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J Am Coll Cardiol, 2006; 48:2077-2084, doi:10.1016/j.jacc.2006.06.072 (Published online 31 October 2006).
© 2006 by the American College of Cardiology Foundation
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STATE-OF-THE-ART PAPER

Cardiovascular Imaging in the Management of Atrial Fibrillation

Oussama M. Wazni, MD*, Hsuan-Ming Tsao, MD{dagger},{ddagger}, Shih-Ann Chen, MD§, Hsuan-Hung Chuang, MBBS*, Walid Saliba, MD*, Andrea Natale, MD*,* and Allan L. Klein, MD*

* Department of Cardiovascular Medicine, Cleveland Clinic, Cleveland, Ohio
{dagger} Taipei Veterans General Hospital, Taipei, Taiwan
{ddagger} I-Lan Hospital, Taipei, Taiwan
§ The Division of Cardiology and Cardiovascular Research Center, National Yang-Ming University, School of Medicine, Taipei, Taiwan.

Manuscript received March 1, 2006; revised manuscript received May 31, 2006, accepted June 6, 2006.

* Reprint requests and correspondence: Dr. Andrea Natale, Head, Section of Pacing and Electrophysiology, Director, Electrophysiology Laboratory, Co-Director, Center for Atrial Fibrillation, Department of Cardiovascular Medicine, Cleveland Clinic Foundation, Desk F 15, 9500 Euclid Avenue, Cleveland, Ohio 44195. (Email: natalea{at}ccf.org).


   Abstract
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 Abstract
Role of echocardiography in...
 References
 
Atrial fibrillation (AF) is he most commonly encountered arrhythmia in clinical practice, with an overall prevalence of 0.4% in the general population. Recent advances in technology and in the understanding of the pathophysiology of AF have led to more definitive and potentially curative therapeutic approaches. Echocardiography has a well-established role in the assessment of cardiac structure and function and risk stratification, and has become an essential part of the guidelines for management of AF. The development of intracardiac echocardiography has led to real-time guidance of percutaneous interventions, including radiofrequency ablation and left atrial appendage closure procedures for patients with AF. Other imaging modalities, including computed tomography and magnetic resonance angiography, have allowed for more accurate measurement and better understanding of the cardiac anatomy. We review the impact of various imaging modalities in the evaluation and management of AF.

Abbreviations and Acronyms
  AF = atrial fibrillation
  CT = computed tomography
  ICE = intracardiac echocardiography
  LA = left atrium
  LAA = left atrial appendage
  LV = left ventricular
  MDCT = multidetector computed tomography
  MRA = magnetic resonance angiography
  NSR = normal sinus rhythm
  PV = pulmonary vein
  RF = radiofrequency
  SEC = spontaneous echo contrast
  TEE = transesophageal echocardiography

Atrial fibrillation (AF) is the most commonly encountered arrhythmia in clinical practice, with an overall prevalence of 0.4% in the general population (1). Recent advances in technology and in the understanding of the pathophysiology of AF have led to more definitive and potentially curative therapeutic approaches. Echocardiography has a well-established role in the assessment of cardiac structure and function, risk stratification, and has become an essential part of the guidelines for management of AF (2). The development of intracardiac echocardiography has led to real-time guidance of percutaneous interventions, including radiofrequency ablation and left atrial appendage closure procedures for patients with AF. Other imaging modalities, including computed tomography (CT) and magnetic resonance angiography (MRA), have allowed for more accurate measurement and better understanding of the cardiac anatomy. In this work, we review the impact of various imaging modalities in the evaluation and management of AF.


   Role of echocardiography in AF
Top
 Abstract
 Role of echocardiography in...
References
 
Transthoracic echocardiography.   Transthoracic echocardiography (Tables 1 and 2)Go identifies conditions that predispose patients to AF. This information can influence the subsequent management strategies; a structurally normal heart may suggest a triggered mechanism for AF that may be amenable to radiofrequency (RF) ablation whereas the presence of severe mitral stenosis makes long-term maintenance of normal sinus rhythm (NSR) unlikely. Left ventricular (LV) systolic function helps to guide the choice of pharmacologic therapy for rate and rhythm control in chronic AF.


