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elevance of imaging structures adjacent to the left atriumuring catheter ablation for atrial fibrillationavid M. Donaldson, MD,* Amar Shah, MD,† E. Kevin Heist, MD, PhD,* Conor D. Barrett, MD,*odtfred Holmvang, MD,* Suhny Abbara, MD,† Jeremy Ruskin, MD,* Moussa Mansour, MD*

rom the *Heart Center and the †Department of Radiology, Massachusetts General Hospital, Boston, Masschusetts.

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ntroductionatheter ablation has become one of the primary treatments

or symptomatic drug-refractory atrial fibrillation (AF).his procedure can be associated with complications includ-

ng the risk of injury to structures adjacent to the left atriumLA), such as the esophagus, the aorta, the left circumflexoronary artery, the bronchial tree, and the phrenic nerve.he objective of this article is to review the above anatomynd assess the ability of imaging to detect the proximity andrientation of these secondary structures in relation to theA and the pulmonary veins (PVs). Lastly, we will discussblation strategies to avoid damage to these secondarytructures. The imaging techniques described in this reviewnclude computed tomographic (CT) angiography, magneticesonance angiography, and intracardiac echocardiographyICE).

sophagusatheter ablation within the LA may be associated with

hermal injury to the esophagus and possibly atrioesopha-eal fistula formation.1,2 This complication is rare but po-entially devastating and often fatal. It is believed that itsccurrence is underreported, but current estimates suggesthat 0.01% of all endocardial LA ablation procedures areomplicated by atrioesophageal fistulas.3 Therefore, tech-iques to reduce the risk of this complication are of greatmportance.

Imaging techniques can be used to identify areas of theA that are in direct contact with the esophagus. Contrast-nhanced CT (Figures 1A and 1B) and magnetic resonancemaging (MRI; Figure 1C) provide detailed imaging of thesophagus. The key anatomic variants of this region and

EYWORDS Atrial fibrillation; Left atrial mapping; Computed tomography;agnetic resonance imaging; Transesophageal echocardiography; Ablation;magingBBREVIATIONS AF � atrial fibrillation; LA � left atrium; PVs � pulmonaryeins; ICE � intracardiac echocardiography (Heart Rhythm 2010;7:69–275)

ddress reprint requests and correspondence: Moussa Mansour, M.D.,ardiac Arrhythmia Center; GRB109, 55 Fruit Street, Massachusetts Gen-ral Hospital, Boston, Massachusetts 02114. E-mail address: mmansour@

rartners.org. (Received February 26, 2009; accepted September 11, 2009.)

547-5271/$ -see front matter © 2010 Heart Rhythm Society. All rights reserved

ossible determinants of the risk of thermal damage to thesophagus include absence of a fat pad surrounding the esoph-gus, wall thickness of the LA and esophagus, and the spatialelationship of the esophagus and LA.4 A majority of patientsave direct contact between the esophagus and the inferior toid portions of the posterior LA.5–7 There is a protective layer

f fat between the esophagus and the posterior LA, yet thisayer is often discontinuous and, in 2% of normal subjects,ompletely absent.8 MRI of the esophagus can readily de-ect this fat pad and alert the operator if it is absent (FigureC). The wall thickness of the esophagus and the LA variesreatly (Figure 1). In one study, the average thickness of thenterior esophageal wall ranged from 1.9 to 5.3 mm, whilehe average thickness of the posterior LA ranged from 1.3 to.1 mm.8 The minimum distance separating the esophagusnd the LA can be as small as 3.3 mm when measured fromuter wall to outer wall.4 If one considers minimal dimen-ions for LA and esophageal wall thickness, 1.3 and 1.9m, respectively, and 3.3 mm for the separation of these

tructures, the minimal total separation from esophaguso LA inner wall is 5.5 mm. This is well within thechievable depth of radiofrequency (RF) ablation lesions,ven with relatively conservative power and time set-ings.

