Prevention of Esophageal Damage During Ablation of Atrial Fibrillation by the Esophagus Mechanical Deviation

Atrial fibrillation is the most prevalent arrhythmia in the world population. Despite the use of antiarrhythmics, it is difficult to control clinically, causing symptoms and mainly generating risk of a thromboembolic event. Since 1998, by means of radiofrequency ablation, the treatment of atrial fibrillation has completely changed, but together with this important evolution complications from this ablative treatment technique have also started. In addition to the pulmonary vein stenosis caused by the ablation and later corrected with the change in the technique, atrioesophageal fistulas appeared due to the application of radiofrequency in the posterior wall of the left atrium. This wall is very close (0.5 cm onaverage) to the esophagus, which facilitates the formation of the fistula that leads to the death of almost 100% of the affected patients, despite the various treatment measurements already developed. To avoid this serious complication, several authors have created techniques to protect the esophagus including its mechanical deviation to a region opposite to the radiofrequency application, taking advantage of its mobility and easiness of handling. The mechanical deviation of the esophagus has proven to be the simplest, cheapest and most efficient way to protect this organ from radiofrequency thermal damage during atrial fibrillation ablation.


INTRODUCTION
The treatment of atrial fibrillation (AF) by catheter was described in 1998. Haïssaguerre 1 described at that time that the triggers of atrial fibrillation were located inside the pulmonary veins (PVs), in muscle bands from the atrial walls that penetrated these veins in a disorganized and random way. The application of radiofrequency within the PVs resulted in high incidence of stenosis and recurrence of arrhythmia 2 .
As a consequence, the technique was modified and the target of ablation by radiofrequency (RF) changed from the interior of the ostia to the antrum of PVs 3  Catheter ablation of AF using different energy sources has become routine in most electrophysiology services due to better control of sinus rhythm when compared to antiarrhythmic drug therapy 4 . The pillar of the procedure is the isolation of PVs 5 . However, to improve results, several of these services started to use an extended approach to perform ablations, mainly in persistent forms of AF, implying the mapping and ablation of fractionated atrial electrograms (FAEs) 6 , AF nests 7 , in background tachycardia (BKT) 8.9 , as well as the creation of additional blocking lines 10 , in addition to the standard isolation of PVs 5 .
In addition to RF ablation, new energy sources have been made available (cryoablation, laser and ultrasound) as well as technically more elaborate catheters, which are capable of producing deeper lesions, creating the potential risk of damaging structures near the left atrium (LA), particularly the esophagus 11 .
One of the greatest concerns and possible limitations during the procedure is the warming of the esophagus, which can occur when the RF is being released into in the LA posterior wall. As a consequence, erosions (esophagitis) and ulcers are described, ranging from 2.9 to 47% 12 and atrioesophageal fistula (AEF), which is fortunately rare 13 . Atrioesophageal fistula has been described in patients undergoing surgical AF ablation 14 and in patients undergoing catheter AF ablation using radiofrequency or cryoablation 15 .
Although the incidence of AEF is 0.1-0.25% 16-18 , this complication increases the mortality rate by 80 to 100%, being the second most frequent cause of death (16%) related to the AF ablation procedure, behind only the acute cardiac tamponade 19,20 . The data available in the literature suggest that the occurrence of EAF appears to be independent of operator experience 21 .
Although several strategies are being developed to avoid thermal damage to the esophagus, they still occur and have been reported in the literature 13,22 .
The objective of this article is to review the fundamental aspects of esophageal thermal lesions, their complications, and which techniques are currently being used to prevent them.