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  		Table 1. Echocardiography in Atrial Fibrillation
 

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  		Table 2. Echocardiographic Predictors of Stroke in Patients With Atrial Fibrillation
 
The size of the left atrium (LA) can be readily assessed by transthoracic echocardiography. Sustained AF can lead to progressive increase in LA size, termed atrial remodeling. This remodeling may be reversed with the restoration of NSR. Diameter measurements of the LA may not be reflective of the true extent of LA enlargement as measured by LA volume (3,4). Increased LA volume is associated with a low probability of successful cardioversion for chronic AF or maintenance of NSR (5–7).

Disordered atrial contractions in AF result in a 20% to 30% reduction in stroke volume and cardiac output, which is further accentuated in those patients with heart disease. The ratio of peak early transmitral velocity (E) to early diastolic annular (Ea) velocity (the E/Ea ratio) has been shown to reliably estimate LV filling pressures in patients with AF (8). Cardioversion may lead to improved LV filling because of synchronized atrial activity and function. Van Gelder et al. (9) found that improvement in ejection fraction and peak oxygen consumption lag behind improvement in atrial function after cardioversion. The improved ventricular function after conversion to NSR leads ultimately to improved exercise tolerance, less fatigue, resolution of dyspnea and chest discomfort, and overall decreased morbidity (9,10).

Atrial mechanical function is difficult to assess in patients with AF. Upon conversion to NSR, one can reassess the velocity and duration of transmitral atrial wave and pulmonary venous atrial reversal using pulsed-wave Doppler, as well as the mitral late diastolic annular velocity from tissue Doppler imaging. During AF, early transmitral velocities are abnormal and reduced. Atrial stunning has been documented with spontaneous and pharmacological cardioversion (11). Depending on the duration of AF and degree of atrial stunning, the peak transmitral atrial wave velocity may remain reduced for up to 4 weeks after successful cardioversion, which is the basis for the anticoagulation guidelines after conversion (12).

In our experience, atrial volume diminishes and mechanical function improves or is unchanged after isolation of the pulmonary vein (PV). On the other hand, procedures targeting the anterior and anterolateral aspects of the LA may result in decreased LA function (13). Recently, LV function has been shown to improve with PV isolation in patients with AF and heart failure (14,15).

Transesophageal echocardiography.   Transesophageal echocardiography (TEE) is the modality of choice for detecting LA or left atrial appendage (LAA) thrombi with a sensitivity and specificity of approximately 95% to 100% (11) (Fig. 1). In patients with pre-existing thrombus, TEE should be used to confirm thrombus resolution before attempts at cardioversion. Varying degrees of blood stasis have been described, ranging from spontaneous echo contrast (SEC) to "sludge." Spontaneous echo contrast, identified as swirling echodensity, reflects increased erythrocyte aggregation, the presence of fibrinogen, and a low flow state and is associated with later development of thrombus and with systemic embolization (8,11). Sludge is a viscid echodensity in the LA or LAA without clear thrombus formation. Sludge represents thrombus in situ that is a stage farther along the continuum toward thrombus formation. A negative correlation between peak LAA emptying velocity and SEC highlights the relation of LAA dysfunction to thrombus formation (16–18). It is important to note that anticoagulation does not influence the presence of SEC because it does not change the underlying hemodynamic abnormalities.


Figure 1
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  		Figure 1 Transesophageal echocardiography images of a laminated left atrial appendage (LAA) thrombus (left) and mobile protruding left atrial appendage thrombus (right). Note the presence of severe spontaneous echo contrast.

 
The finding of thrombus on TEE portends a very poor outcome. In one observational study, it was associated with an embolic risk of up to 10.4% per year and a death risk of 15.8% in a series of patients, most of whom were receiving oral anticoagulation (19). Despite the sensitivity of multiplane TEE being >95%, thrombi <2 mm may still be missed, given the complex morphology of multilobed LAA. The percentage of thrombus resolution after 4 to 6 weeks of anticoagulation has varied widely from 5% to 90% (20–23). It is generally accepted that 20% of thrombus may still be present.