A second consideration is the length and width of contactetween the LA and the esophagus. The contact length isariable and has been measured at 58 � 14 mm, and theidth has been measured at 13 � 6 mm.8 As the surface

rea of contact between these two structures increases, sooo does the potential for thermal injury propagated to thesophagus and the risk of atrioesophageal fistula forma-ion.

There is also considerable heterogeneity in the anatomicrientation between the esophagus and the LA. Variationccurs in the medial to lateral position of the esophagusith respect to the LA. The esophagus can be located in

hree general areas: near the left superior and inferior PVsFigure 2A), in a midline position between the right and lefteins (Figure 2B), or close to the right superior and inferioreins (Figure 2C). Lemola et al8 found that the esophagusravels parallel to the left PVs in 56% of patients, with the

emaining 44% having either a predominantly right or me-

. doi:10.1016/j.hrthm.2009.09.020

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270 Heart Rhythm, Vol 7, No 2, February 2010

ial directed esophagus. In 36% of patients with a left-sidedsophagus, the esophagus had an oblique course from theeft superior to the right inferior PV.8 This variability isritical for the operator to understand, as an irregular esoph-geal-to-atrial relationship can markedly alter the safety ofn AF ablation procedure.

Taking into consideration these anatomic features,everal ablation strategies can be adopted to minimize theisk of esophageal injury. The lesions generated by RFblation have a depth that is proportional to the durationf lesion application, power, electrode temperature, elec-rode-tissue stability, and electrode-tissue contact pres-ure. It is established that long-duration and high-powerblation lesions in the posterior aspect of the LA canause lesions that extend beyond the epicardium andotentially cause esophageal injury.2 The operator couldppropriately adjust ablation duration and power basedn images demonstrating a close relationship between theblation site and the esophagus or could select an alter-ate site for ablation to reduce this risk.

Imaging of the LA and esophagus has the potential toeduce the risk of inadvertent ablation adjacent to thesophagus with subsequent esophageal thermal injury.hick areas of esophagus and the LA would be expected

o be less vulnerable to thermal injury than thin areas. Ithould be noted, however, that no technique has yet been

onclusively proven to reduce or prevent atrioesophageal v

stula formation, given the infrequent occurrence of thisomplication.

As the esophagus is mobile,9 real-time monitoring of thesophagus may be helpful in avoiding the placement ofesions in regions where the esophagus is in direct continu-ty with the LA. To accomplish this, real-time monitoring ofsophageal temperatures10 or visualization of the esophagusuring the ablation procedure may help to avoid thermalnjury.9 Direct visualization of the esophagus during theblation is possible with ICE,11,12 which can be facilitatedy the use of an electroanatomic mapping system that per-its integration with ICE imaging. Preablation CT image

ntegration with real-time ultrasound imaging (Figures 3And 3B) can detect changes in the esophageal locationetween the time of CT imaging and the ablation procedure.ccurate localization of the esophagus can also be aided by

ntraprocedural swallowing of oral contrast paste, whichines the lumen of the esophagus and provides the operatorith real-time fluoroscopic visualization of this structure.9

he cumulative use of preprocedural imaging with CT orRI coupled with intraprocedural ICE may allow for a

uccessful ablation while limiting the risk of esophagealnjury.

ortahe aorta lies in close proximity to the LA, making it

Figure 1 Two-dimensional thoracic CTimage with barium paste lining the luminalsurface of esophagus. A: Axial view; B:sagittal view. Ao � aorta; AV � aorticvalve; Esoph � esophagus; LIPV � leftinferior PV; RIPV � right inferior PV. C:MRI sagittal view showing the LA rela-tionship to the esophagus, with evidence ofregional distribution of esophageal fat. D:Normal anatomic relationship of LA toaorta as visualized with CT. Volume-ren-dered imaging of the LA, PVs, and aorta.Ao � aorta.

