Localizing the Esophagus
Variable Course of the Esophagus
Any strategy to limit or avoid RF energy delivery in close proximity to the esophagus requires that the clinician have accurate information about exactly where the esophagus is located relative to intended sites of ablation. Between patients, the course of the esophagus is variable. We have observed the esophagus to be located rightward of the spine, compressed between the spine and left atrium, behind the atrium in the groove bounded by the aorta and the spine, compressed between the aorta and the left atrium, and leftward of the aorta. Furthermore, the esophagus position relative to the left atrium can be adjacent to the right PV antra and ostia, the posterior left atrium between the right and left PVs, or the left PV antra and ostia.[13,15,17,18] The esophagus is often compressed between the left atrium and surrounding structures, causing the esophagus to take a flattened and ovoid shape with a broad contact patch abutting the atrium, and we have observed cases where this contact patch spans most of the posterior left atrial wall,[17] consistent with a previous CT study that reported an average contact patch width of 1.9 cm.[19] In the individual patient, the esophagus can be adjacent to or overlay the right or left PV antra, or can course diagonally to overlay both the left and right PV antra.[15] Figure 1 depicts some clinical examples of the variable relationship between the esophagus and left atrium. Thus, the esophagus could be at risk for thermal injury during RF ablation from virtually anywhere at the posterior left atrial endocardium or within the PV near the os, depending on the individual's anatomy.
Figure 1.
Examples of the variable anatomic relationship between the esophagus and left atrium. Panels A through F are transverse bright blood MRI images from different patients. The esophagus appears immediately posterior to the left atrium in each panel. The arrows indicate the span of the atrium-esophagus contact area. Panel A, the esophagus is rightward of the spine adjacent to the right PV antrum; panel B, the esophagus is compressed between the spine and atrium spanning the region from the right PV antrum and os to the posterior wall near the left PV antrum; panel C, the esophagus spans the entire posterior wall of the atrium; panel D, the esophagus is adjacent to the left PV antrum and os; panel E, the esophagus is partially compressed by the descending aorta and is adjacent to the left PV os; panel F, the esophagus is compressed by the aorta and spans a broad region adjacent to the left PV, the PV os, and the PV antrum.
Movement of the Esophagus
In addition to having a variable course, it has been reported that the esophagus position relative to the left atrium can be dynamic. Peristalsis and deglutition in the awake but sedated patient,[20] or changes in patient position from the left to the right recumbent position,[21] have been reported to cause the esophagus to shift laterally up to several centimeters in relationship to the atrium. However, other evidence suggests the esophagus may have a consistent "resting" position. For example, stability in the location of the esophagus during a second ablation procedure, as compared to that during the first, has been reported.[22] Further, when esophageal position defined by preprocedure CT scan with a barium swallow was compared with the intraprocedure position defined using an electroanatomic mapping system, the esophagus position corresponded to that identified with the CT scan done on another day, and did not change during the procedure.[23] Likewise, another group reported stable intraprocedure esophagus position using real-time monitoring with an electroanatomic mapping system in patients undergoing catheter ablation while under general anesthesia.[24] It is unclear what role, if any, instillation of barium paste into the esophagus has in stimulating or accentuating esophageal movement, whether general anesthesia reduces the likelihood that the esophagus will move during catheter ablation, and what other factors may favor significant lateral excursion of the esophagus during catheter ablation. Our experience is that most patients demonstrate a stable and fixed esophagus position during the catheter ablation procedure when general anesthesia is employed, and that the esophagus position observed during the procedure corresponds closely with that defined by pre-procedure magnetic resonance imaging (MRI) or CT done days to weeks beforehand. The esophagus position revealed by follow-up imaging done months later is similarly stable in most patients. Nonetheless, the potential for significant lateral displacement of the esophagus remains, recommending the use of "real-time" imaging during catheter ablation to confirm the location of the esophagus during ablative energy delivery.