The mechanical function of LAA is best assessed using TEE. Low LAA emptying velocities (<20 cm/s) correlate strongly with the presence of SEC and thrombus formation (24,25), whereas LAA emptying velocities >40 cm/s predict greater likelihood of sustained NSR 1 year after cardioversion (19). In AF without thrombus, LAA appears to passively empty and fill with multiple small fibrillatory contractions that do not contribute to LV filling. There is decreased or no demonstrable flow observed in those with thrombus. In contrast, LAA function during NSR has the normal characteristic pattern of emptying, resulting in a well-defined, pulsed-wave Doppler signal (26,27).

Transesophageal echocardiography-guided cardioversion.   The ACUTE (Assessment of Cardioversion Using Transesophageal Echocardiography) trial (28) compared a TEE-guided strategy combined with short-term anticoagulation using a conventional 3-week oral anticoagulation precardioversion strategy. Although there was a significant difference in the composite end point of major and minor bleeding and a shorter time to cardioversion, there was no difference in the composite end point of stroke, transient ischemic attack, and peripheral embolism. The ACUTE II pilot trial compared TEE-guided cardioversion using low molecular weight heparin with intravenous unfractionated heparin in 155 patients and showed similar safety and efficacy with a lower length of stay and more sinus rhythm in the low molecular weight heparin group (29). Kinnaird et al. (30) also used TEE to evaluate the PVs before and after RF ablation.

Intracardiac echocardiography.   Intracardiac echocardiography (ICE) is an essential tool for safe trans-septal access, identification of anatomical structures relevant to the ablation, placement, navigation of the circular mapping catheter, titrating energy delivery during ablation, and the early diagnosis of complications (31,32).

Traditional fluoroscopic landmarks occasionally might be misleading when there is distortion of the atrial or septal anatomy in conditions such as LA dilation, interatrial septal aneurysm, and previous cardiac surgery, thereby increasing the risk of complications. The use of ICE allows a safer approach for trans-septal puncture and facilitates earlier administration of heparin to prevent clot formation on the trans-septal sheath that can be visualized by ICE. Compared with pulmonary venography, ICE has shown greater accuracy to define the antrum. Correct identification of the anatomy is important to allow for the appropriately sized catheter selection and proper vein treatment due to significant variation, including a common vestibule of the left PVs and additional small branches (33,34).

Intracardiac echocardiography can be used to evaluate catheter movement during lesion delivery without the need for intermittent fluoroscopy and is helpful in monitoring the catheter-tissue interface during energy delivery (35). Conventionally, temperature, power, and impedance are monitored and RF energy discontinued if they are greater than the safety thresholds. Using ICE, we have learned that RF delivery generates microbubbles. By guiding our ablation titrating power output to limit microbubble formation, we have seen improved success rates and reduced risk of complications, including thromboembolic episodes (36). The use of ICE guidance in such a way may also prevent atrioesophageal fistulas.

The esophagus has a variable relationship to the PVs and posterior LA wall. Visualization of this thin-walled structure can be enhanced by the ingestion of a carbonated beverage. Identification of the esophageal relationship can prevent occurrence of atrioesophageal fistulas by either controlling or avoiding energy delivery during ablation in this region (37).

Postablation PV narrowing can be quantified with diameter measurement and Doppler flow measurements to assess for flow turbulence. An increase in flow after ablation is likely a marker of extensive swelling, which could theoretically lead to chronic PV stenosis. However, Saad et al. (38) studied PV flow before and after PV ostial isolation and showed that although there were acute changes in the postablation diastolic flows, they do not appear to be strong predictors for chronic PV stenosis. Intracardiac echocardiography can also be used for intracardiac Doppler assessment of LA contraction and for prediction of success after PV isolation (39,40).

Magnetic resonance angiography and multidetector CT.   Evolving techniques in catheter ablation of AF have led to the expansion of the knowledge of LA anatomy (41–44). Understanding the morphological characteristics of LA in detail can achieve a more efficient and successful ablation and prevent potential complications. Noninvasive imaging modalities, including MRA and multidetector computed tomography (MDCT), can depict the PVs and LA and provide a valuable road map before the catheter ablation of AF (45–51). The advantages of cardiac CT/MRA are: 1) imaging the anatomic characteristics of the PV and LA preprocedurally; 2) assessing the anatomic relationship of the LA, esophagus, and adjacent vascular structures; 3) understanding the morphological remodeling of PV and LA in AF; and 4) detecting postprocedural complications. Examples of variations in LA anatomy are depicted in Figures 2 and 3.Go Figure 2 shows an unusual variant of the LA with a roof pouch, and Figure 3 demonstrates a case of an unusually low LAA. Avoidance of both structures during ablation may be critical in preventing complications.


