Located in the front and upper side of RA, where the SVC enters, the sinoatrial (SA) node has a spindle shape (Fig. 6.1). Its upper end is coarse, stretches downward between the superior and IVC, and is covered only by visceral pericardium. The lower end is deep and contacts the endocardium. Nutrition of the SA node is supplied by a branch from the RCA in most (55–60 %) patients or the LCx in others (40–45 %).

The atrioventricular (AV) node is located in the lower right atrial septal plane, downward of the coronary sinus opening (Fig. 6.2). It is cylindrical and approximately 3 mm long and approximately 8–10 mm wide. Blood to the atrioventricular node is supplied by the atrioventricular node artery from the RCA. The AV node is surrounded by an abundance of conductive fiber called the atrioventricular node area.

An electric conduction pathway exists between the SA node and the AV node (Fig. 6.3). Impulses from the SA node to the AV node run through three channels or pathways called internodal tracts, which are divided into the anterior beam, the middle beam, and the posterior beam. The anterior beam extends from the SVC, descending anteriorly into the atrial septum, near the FO to the AV node. Branches from the anterior internodal beam to the LA are called the Bachmann bundle. The middle beam extends from the rear of the SVC and descends into the atrial septum, connecting with the anterior beam near the front of the FO. The posterior beam extends from the SVC and runs along the outer edge of the right atrial wall, continuing downward to the right atrial wall within the crista terminalis, along the valve attachment of the IVC, reaching the AV node.

When an abnormality or damage is associated with the three beams, arrhythmias, such as complete block or nodal rhythm, can occur. In the case of an atrial septal defect, the closure of an open surgery may damage the internodal conduction bundle.

The AV node sends forth a coarse beam called the bundle of His (Figs. 6.4 and 6.5). It is approximately 1–2 mm in diameter and 10–20 mm in length. Connecting with the distal part of the AV node, the bundle of His runs through the right fibrous trigone to the tricuspid septal leaflet until it reaches the lower edge of the membranous septum, where it divides into two branches (left and right) (Fig. 6.4). Because the bundle of His is affixed tightly to the posterior and inferior margin of the membranous septum, it is pushed to the bottom and backward in the cases of membranous VSD and endocardial cushion defect. Its left and right branches will shift and cause different degrees of conduction block (Fig. 6.6). Surgery can cause injuries to the lower edge of the membranous septum, which can result in a complete conduction block.