Which conduction node is most likely injured during a septal approach in Coronary Artery Bypass Grafting (CABG)?

From the Guidelines

The atrioventricular (AV) node is most likely to be injured during a septal approach in coronary artery bypass grafting (CABG). When surgeons use a transseptal approach to access the mitral valve or other posterior cardiac structures during CABG, they must dissect through the interatrial septum, which places the AV node at significant risk. The AV node is located in the floor of the right atrium near the interatrial septum, anterior to the coronary sinus and above the tricuspid valve. Due to its anatomical position, manipulation or direct trauma to this area can disrupt the electrical conduction pathway between the atria and ventricles. Injury to the AV node typically manifests as varying degrees of heart block, from first-degree AV block to complete heart block, potentially requiring temporary or permanent pacemaker implantation.

According to the most recent and highest quality study, the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1, the risk of abnormal atrioventricular conduction varies widely among observational studies, with estimates of 10% to 33% for alcohol septal ablation and 3% to 4% for surgical myectomy. The wide reported range is in part attributable to differences in baseline conduction properties.

Some key points to consider when evaluating the risk of AV node injury during CABG include:

• The anatomical location of the AV node, which is near the interatrial septum and can be easily damaged during septal approaches
• The risk of conduction disturbances, which can range from first-degree AV block to complete heart block
• The potential need for temporary or permanent pacemaker implantation in cases of AV node injury
• The importance of careful dissection techniques and awareness of the node's location to minimize the risk of conduction disturbances

It is also worth noting that other studies, such as the 2008 ACC/AHA guidelines for the management of adults with congenital heart disease 1 and the 2015 scientific statement from the American Heart Association on congenital heart disease in the older adult 1, also highlight the risk of AV node injury during cardiac surgery. However, the 2018 ACC/AHA/HRS guideline 1 provides the most recent and highest quality evidence on this topic.

In terms of minimizing the risk of AV node injury during CABG, surgeons must exercise extreme caution when working in this region, using careful dissection techniques and maintaining awareness of the node's location to minimize the risk of conduction disturbances, which represent a significant complication of cardiac surgery.

From the Research

Conduction Node Injury in CABG

The conduction node most likely injured during a septal approach in Coronary Artery Bypass Grafting (CABG) is the atrioventricular (AV) node.

• The AV node is located in the septal area of the heart, near the tricuspid and mitral valves 2.
• The lower pole of the AV node is situated vertically above the tricuspid septal leaflet (TSL) in 100% of cases, and at the level of the muscular interventricular septum in 65% of cases 2.
• The penetrating bundle of His, which is part of the AV conduction system, is located at the level of the TSL 2.
• Injury to the AV node or the bundle of His can result in cardiac conduction disorders, such as atrioventricular block 3, 4.

Anatomical Considerations

The septal area of the heart is a complex region, with important structures such as the membranous septum, the central fibrous body, and the Koch triangle 5.

• The AV node lies within the triangle of Koch, and the bundle of His exits the AV node and penetrates the right fibrous trigone 5.
• The septal atrioventricular junction is a common location for intracardiac shunts, such as membranous and perimembranous septal defects 5.
• Imaging techniques such as computed tomography (CT) and magnetic resonance imaging (MRI) can provide detailed information about the anatomy of the septal area 5.

Surgical Implications

Surgical procedures in the septal area, such as septal myectomy and alcohol septal ablation, can have procedure-specific effects on AV conduction and the His-Purkinje system 3.

• Septal myectomy is associated with the development of left bundle branch block (LBBB) in 50-100% of patients, while alcohol septal ablation is associated with right bundle branch block (RBBB) in 37-70% of patients 3.
• AV block requiring permanent pacing occurs in approximately 2-3% of patients after septal myectomy and 10-15% of patients after alcohol septal ablation 3.