Methods to Localize the Esophagus Before RF Energy Delivery
Specific imaging methods to define the esophagus-atrium relationship can be grouped into real-time methods that yield images during RF-energy delivery, and non-real-time methods that yield static images generated sometime before RF energy delivery. The later category includes the following: (1) preprocedure CT[25] or MRI[26]; (2) importation of registered CT-generated shells of left atrium and esophagus (Figure 2); (3) tagging the esophagus location at the start of the procedure on an electroanatomic map generated with Carto™ (Biosense Webster, Diamond Bar, CA, USA) or NavX™ (St. Jude Medical, Sylmar, CA, USA) using fluoroscopic visualization of the esophageal lumen containing barium or some other luminal marker as a guide (e.g., NG tube, esophageal temperature probe, or catheter)[17,24]; or (4) using an electroanatomic mapping system to generate a static representation of the esophagus. The last method could involve using a luminal catheter recognized by the NavX™[24] or Carto™ systems, or using intracardiac ultrasound to render a static 3-D representation of the esophagus together with a rendering of the left atrium[27,28] (Carto SoundStar™; see Figure 2). The Carto SoundStar™ systems renders 3-D structures like the esophagus and left atrium by merging contours of these structures, as annotated on each of multiple separate 2-D intracardiac echocardiography (ICE) imaging planes to produce the 3-D image. Real-time imaging features of the Carto SoundStar™ system are discussed below.
Figure 2.
Different imaging modalities reveal the anatomic relationship between the esophagus and left atrium. Panels A-D are from one patient with the esophagus adjacent to the right PV antrum and os, and show (A) a transverse MRI image, (B) a rendering of the left atrium and esophagus generated with intracardiac ultrasound (Carto-Sound™) together with a segmented MRI scan of the left atrium registered to the ICE-rendered structures, (C) a still frame of a real-time 2-D ICE image superimposed on the rendered structures and registered MRI shell, (D) the same as C with removal of the ICE-rendered esophagus to allow confirmation in real time of the esophagus position and registration of the MRI shell. Note that the 2-D ICE image is correctly displayed relative to the registered and rendered images due to location sensors in the ICE probe. Yellow tags represent biplane fluoroscopic determination of the location of the esophageal luminal probe, blue tags represent cryothermy ablation sites, and red tags represent RF ablation sites. Panels E through G are from another patient with the esophagus adjacent to the left PV antrum and os as depicted in panel E. In panel E, white arrows indicate the course of the luminal esophageal probe in the left anterior oblique view (43°), and the left superior PV is opacified with hand injection of Isovue via a pigtail catheter in the vein. Two transseptal sheaths, a permanent pacing lead, and the coronary sinus catheter are also visible. Panel F shows a posterior view with leftward angulation of the segmented left atrial shell and esophagus from a prior CT scan, registered to the ICE rendering of the left atrium (hidden in this example). The esophagus shell is incomplete due to lack of esophageal contrast during CT imaging. Panel G is the same except for rightward angulation. Real-time confirmation of the registration and of the position of the esophagus is depicted with the overlying 2-D ICE image.
Of these non-real-time imaging methods, "bright blood" axial MRI images or CT can be useful to depict the breadth and location of the esophagus, including information about the extent of the esophagus-atrium contact patch. For example, some patients have a very broad esophagus-atrial contract patch, which, if identified, would recommend caution in relying solely on a discreet luminal esophageal marker for designation of regions of the posterior left atrium (LA) wall that are adjacent to the esophagus (Figure 1 and Figure 3). Whereas near real-time intraprocedural tagging or rendering of the esophagus on an electroanatomic map can confirm findings from CT or MRI and provide complimentary information about potential proximity of intended ablation target sites to esophageal tissue, these methods do not provide reliable information about the extent of the esophagus-atrial contact region or thicknesses of intervening tissue between catheter tip and esophagus at the moments of ablation. Furthermore, none of these methods can detect motion of the esophagus that might occur after image acquisition.
Figure 3.
