusively within the triangle of Koch. This landmark is delineated by the tendon of Todaro, the septal attachment of the tricuspid valve, and the mouth of the coronary sinus. The atrial specialized tissues within the triangle are composed of a component transitional between atrial and nodal cells together with the compact atrioventricular node. The axis passes through the atrioventricular membranous septum at the apex of the triangle of Koch as the penetrating atrioventricular bundle of His. It then immediately emerges in the subaortic outflow tract beneath the commissure between the non-facing and right coronary leaflets of the aortic valve. The axis then branches almost immediately in the normal heart, usually on the crest of the muscular septum but sometimes to its left side. The left bundle branch fans out on the smooth aspect of the septum in a continuous cascade, splitting into three divisions (anterior, septal and posterior) towards the ventricular apex. The right bundle branch turns back through the septum and continues through the substance of the septomarginal trabeculation before crossing in the moderator band and ramifying into the right ventricular myocardium. Sometimes, the axis itself continues as a dead-end tract on the septal crest and ascends towards the facing commissure of the aortic valve. The function of this part of the axis is unknown.

Although grossly not visible, the atrioventricular (AV) node and the bundle of His are located on an approximately straight line drawn between the coronary sinus ostium and the membranous septum. Whereas, the location of the coronary sinus ostium is obvious, the location of the membranous septum is not. A key to the location of the membranous septum lies in studying the tricuspid valve leaflets. The membranous septum is located at the commissure between the anterior and septal leaflets of the tricuspid valve. Hence, the AV node and the bundle of His are located in a line between the coronary sinus ostium and the commissure between the anterior and septal leaflets of the tricuspid valve. The atrioventricular node and bundle of His are located within the triangle of Koch, which is formed by 1) the origin of the septal leaf of the tricuspid valve, the Thebesian valve, and the Eustachian valve of the vena cava and its anterior extension, known as the tendon of Todaro. Since the Thebesian and Eustachian valves are quite variable and the tendon of Todaro is difficult to see grossly, although as well seen histologically, the most optimal method to localize the atrioventricular conduction system is by means of the coronary sinus to membranous septum line.

The atrioventricular node and bundle of His are not grossly visible; hence, it is necessary to know where these structures are, in order to avoid surgically induced heart block. The atrioventricular node is located directly in front of the ostium of the coronary sinus. If one mentally draws a line between the ostium of the coronary sinus and the commissure between the anterior and the septal leaflets of the tricuspid valve, this is where the atrioventricular node and the bundle of His are located. The membranous septum is just medial to the commissure between the anterior and septal leaflets of the tricuspid valve. The membranous septum has an atrioventricular portion between the right atrium and the left ventricle, and an interventricular portion between the right and left ventricles. The atrioventricular bundle penetrates just behind the membranous septum to pass from the atrial to the ventricular level, this portion being known as the penetrating bundle. The atrioventricular node and bundle consist of specialized muscle which is located within the triangle of Koch. The sides of this triangle are formed by the origin of the septal leaflet of the tricuspid valve, the Thebesian valve of the coronary sinus, and the tendon of Todaro. The tendon of Todaro is formed by the anterior prolongation of the Eustachian valve of the inferior vena cava and the Thebesian valve of the coronary sinus, which fuse and run anteriorly as one tendon beneath the right atrial endocardial surface. Thus, although well seen histologically, the tendon of Todaro is not visible grossly. Moreover, the Thebesian valve of the coronary sinus is quite a variable structure. Consequently, the most practical way to localize the invisible atrioventricular node and bundle is to draw a line mentally between the ostium of the coronary sinus and the membranous septum at the anteroseptal commissure of the tricuspid valve.

Although the presence of specialized pathways between the sinus and atrioventricular nodes has been postulated, there is yet no convincing evidence of their existence. Careful studies of the entirety of the atrial tissues show that the histologically specialized tissues are confined to the sinus and atrioventricular nodes and the rests of nodal tissue that are found around the atrioventricular junctions in the distal insertions of the atrial myocardium into the valve leaflets. These rests around the atrioventricular junction are of interest as they are found in most normal hearts. They are, however, sequestrated in the atrial tissues. In normal circumstances, they do not make contact with the ventricular myocardial tissues. Presumably, these were the structures observed by Kent when he suggested that the possibility for atrioventricular conduction in normal hearts existed at several points around the atrioventricular junction. In terms of interatrial conduction, it is now established that it is the structure of the bundles of working myocardium which determine the preferential spread of activity.