Figure 2
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  		Figure 2 Left atrial roof pouch in the midsuperior posterior wall of the left atrium.

 

Figure 3
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  		Figure 3 Low left atrial appendage (LAA), posterior view.

 
Anatomy of the PVs.   The interventional therapy of AF has been focused on the interruption of electric conduction by isolating the AF initiators of PVs from LA tissues. The detailed information of PV anatomy and the relationship between PV and LA is mandatory for the mapping and ablation procedures.

The PV ostia are ellipsoid with a longer superior-inferior dimension, and the funnel-shaped ostia are frequently noted in patients in AF (46). The right superior PV is close to the superior vena cava or right atrium, and the right inferior PV projects horizontally. The left superior PV is in close vicinity to the LAA, and the left inferior PV courses near the descending aorta. These observations are essential for the trans-septal procedure, placement of a circular mapping catheter, and the application of energy around or outside the PV ostia.

Morphology patterns of PV trees.   Although the morphologies of PVs have a certain basic pattern, they are more variable than arteries. Variations of PVs can be readily demonstrated by cardiac CT/MRA (Table 3). The variability can substantially influence the success rate of catheter ablation if the variant veins are inadequately treated. Several studies reported the existence of supernumerary right PVs, with the incidence ranging from 18% to 29% (45,48,51–54) (Fig. 4A). Tsao et al. (47) used MRA to demonstrate the PV variant of a discrete right middle PV with an independent orifice rather than the typical 2 PV ostia in the right side. The ectopic focus originating from the right middle PV could initiate AF, which is cured by catheter ablation of right middle PV. In addition, a significantly longer distance between the PV ostium and first branch was demonstrated for left versus right PVs (44). Perez-Lugones el al. (54) showed that multiple ramifications and early branching of the right inferior PV were observed in the study, which might explain the low incidence of firing of the right inferior PV.


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  		Table 3. Anatomic Variations of Pulmonary Veins (PVs)
 

Figure 4
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  		Figure 4 (A) Supernumerary right pulmonary veins (PVs) (arrows). (B) Broad PV-atrial junction. LIPV = left inferior pulmonary vein; LSPV = left superior pulmonary vein.

 
A common trunk of left or right PVs also was imaged using CT/MRA. A common ostium is found more frequently on the left-sided PVs (6% to 35%) and results in a broad PV-atrial junction (Fig. 4B). The common left PV is a consistent origin of arrhythmogenic ectopy (55). Localization of the true PV atrial junction in these patients can be more accurately determined with the assistance of the 3-dimensional images before mapping and ablation procedures.

Anatomic relationship between LA and adjacent structures.   Atrioesophageal fistulas have been reported during intraoperative RF ablation of AF using the endocardial approach, percutaneous PV isolation, and LA ablation (56–59). An atrioesophageal fistula can cause an air embolism with a stroke, mediastinitis, or gastrointestinal bleeding and is associated with a high rate of mortality. Understanding the anatomic relationship of the esophagus and PV/LA provides useful information for avoiding esophageal injury during the catheter ablation. Several studies have demonstrated the close relationship of the posterior LA, coronary sinus, PVs, and esophagus by CT scan (Figs. 5A to 5C) (60–63). Although the peristalsis and dynamic movement of the esophagus was suspected to influence the results, the anatomic parameters of the relationship of the esophagus, PVs, and LA posterior wall are useful in determining the location of the ablation lesions in the LA and to understand the possible risk of esophageal injury.


Figure 5
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  		Figure 5 Computed tomography scan depicting the relationship of the left atrium (LA) to adjacent structures. Note proximity of esophagus to left inferior pulmonary vein in A and to the antrum of the right inferior pulmonary vein (RI) in B. (C) The esophagus (Eso) abuts the coronary sinus (CS). Ao = descending aorta; LI = left inferior pulmonary vein; S = spine.

