Strategies to Minimize the Risk of Esophageal Injury During Catheter Ablation for Atrial Fibrillation

Specific Methods to Minimize Esophageal Injury During Ablation

Fundamentally, there are three techniques that can be employed to minimize the likelihood of esophageal injury during RF ablation at the posterior left atrium: (1) avoidance of any RF ablation whatsoever when adjacent to the esophagus, (2) titration of RF energy delivery or other techniques to limit or reduce heat transfer to and thermal injury of the esophagus when ablation is needed at the posterior left atrium near the esophagus, and (3) use of methods other than RF energy that may have a lesser tendency to produce esophageal injury leading to fistula formation when targeting tissue near the esophagus. Each will be considered in turn.

Avoidance -- Moving the Ablation Lines

Kottkamp et al. reported a strategy of altering the lesion set for circumferential isolation of the PV and PV antral regions in order to avoid the esophagus.^[35] In this report, ablation lines were moved away from the PV antral region in some and closer to the PV os in others to avoid delivery of RF pulses very near the esophagus. This can be an effective approach in some instances depending on the specific location and breadth of the esophagus-atrium contact region. Two other groups have reported a lesion set surrounding all PVs to minimize the need for ablation at the posterior left atrial wall, thus minimizing RF energy delivery at sites potentially adjacent to the esophagus.^[36,37] A potential pitfall of these approaches is that expansion of the circumferential ablation lines away from the PV antrum increases the technical difficulty to achieve acute and/or persistent electrical isolation of the PV and PV antrum due to the need to ablate regions with thicker myocardium^[16] and to create uninterrupted ablation lines that span a greater total distance. Our experience is consistent with increased difficulty in achieving PV isolation when the ablation line is moved farther away from the PV ostium.

Avoidance -- Moving the Esophagus

Another approach to avoid delivery of RF pulses in close proximity to the esophagus is to move the esophagus out of the way with a luminal transesophageal echo probe.^[38] When successfully implemented, ablation sites, otherwise presenting risk, might be safely targeted with RF energy. However, there is always a potential risk associated with placement of a large luminal device into the esophagus during RF energy delivery at the posterior left atrium, whereby there could be unintentional displacement of the esophagus or passive application of pressure toward the left atrium ablation sites that in turn reduces the distance between the ablation catheter tip and esophagus and enhances heat transfer during ablation.^[8] The ability of an esophageal luminal device to displace the esophagus toward the atrium and consequently enhance heat transfer to and injury of the esophagus has been reported in animal models when an expandable esophageal balloon device was present in situ during endocardial RF energy delivery.^[39,40] Another potential limitation of this approach is that deflection of a luminal device may simply deform portions of the esophagus wall without necessarily moving the portion of the esophagus immediately adjacent to the left atrial ablation sites.

RF Energy Titration -- Use of Luminal Esophageal Temperature Monitoring

RF energy titration is likely the most common approach to minimize risk of esophageal injury during catheter ablation for AF. However, the challenge of this approach is in knowing how much delivered RF energy is "safe." Ideally, some real-time method to assess heat transfer to or lesion formation within the esophagus could guide RF energy titration during ablation; however, no method has yet been convincingly demonstrated to achieve this. Observational studies employing real-time luminal esophageal temperature measurements during left atrial catheter ablation reveal that esophageal heating can indeed be observed during some RF energy applications.^[17,29,30] Preliminary evidence^[41] suggests that rapid esophageal heating (e.g., rates >0.05-0.1°C per second) may herald instances of efficient heat transfer to the esophagus due to some combination of optimal catheter contact for energy transfer, minimal or absent intervening connective tissue,^[12] good contact of the luminal temperature probe with the anterior luminal surface of the esophagus, or other factors. Thus, rapid elevation of luminal temperature is likely specific for mural esophageal heating. Early use of luminal temperature monitoring also suggests an additional factor relevant to energy titration and assessment of esophageal heating, namely that the esophagus cools slowly^[41] and repeated RF energy applications can result in "temperature stacking," whereby a greater degree of mural esophageal heating is achieved with subsequent RF pulses (Figure 4). Further studies are required to characterize the relationship between temperatures reported with a luminal probe and mural temperature of the esophagus.