Complimentary imaging of the esophagus in a single patient. Panels A and B show transverse and sagital "bright blood" MRI images. The esophagus is compressed and spans a broad region adjacent to the left PV antrum. Panel C shows an electroanatomic (E-A) map with a superimposed segmentation of the left atrium from MRI, registered to the map. The left and rightward margins of the esophagus are tagged (yellow). Panels D and E show venograms of the left inferior and left superior PVs in the left anterior oblique projection. The tip of a luminal esophageal probe is indicated with the white arrow, and a nasogastic tube advanced to the stomach is indicated with the black arrow. Panels F (left anterior oblique) and G (right anterior oblique) show the orientation of the temperature probe tip within the broad esophageal lumen opacified with Gastrografin introduced via a nasogastric tube withdrawn to the mid esophagus. The probe tip is seen to be displaced to the leftward margin, and is anteriorly located immediately adjacent to the left PV antrum. Without opacification of the esophageal lumen, the rightward extent of the esophagus, spanning more than 1 cm from the probe tip position, would not be appreciated.
Methods to Localize the Esophagus During RF Energy Delivery
Real-time visualization of the esophageal position can be achieved with repetitive fluoroscopic visualization of the esophageal lumen containing barium or another luminal marker during the procedure,[17,29,30] with real-time display of a multi-polar catheter in the esophagus lumen recognized by the NavX system,[24] or with real-time intracardiac ultrasound from the right[26,31] or LA.[32] Ultrasound is the only real-time imaging modality that allows visualization of the extent or thickness of the esophagus wall, especially when imaging is done from the left atrium.[33] Barium paste can allow visualization of the true width of the lumen, but information about esophageal wall thickness is difficult to infer as only the lumen is visualized. Likewise, a luminal marker like a temperature probe cannot provide information about esophageal wall thickness. In addition, a luminal probe or catheter can provide misleading information about the medial to lateral extent of the esophagus-atrium contact patch because it may be positioned eccentrically to one side of a broad and flattened lumen whereby ablation-directed 1-2 cm more medially or laterally from the probe based on fluoroscopy may in fact be immediately adjacent to the opposite margin of the esophagus[17] (see Figure 3). Real-time imaging can also confirm information about the esophagus provided by preprocedure imaging and by the near real-time imaging techniques discussed above (see Figure 2 and Figure 3). Thus, intracardiac ultrasound can accurately reveal the location and extent of the esophagus and is uniquely well suited to assess true proximity of actual ablation sites to esophageal tissue.[26] Without use of real-time intracardiac ultrasound with good visualization of the esophagus, it may be best to assume that the "danger zone" for RF-induced esophageal injury extends 1-2 cm to either side of a discreet marker like a luminal temperature probe.
The most significant limitation of intracardiac ultrasound relates to the technical difficulty of maintaining orientation of the 2-D imaging plane to visualize the catheter tip and/or esophagus during RF pulses owing to the limited view of a 2-D ICE imaging plane in combination with movement of the imaging plane relative to the heart during the cardiac cycle. In addition, "shadowing" is often seen to extend from the catheter tip, making it difficult to assess the exact tip location, especially when the catheter shaft is imaged and oriented parallel to the radiating ultrasound beam. Intracardiac ultrasound imaging from the left atrium may be useful to overcome some of these limitations and can facilitate imaging of both the esophagus and ablation catheter tip during lesion delivery.[33]
Aside from rendering a static 3-D representation of the esophagus, the Carto SoundStar™ system has important real-time imaging features to help address some of these limitations of 2-D ICE imaging during left atrial catheter ablation. First, the system annotates in real time the exact location of the ablation catheter tip on the 2-D ICE image to facilitate orientation of the 2-D imaging plane to allow accurate visualization of the catheter tip. Second, the 2-D imaging plane is displayed as a "fan-like" image, superimposed on the 3-D rendering of the esophagus and left atrium in real time to facilitate optimal orientation of the ICE imaging plane relative to structures of interest such as the PV antra or esophagus (Figure 2).
Real-time 3D ultrasound is a new technology that has potential to enhance catheter ablation for AF by reducing the difficulty of maintaining structures of interest such as the ablation catheter tip and Ligament of Marshall within the ultrasound imaging field during catheter ablation. Whereas this technique has been employed to guide catheter ablation for AF,[34] the potential utility of real-time 3-D ultrasound imaging to define the course of the esophagus during catheter ablation has not 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.