ulnerable to injury during catheter ablation for AF, espe-

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271Donaldson et al Imaging Structures during Ablation for AF

ially during the transseptal puncture. The descending aortas located posterior to the LA, (Figure 1), while the ascend-ng aorta is located anterior to the LA and somewhat to theight of the spine (Figure 1). In normal subjects, the poste-ior LA is directly opposed by a large portion of the anteriorurface of the descending aorta, with an average contactrea of 18.9 � 4.4 mm2.13

Although the relationship of the aorta to the LA is rela-ively homogeneous, key anatomic variants occur. The LAs commonly dilated in patients with AF,13 and the LA is

A

C

LIPV

LSPV

Barium Enhanced Esophagus

Barium Enhanced Esophagus

igure 2 Volume-rendered three-dimensional CT image of the LAontrast-filled esophagus demonstrates normal physiologic peristalsis. A:

Esophagus

LA A

igure 3 Carto sound imaging ofsophagus and LA. A: Two-dimensionaliew of the LA in relation to the esopha-us. B: Carto sound image of esophagus inelationship to CT imaging of the LA andVs.

oted to be in contact with the aorta more frequently inatients with AF than in controls.14 One report showed thatn 10 of 42 patients with AF, the aorta ran in an indentationf the posterior LA wall.14 In another study, four of 21atients exhibited compression of the posterior aspect of theA by the descending the aorta.15 Ho et al16 showed that aondilated aorta can compress the anterior and the posteriorA as well as the PVs (Figures 4A and 4B). Although itould seem that anterior compression of the LA would

ncrease risk of injury during the transseptal puncture, there

B

LPV RPV

Barium Enhanced Esophagus

RIPV

RSPV

with oral barium enhancement of the esophagus. Irregularity of theed esophagus; B: midline esophagus; C: right-sided esophagus.

Esophagus

LA

B

and PV

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272 Heart Rhythm, Vol 7, No 2, February 2010

ave been no reported cases. Similarly, posterior LA com-ression would seem a risk factor for PV stenosis by nar-owing the functional ostium of the veins, but there haveeen no documented cases of this either.

Imaging performed preprocedure with CT or MRI canetect these key anatomic differences and can be useful touide the ablationist when performing the transseptal punc-ure. In the presence of compression of the posterior wallith the descending aorta, the transseptal puncture shoulde directed in a slightly more anterior direction. Conversely,ompression of the anterior LA by the ascending aortahould direct the transseptal puncture slightly posterior andway from the anterior wall. Knowledge of these anatomicariants is most critical for the transseptal puncture, as thereave been no reported incidences of direct aorta damage byeating during ablation in the LA, which may be related toigh blood flow in the aorta causing rapid dissipation of RFnergy. Although the risk of clinically important damage tohe aorta by AF ablation is likely extremely low, the aortaan exert a compressive effect on the left PVs, which mayncrease the risk for PV stenosis.

eft circumflex arteryhe left circumflex (LCx) coronary artery is located in theosterior atrioventricular groove (Figure 5C) in continuityith the coronary sinus (CS; Figures 5A and 5B), and there

s commonly a separating layer of fat. The circumflex cor-nary artery is also located in close proximity to the inferiorspect of the LA appendage (LAA; Figure 5C). In oneeport, the mean distance between the LCx and the LAAas 3.3 mm (0.7–19.6 mm), and the median distance be-

ween the CS and the LCx was 2.0 mm (0.4–9.7 mm).17 Asblation in the CS is sometimes performed during AF ab-ation procedures, a comprehensive understanding of thenatomical relationship between the CS and the circumflexrtery can prevent damage to this structure and the resultantisk of myocardial infarction.