Limitations of Luminal Esophageal Temperature Monitoring

The sensitivity of luminal temperature monitoring to recognize all instances of esophageal heating and to accurately report the degree of significant heating is questionable. Initial clinical studies suggested that there is a poor correlation between total energy delivered during an RF pulse and elevation of luminal esophageal temperature,^[30,41] implying that factors other than total energy delivered determine esophageal heating or that a luminal probe does not report true mural esophageal heating with fidelity. It is likely that both explanations are true.

Tissue thicknesses and composition between atrial endocardium and esophagus are variable between patients^[13,14,15] and can significantly alter heat transfer to the esophagus,^[12,42] as discussed above. Further, a preliminary animal study suggests that actual tissue heating and esophageal injury may be critically dependent on catheter tip-tissue contact force.^[39] In aggregate, these variables cannot be reliably measured during the catheter ablation procedure, and would result in very different effects on the esophagus of otherwise equivalent RF energy pulses delivered to the endocardial surface. In addition, the fidelity of luminal temperature monitoring to estimate mural esophageal temperature can be poor because there is the potential for large offsets between actual mural esophagus temperature and that reported by a probe located in the esophageal lumen as described in a preliminary report in animals,^[43] and using other model systems.^[11,41,42] The potential for significant underestimation of RF-induced esophageal heating using luminal esophageal temperature monitoring may be related to the potential for variable orientation of a temperature probe within the esophageal lumen^[17] (Figure 3), and has been borne out by two reports of atrioesophageal fistula formation after catheter ablation for AF despite continuous intraprocedural esophageal temperature monitoring without evidence of significant esophageal heating.^[4,44] Accordingly, lack of evident esophageal heating via a luminal temperature probe might not exclude RF-induced esophageal injury and may have poor sensitivity to detect thermal injury. When a luminal temperature probe is employed to identify esophageal heating, it should be emphasized that it is important to adjust the rostral-caudal position of the probe repeatedly during RF energy delivery to minimize the distance between probe tip and ablation target site to minimize so much as possible temperature offsets between maximum mural esophageal temperature and that reported by a luminal probe.

Alternate Methods to Monitor for Thermal Injury of the Esophagus During Catheter Ablation

Methods other than luminal temperature monitoring have been proposed to monitor for thermal injury of the esophagus during catheter ablation. Intracardiac ultrasound imaging can often visualize the ablation catheter tip and esophagus simultaneously in real time when endocardial regions adjacent to the esophagus are targeted.^[26,31,32] Furthermore, ICE has been reported to reveal thickening of the atrial myocardium during RF lesion formation with increased risk of thermal injury to the adjacent esophagus inferred. However, utility of ICE to directly monitor for esophageal injury may be limited, at least when imaging is done from the right atrium, because no clear changes in the esophagus during RF ablation have been identified with ICE imaging.^[31] Accordingly, the primary utility of ICE imaging appears to be to identify ablation target sites that are in fact immediately adjacent to esophagus tissue rather than to provide real-time monitoring for thermal injury of the esophagus.

Approaches for RF Energy Titration -- How Much Power Should Be Applied During RF Ablation at the Posterior LA?

A general strategy for titrating RF energy during catheter ablation might well include multimodal imaging to define the location of the esophagus, use of a luminal temperature probe, avoidance or significant limitation of RF energy delivery whenever ablation targets are potentially near the esophagus regardless of luminal temperature probe data, and termination of RF energy delivery whenever a temperature probe reports rapid elevation of luminal esophageal temperature. Data from a luminal probe can provide added information about esophageal heating when these data are properly acquired and interpreted, and have been reported to help guide RF energy titration to minimize thermal injury to the esophagus during catheter ablation.^[45,46] Placement of a luminal temperature probe also allows for fluoroscopic localization of the esophagus (again, with limitations; see Figure 3).