ANATOMICAL CORRELATION BETWEEN ESOPHAGUS AND LEFT ATRIUM
The middle esophagus is a fibromuscular tube 25 cm long and only 2 cm wide. The esophagus position is often closer to the left PV ostia compared to the right PV ostia 23,24 .
According to Cury et al. 25 , the mean transverse diameter of the LA in contact with the esophagus is 18.9 ± 4.4 mm, while the mean distance between the two structures is 1.9 ± 0.7 mm (Fig. 1).
Regarding the innervation, the anterior vagal trunk passes externally to the pericardium, from 2.5 to 6.5 mm from the posterior wall of the left atrium or pulmonary veins 26 . Another very important point is that both irrigation and esophageal innervation occur from the outer portion (adventitia) to the innermost portion of its wall (mucosa).
Thus, any injury to the outer portion of the anterior wall has a high potential to compromise both irrigation and esophageal innervation, and may determine several unwanted consequences.

Risk factors
There are some risk factors mentioned in the literature

CLINICAL FEATURES
Usually, the AEF diagnosis occurs between 2 days and 6 weeks after the ablation procedure. Han et al. 32  presence of digestive bleeding, with PR of 4.22 (p < 0.047).
Another complication described is gastroparesis, which is gastric obstruction due to the absence of a mechanical component 33,34 . These symptoms may begin within 3 h to 3 weeks after the ablation procedure has been performed. The mechanism of injury occurs through damage to the vagal plexus that compromises motility and gastric emptying.   They are prescribed a few days before the ablation procedure until 4 to 6 weeks after the procedure, mainly in cases where the lesion of the esophageal mucosa is identified 36,37 .

DIAGNOSIS
Once the diagnosis has been established, treatment should be carried out early. There are two techniques that can be used, correction with the use of endoscopic stenting or surgical correction, which is preferable because it significantly reduces the mortality rate 13,20 .
In a retrospective analysis with 29 patients presenting AEF after the AF ablation procedure, it was shown that 100% of the patients who were treated with stent died, while 59% of the patients who underwent the surgical procedure survived 38 .
In the case of a diagnosis of gastroparesis, depending on the degree of the lesion, treatment can be performed through diet modification and nutritional support, prokinetic agents, intrapiloric injection of botulinum toxin and also using gastric electrical stimulation. In these cases, positive results were reported within weeks 38 .
Patients should be well advised about the wide range of symptoms related to esophageal injury before discharge from hospital in order to anticipate possible diagnosis of complications.

Electroanatomical mapping combined with computed tomography or magnetic resonance imaging
The use of computed tomography or magnetic resonance imaging before a procedure allows to generate images of the LA and esophagus. These images can be fused with the anatomy built by the 3D electroanatomic mapping system. This method has a restriction already mentioned before, because it generates static images 39,44 . Thus, changes in the position of the esophagus during the procedure may occur and may not be identified at the time of applying RF to the LA posterior wall. Consequently, a false sense of security can be produced during RF application.

RF power titration (W)
The

Continuous esophageal cooling
One way to prevent the esophageal warming can be through

Mechanical deviation
Considering that the esophagus is anatomically very  (Fig. 4). According to the region where RF is applied in the LA, the esophagus is moved laterally to the opposite position by manipulating the TEE transducer, obtaining, in the vast majority of cases, a sufficient distance to avoid heating this organ. In addition, a multichannel thermometer can be inserted into the esophageal lumen to monitor temperature during RF application 55 . If any temperature rise is found, the esophageal deviation is reassessed and optimized to allow the application in a safe and complete way. According to these studies, no cases of atrioesophageal fistula or major esophagusrelated complications were observed, and this technique was used in a large number of cases (704 patients) 54 .
Later, Chugh et al. 55   The radiological image on the left shows the esophagus filled with barium contrast and with the transesophageal echocardiogram (TEE) probe inserted. In this case, the right PV approach would be highly risky to produce the AEF due to their overlap with the esophagus. On the right, the same patient having his esophagus displaced by the TEE transducer itself, allowing the approach of the right PVs without the risk of warming the esophagus.   (a) In ation port   More than 3,000 procedures have been performed with this method without a single atrioesophageal fistula, proving that the mechanical displacement of the esophagus is a highly simple, cheap and safe way to avoid this serious complication. Some manufacturers have presented solutions to perform esophageal diversion, which are available for services that do not routinely use transesophageal echocardiography during the procedure.