London, UK - An anatomic study of cadavers and human heart specimens has shown that the thickness of the posterior left atrial wall and the distance between the esophagus and left atrium varies [1]. According to investigators, the heterogeneous structures and relationships between the esophagus and posterior left atrial wall should be considered during the ablation of atrial fibrillation (AF) because of the potential risk of esophageal injury during the increasingly popular procedure.

"The problem relates to when the electrophysiologist, or even the surgeon, enters the left atrium and creates lesions within the left atrial wall," Dr Siew Yen Ho (National Heart and Lung Institute, London, UK) explained to heartwire. "In humans, the left atrial wall varies in thickness and, on occasion, when the energy applied is a little bit too much for that particular tissue, it can burn important structures behind the left atrial wall, such as the esophagus, as well as blood vessels and nerve bundles."

The results of the study are published online August 29, 2005 in Circulation.

With catheter-ablation techniques increasingly being used to treat paroxysmal and persistent AF, the development of a fistula between the esophagus and the left atrium has been reported as a complication of intraoperative and radiofrequency ablation procedures. Because the complication can be fatal, Ho said that a better understanding of the anatomic relationships between the left atrium and esophagus is needed to identify areas of potential high risk.

In this study, the dissection of 15 cadavers revealed that the relationship between the esophagus and posterior wall of the left atrium varied, mainly because the esophagus is displaced by the aortic arch as it descends anteriorly. In some specimens, the esophagus passed along the middle posterior wall, whereas it descended closer to the right or left side of the left atrium in others.

According to Ho, who teaches an "Anatomy for Electrophysiologists" course at the National Heart and Lung Institute in the UK, the study points to the need for real-time imaging during the AF-ablation procedure. While magnetic imaging (MR) and computed tomography (CT) are available to integrate anatomy with 3D electrical maps, a mobile esophagus during ablative procedures makes it particularly vulnerable to injury.

"During the course of the procedure, the esophagus can move around a fair bit," said Ho. "As of yet, there is no way of imaging the ablation procedure in real time, so EPs are essentially going in blind. The esophagus might be closer to the left pulmonary veins, it might be closer to the right pulmonary veins, or it might be closer to the middle part of the left atrial wall. When lesions are created transversely, if the esophagus is running in the middle portion of the left atrial wall, in between the left and right pulmonary veins, and the burn is deeper than intended, the energy source might damage the esophagus directly. It can also damage the blood vessels supplying the esophagus."

The thickness of the atrial wall also affects the potential for injury, noted Ho, especially if radiofrequency ablation is applied to thinner sections of the heart. To address this issue, the researchers examined 30 human heart specimens and found significant differences in the thickness of the left atrial wall. The posterior left atrial wall was found to be significantly thinner at the superior level of the left atrium than the inferior level of the chamber. The left atrial wall was thickest adjacent to the coronary sinus. The space between the esophagus and the atrial wall also showed wide variations, yet despite the varied proximity of the esophagus to the endocardial surface of the left atrium, the distance was <5 mm in 40% of the specimens.

Ho noted that some EPs place a temperature probe in the esophagus to measure temperature changes that might be caused by ablation lesions penetrating too deeply. In addition, she said that there is a need to develop extensive anatomical profiles of the heart to help guide ablation procedures.

"Any surgeon, before going in to operate, must have an extensive understanding of the anatomy," said Ho. "Electrophysiologists, on the other hand, go in and do their procedure without having to learn the anatomy first. It's a problem." Ho added that the important message from this study is that the left atrial wall is not uniform in thickness, and if ablating in that area, EPs really need to be careful. "What I would say to the electrophysiologists is, 'Know your anatomy,' " she said.