There are sparse and largely indirect data available to support specific recommendations regarding how much RF energy is safe when ablation is done near the esophagus. Rapid esophageal heating can occur with even relatively brief and low-energy RF pulses delivered at the posterior left atrial wall, whereby catheter ablation at the right or left PV antra near the PV os with 20 W average for less than 20 seconds resulted in rapid heating at a luminal temperature probe^[17] (Figure 4). Further, we have observed transmural esophageal heating rates of greater than 0.2°C per second during select RF energy application in patients undergoing ablation with slightly higher power settings of 30-35 W. Preliminary data using both a 2-D finite element model and an in vitro model of heat transfer to the esophagus during RF catheter ablation suggested that, in the absence of an adipose tissue layer between the atrium and esophagus, transmural heating of the esophagus to greater than 50°C could occur in as little as 50 seconds.^[12] By definition, a luminal temperature probe will report the minimum transmural temperature; actual magnitudes and rates of temperature rises in the esophagus wall are, if anything, higher than that reported by a luminal temperature probe. Delayed cooling of the esophagus and temperature stacking (Figure 3) has also been observed using luminal temperature monitoring during RF ablation at the posterior left atrial wall (Figure 4). Pending more definitive data, it would seem reasonable to limit RF energy to under 20 W for less than 15-20 seconds when delivering RF energy near the esophagus, and to allow at least 180 seconds for esophageal cooling between limited additional RF energy applications within a single region adjacent to the esophagus. These recommendations are speculative and derive from the practical need to titrate in some way RF energy delivery to the left atrial posterior wall during catheter ablation for AF. New approaches to titrate RF energy during ablation have been proposed, including use of high-energy pulses for very short periods.^[47] Evidence-based guidelines for RF energy titration have yet to be established.

Another novel method to limit heat transfer to the esophagus having bearing on RF energy titration is to place a cooling device such as a cold saline-irrigated balloon into the esophageal lumen to counteract conductive heat transfer to the esophagus during RF ablation.^[40,48,49] Because esophageal heating during RF ablation is likely due to conductive heat transfer,^[7] cooling from the luminal surface of the esophagus would be expected to limit transmural thermal injury.^[49] Theoretically, this technique might allow for safe delivery of higher levels of RF energy and minimize the need to titrate energy delivery. However, as discussed above, animal studies have also demonstrated that inflation of a balloon in the esophagus can enhance heat transfer to the esophagus and increase thermal injury, presumably by displacing the esophagus toward the endocardial ablation site.^[39] Accordingly, additional investigation will be required before the safety and utility of approaches employing active esophageal cooling and optimal RF energy titration, while using such systems, are established.

In summary, heat transfer to the esophagus is likely dependent on variables including catheter tissue contact pressure, catheter orientation, atrial wall thickness, thickness of intervening connective tissue, exact location and extent of the esophagus-atrial contact patch, and esophageal wall thickness. These variables are difficult to define in the clinical setting. Nonetheless, general characteristics of RF-induced esophageal heating such as latency to heating, delayed cooling, temperature-stacking, inter-individual variability in the efficiency of heat transfer to the esophagus, variable fidelity of luminal temperature monitoring to reflect mural esophageal heating, and the ability of brief low-energy RF pulses to heat the esophagus under select conditions should be considered when making clinical decisions regarding RF energy titration during ablation at the posterior left atrial endocardium.