Ablation is commonly performed in the CS while target-ng complex fractionated atrial electrograms, during CSsolation and with completion of a mitral isthmus line of

Ao

Ao

LA

LIPV

LIPV

A B

lock. Although usually safe, the absence of fat surrounding L

he middle cardiac vein and in CS diverticulae can makeblation in these structures somewhat riskier. Ablationithin the LAA, although rarely performed during AF pro-

edures, raises the possibility of damage to the circumflexrtery, especially when lesion sets are delivered to the an-erior border of the LAA. Long-duration and high-power RFnergy applications in the LAA and CS can cause damage tond occlusion of the LCx artery.17 Therefore, to reduce theisk of damage to the LCx while ablating within the CS,he operator should start with low power and graduallyitrate up. Alternatively, cryothermal ablation may benother option and has been reported to be safely per-ormed in the CS.18

Preprocedural imaging with CT of the anatomic relation-hip between the LCx and the CS can inform the operatorbout whether a superior or inferior directed ablation shoulde performed within the CS to avoid ablating in the direc-ion of the LCx and thereby reduce the risk of thermalnjury. In addition, for patients with key anatomic variantsuch as persistent left superior vena cava (SVC), preproce-ural imaging is particularly important to identify thesenomalies and thereby facilitate planning for an optimallyafe ablation procedure. A persistent SVC can be a source ofocal triggers for AF, making ablation in this region impor-ant.19 In one study of six patients with AF and persistenteft SVC, four patients had successful isolation of foci fromhe left SVC. In these cases, imaging was critical to makinghe ablation procedures both safe and effective in eliminat-ng the source of AF.20

ronchial treetriobronchial fistula (ABF) is a rare but life-threatening

eported complication of AF ablation procedures.21 Lesionserformed deep within the PV increase the risk of ABF, ashe more distal PV is in closer contact with the distalronchi.

The intrapulmonary bronchial tree travels in an inter-wined course in close proximity to the distal PVs, joiningroximally to form larger bronchi (Figure 6). The left andight main stem bronchi are located above the roof of the

o

Ao

RSPV

Figure 4 Thoracic CT showing aortacompressing the LA and left inferior PV,as visualized in (A) the axial view and (B)the volume-rendered three-dimensionalimaging in the superior view.

A

A. The main body of the LA is located far from the

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273Donaldson et al Imaging Structures during Ablation for AF

ronchial tree (Figure 6). In comparison, the PVs, particu-arly the superior PVs, are in close contact with the bron-hial tree. As Wu et al21 reported in a case series of 70atients, the bronchi are in direct contact with the four PVsn the vast majority of patients.

The anatomic relationship among the bronchial tree, theA, and the PVs can be well seen on CT (Figure 7),roviding the operator clear visualization of key structureshat may reduce the risk of ABF. It possible to segment outhe bronchi in the preprocedural CT, and this may be infor-ative in rare cases, although this is not frequently per-

ormed during routine AF ablation procedures. The safestractical strategy to reduce ABF without the use of bron-hial imaging is to avoid lesions within the PVs, particularlyhe superior veins. This strategy is also obviously useful ineducing PV stenosis.

hrenic nervehe phrenic nerve is an extrapericardial structure that liesdjacent to the atrium and can be injured during AF abla-ion. The right phrenic nerve travels in the superior medi-stinum adjacent to the SVC over the right atrium andoward the inferior vena cava to innervate the right hemi-iaphragm. The left phrenic nerve travels in the superiorediastinum over the left ventricle and innervates the left

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Coronary Sinus

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Small Cardiac

Vein

Left Marginal

Vein

A

B

emidiaphragm. t

Damage to the phrenic nerve can result in temporary orermanent diaphragmatic paralysis, which is commonly notetected until intraprocedural respiratory compromise oc-urs or on postprocedure chest radiography. Therefore, it isssential for electrophysiologists to be aware that thehrenic nerve is most commonly injured when ablation iserformed in the region of the SVC or near the right supe-ior and right inferior PVs and, rarely, in deep lesions nearhe crista terminalis. As it is variably visualized duringreprocedural imaging with current techniques, a com-only used strategy to avoid damage when ablating in or

