Postoperative atrioventricular block (AVB) in the correction of complex cardiac defects has been the Damocles’ sword of congenital heart surgery. In the early days of this specialty, one in ten patients developed AVB after surgery, which required treatment with adrenaline, atropine, and the newly discovered isoproterenol. These treatments were useful but temporary, condemning patients with permanent AVB. Temporary epicardial pacemaker wires were designed and first used by Walton Lillehei in the 1950s. However, they were connected to a bulky generator, connected to the power grid. If the grid failed, so did the generator and, consequently, cardiac stimulation. Unfortunately, Dr. Lillehei faced this tragic situation with one of his pediatric patients, with fatal results. This led him to seek Earl Bakken, an electrical engineer and hospital technician, who developed the first portable pacemaker in his small garage. Earl Bakken likely never imagined the significance of this milestone for the company he co-founded, Medtronic®.

We have made significant progress since then, but postoperative AVB remains the Achilles’ heel of our interventions. In less complex congenital surgeries, the incidence is approximately 1%. However, with greater complexity comes a higher risk, reaching up to 20% in conditions such as heterotaxy/isomerism syndrome.

The Boston Children’s study reports its experience with intraoperative conduction system mapping in various congenital heart defects. Their objective is to develop a predictive model of conduction tissue anatomy based on the specific congenital condition. From February 2020 to December 2021, mapping was performed in all complex congenital surgeries. Using a multielectrode catheter, electrograms were obtained in a decompressed, beating heart. They recorded patient anatomy, surgical procedures, the location of the His bundle, and conduction system status post-surgery. A classification and regression tree analysis was used to develop a predictive model of conduction tissue location based on the type of congenital defect.

A total of 109 patients were mapped, with median weight and age of 10.8 kg and 1.8 years, respectively. The conduction system was identified in 96% of cases, with a median mapping time of 6 minutes. The anatomies evaluated included atrioventricular canals, double outlet right ventricles, complex transposition of the great arteries, and multiple ventricular septal defects. The classification and regression tree analysis identified ventricular looping and visceroatrial situs as key discriminators for conduction system location. As a result, 89.5% of patients were free of AVB postoperatively. Only one patient (2.9%) with heterotaxy syndrome developed postoperative AVB.

The authors concluded that intraoperative mapping allows identification of the His bundle location and enhances understanding of the anatomical factors influencing its position. The predictive model provides additional insight into the conduction system in these patients, aiding surgeons in anticipating its location and avoiding injury.

COMMENTARY:

It has been just over a century since the anatomical localization of the cardiac pacemaker by Keith and Fick and the identification of the atrioventricular node by Sunao Tawara. Over recent decades, knowledge of the cardiac conduction system has advanced significantly thanks to genetic and molecular developments. For simple congenital defects, the location of the AV node and His bundle is relatively predictable; however, in complex cases, it becomes a guessing game. Experience has shown that the conduction system rotates with ventricular looping. This explains its anterosuperior position in patients with congenitally corrected transposition of the great arteries. However, ventricular looping is not the only factor; misalignment of the atrial and ventricular septa, as well as the atresia of a cardiac chamber, also play a role. In some cases, multiple AV nodes exist, and determining the functionally relevant one remains a mystery.

This novel study from Boston Children’s is the first large-scale investigation into the conduction system in patients with complex congenital heart defects. It is conducted in a beating, decompressed heart without complications related to the procedure. Despite their efforts, 4% of patients could not have their conduction system mapped, and in 10%, AVB could not be avoided. However, in heterotaxy patients, this technique reduced AVB from a reported 14% in their series to 3%.

This technique has several limitations, including catheter design, which was initially intended for percutaneous studies. This is evidenced by the 4% of patients whose mapping was unsuccessful due to size issues. Another limitation lies in logistics and resources; not all hospitals can afford to have an electrophysiologist in the operating room for intraoperative studies.

The beginnings of any innovation are cumbersome, but as more experience is gained in mapping, I am confident it will become more efficient with devices specifically designed for this purpose.

In conclusion, while we may not achieve a universally predictable conduction system pattern for all congenital defects, studies like this one provide roadmaps to avoid stepping on mines with our sutures.

REFERENCE:

Feins EN, O’Leary ET, Davee J, Gauvreau K, Hoganson DM, et al. Conduction mapping during complex congenital heart surgery: Creating a predictive model of conduction anatomy. J Thorac Cardiovasc Surg. 2023 May;165(5):1618-1628. doi: 10.1016/j.jtcvs.2022.11.033.