Alternate Ablation Modalities

The ability of an operator to define all variables that determine RF energy transfer into heart tissue or the esophagus, or to monitor RF-induced esophageal heating, is limited. Thus, RF energy titration to avoid esophageal injury is empiric and would be expected to carry some risk. Cryothermy is an alternate tissue ablation method that has been used extensively for arrhythmia surgery, and more recently for endocardial catheter ablation of supraventricular arrhythmias with a 4-mm-tipped catheter.^[50] Larger-tip cryothermy catheters with similar design (CryoCath, Inc. Montreal, Quebec) are available and have been approved for epicardial ablation. These catheters have also been used off label for endocardial ablation for atrial fibrillation.^[17,51] Investigational use of another large-tip cryothermy catheter for endocardial ablation for AF has also been reported.^[52] These studies show that cryothermy can be used to achieve PV isolation and/or to target left atrial tachycardia foci. The question is whether or not cryothermy has the same potential to produce esophageal injury with fistula formation as does RF ablation. Preliminary results from a small clinical series suggested that RF ablation and cryothermal ablation both act on the esophagus through conductive heat transfer because the magnitude and rate of temperature change with each ablation modality was observed to be similar.^[7] Ripley et al. reported that direct cryothermy ablation to the outside surface of the bovine esophagus, in vivo, resulted in transmural lesions; however, unlike RF ablation that produced transmural necrosis and ulcer formation in the same model, cryothermy ablation preserved cellular architecture and no chronic ulcerations or fistula formation was evident.^[53] Another in vitro study suggested that cryothermy ablation preserved the structural integrity of porcine esophageal tissue whereas RF ablation did not,^[54] consistent with preservation of tissue architecture and absence of coagulation necrosis and destruction of interstitial structural proteins. Preliminary evidence in an animal model of focal endocardial cryothermy ablation suggested that transmural injury to the esophagus could occur after cryothermy ablation but the injury did not progress to deep ulcers or fistula formation.^[55] Trials of cryothermy balloon therapy for PV isolation in humans are ongoing. This technology results in cryothermal tissue injury over broad regions of the posterior left atrial wall and PV antrum, and significant overlap with the esophagus is likely to have occurred in many patients. A recent preliminary report of 346 patients undergoing cryoballoon ablation for AF did not identify any patient with clinical evidence of esophageal injury or atrioesophageal fistula despite rigorous patient follow-up.^[56] A separate report of four patients undergoing endoscopy after cryoballon catheter ablation revealed that one of the patients had superficial ulceration at the retrocardiac esophageal lumen that healed completely within a month.^[57] Yet another preliminary report suggested that focal endocardial cryothermy at regions immediately adjacent to the esophagus was apparently safe in a small number of patients.^[7] Taken together, these data suggest that cryothermy can cause transmural injury to the esophagus but may be less likely to result in deep ulceration and fistula formation. Though definitive data are lacking, this preliminary and indirect evidence suggests that cryothermy may be the preferred ablation modality when targeting left atrial posterior wall endocardial sites adjacent to the esophagus, either alone or in combination with RF ablation. In addition to the apparent reduced risk of transmural esophageal necrosis with fistula formation, cryothermy ablation appears also to have a lesser tendency to produce PV stenosis.^[51,52,58] Given that it is preferable not to deliver ablation lesions immediately adjacent to the esophagus even with cryothermy, an option to redirect cryothermy lesions to the vein ostium or just within the vein and away from the esophagus in select patients may further enhance safety of cryothermy vis-à-vis the esophagus; this is not an option with RF energy-based ablation due to the risk of PV stenosis. An important limitation of these data regarding use of cryothermy for left atrial ablation in humans is that the occurrence of A-E fistula, even with RF energy, is quite low, and would not be expected to be higher with cryothermy. Thus, many ablation procedures employing cryothermy at the posterior wall would be required to realize statistical power to detect clinically significant cryothermy-induced esophageal injury. Alternate endpoints after cryothermal left atrial ablation such as upper endoscopy 1-3 days after the procedure could provide valuable information about the safety of cryothermy (or any other method to reduce risk) because early mucosal changes recognized with endoscopy are thought to be requisite harbingers of A-E fistula formation; however, such studies have not yet been reported.

Pacing Clin Electrophysiol. 2009;32(2):248-260. © 2009  Blackwell Publishing

Cite this: Strategies to Minimize the Risk of Esophageal Injury During Catheter Ablation for Atrial Fibrillation - Medscape - Feb 01, 2009.