ear the right-sided PVs and SVC is to pace and ensureoncapture of the phrenic nerve before RF or other forms ofF ablation, including balloon-based technologies. Most

mportantly, phrenic nerve damage can be limited signifi-antly by limiting RF lesion sets to antral sites. Addition-lly, as seen on CT, the left phrenic nerve travels anterior tohe LA appendage and could be damaged with prolongednergy applications deep within the LAA (Figure 8A). Ab-ation performed in the anterior region of the LAA carries aignificant risk of phrenic nerve injury and should thereforee performed with great care and with preablation pacing todentify the phrenic nerve. In addition, it is well establishedhat there is a relatively high risk of right phrenic nerveamage with cryoballoon ablation of right-sided veins, par-

LAD

LA

LCx C

LAA

ure 5 Volume-rendered cardiac CT angiograms.The LCx artery giving off the posterior lateral arterynch and then the posterior descending artery (PDA).The cardiac venous system, showing the CS beingby the small cardiac vein, middle cardiac vein, andmarginal vein. C: Thoracic CT angiogram axial

w (64-slice multirow detector). The left superior PVt shown) and the LAA can lie close to the LCxry.

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icularly if ablation is performed deep within the right PVs

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274 Heart Rhythm, Vol 7, No 2, February 2010

ith a small balloon. This risk can be reduced by pacing theight phrenic nerve during cryoballoon application to theight PVs, with cessation of ablation if diaphragmatic stim-lation is lost during cryo application.

The phrenic nerve is commonly not well visualized withurrent imaging modalities, including standard CT imagingnd MRI. However, 64-slice multidetector CT (MDCT)echnology provides sufficient temporal and spatial resolu-ion to allow visualization of the phrenic nerve and the

CT volume rendered Images of LA and Bro

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Left Main Bronchus

Trache

Left Bronchi

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igure 7 CT angiogram axial view ofronchial tree, LA, and PVs as seen onxial section. Ao � aorta; Bronch � bron-hus; LS � left superior PV;A � pulmonary artery.

ericardiophrenic bundles as they course around the peri-ardial surface and surrounding structures (Figures 8A andB). Matsumoto et al22 reported on 106 patients undergoingDCT with successful visualization of the right phrenic

erve in 47% and left phrenic nerve in 74% of subjects, asell as the relationship of the phrenic nerves to the LAA,S, and PVs. Of these, the left phrenic common bundle was

een to cross the LAA in 95%, the great cardiac vein in0%, the posterior vein in 49%, and, rarely, the posterior

ial TreeRight Main Bronchus

LSPVLSPV

LIPV Left Bronchi

Figure 6 CT three-dimensional volume-rendered view of bronchial tree, LA, andPVs. A: Anterior-posterior view; B: poste-rior-anterior view with cranial angulation;C: inferior view; D: superior view.

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275Donaldson et al Imaging Structures during Ablation for AF

nd anterior interventricular veins. The right phrenic com-on bundle was visualized less often, owing to variablebrofatty tissue envelopment and contrast opacification.

onclusionhe LA is a complex structure that lies in close proximity

o the esophagus, aorta, circumflex coronary artery, bron-hial tree, and phrenic nerves. Advances in imaging tech-iques and a clear understanding of the anatomic rela-ionships of the LA and PVs to these critical structuresre essential to optimizing the safety and efficacy ofatheter ablation for AF.

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A

L. Phrenic Nerve

igure 8 MDCT of the phrenic nerve.: Volume-rendered three-dimensional

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1. Wu MH, Wongcharoen W, Tsao HM, et al. Close relationship between thebronchi and pulmonary veins: implications for the prevention of atriobronchialfistula after atrial fibrillation ablation. J Cardiovasc Electrophysiol 2007;18:1056–1059.

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AA LSPV

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raphy. Am J Cardiol 2007;100:133–137.