 
The close proximity of the LA roof and right pulmonary artery was also revealed by CT imaging (Fig. 6A). Although there were no reported cases of injury to the right pulmonary artery, it may be that another structure could be damaged when more powerful energy sources are introduced to make a deep lesion in the LA roof. It is likely that the cooling effect of the rapid blood flow may protect the right pulmonary artery from heat injury. However, to avoid the potential hazard of right pulmonary artery injury, the ablation at the LA roof (especially near the right superior PV orifice where the distance to the right pulmonary artery was the shortest compared to the other parts of the roof) should be performed with care.


Figure 6
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  		Figure 6 Computed tomography scan depicting the relationship of the left atrium to adjacent structures. Note the course of the pulmonary artery (PA) along the roof of the left atrium (A) and of the left circumflex artery (LCX) close to the left atrial appendage (LAA) (B). LS = left superior pulmonary vein; roof = left atrial roof; RS = right superior pulmonary vein.

 
In addition, the very close vicinity between the LAA orifice and proximal left circumflex artery was demonstrated by CT images (Fig. 6B). Takahashi et al. (63) reported a case wherein the left circumflex coronary artery was acutely occluded during ablation within the coronary sinus. Due to energy applications around the LAA that have been recently proposed to increase the success rate in treating persistent AF, ablation near the anterior base of the LAA orifice must be conducted with caution to avoid any risk of injury to the left circumflex artery.

Morphological remodeling of PVs and LA in patients with AF.   The ostial geometries of PVs have been comprehensively evaluated by the use of CT and MRA. Tsao et al. (64,65) first reported the different sizes of PVs among controlled AF, paroxysmal AF, and chronic AF patients by MRA images. Furthermore, the significant dilatation of both superior PVs with simultaneous LA enlargement was demonstrated among patients with paroxysmal AF and chronic AF. After successful ablation of arrhythmogenic PV, the dilated (nonablated) PVs could regress during long-term follow-up (65). Several reports demonstrated the morphological remodeling and reverse remodeling process of LA and PVs in patients in AF (66–68).

Detection of complications after catheter ablation.   The feasibility and safety of catheter ablation of the PVs and LA have been well documented. However, the procedure-related major complications, including cerebral emboli, PV stenosis, and pericardial effusion with tamponade, could occasionally be encountered. The use of MRA and CT can detect PV stenosis after ablation of AF (69–72). Acquired PV stenosis after PV ablation was a major concern when RF energy was applied around or inside the PV ostia. Although a single PV stenosis can be asymptomatic, the severity of clinical symptoms may be related to the numbers and stenotic degree of the involved veins. The use of MDCT and MRA can effectively delineate the lesions and provide the information for justification of treatment (70–72). The ROTEA (Role of TEE in Pulmonary Vein Ablation: Comparison with Computed Tomography) study is an ongoing prospective study assessing the incidence of PV stenosis detected by TEE compared with MDCT.

Conclusions.   Echocardiography continues to be the foundation of clinical management of AF. TEE continues to be the gold standard to exclude LAA thrombus pre-procedurally; however, MDCT is increasingly being used to exclude thrombus and shows excellent negative predictive value (73). However, successful catheter ablation of AF highly depends on understanding the LA and PV anatomy. Advances in imaging technology have improved the quality of ICE and CT/MRA, providing crucial information for electrophysiologists to perform ablations within the LA. Familiarity with the normal and variant patterns of PVs, the important landmarks within the LA, and the topographic relationship of the LA and the surrounding structures is important before the ablation procedure.


   References
Top
 Abstract
 Role of echocardiography in...
 References
 

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H. Calkins, J. Brugada, D. L. Packer, R. Cappato, S.-A. Chen, H. J.G. Crijns, R. J. Damiano Jr, D. W. Davies, D. E. Haines, M. Haissaguerre, et al.
HRS/EHRA/ECAS Expert Consensus Statement on Catheter and Surgical Ablation of Atrial Fibrillation: Recommendations for Personnel, Policy, Procedures and Follow-Up: A report of the Heart Rhythm Society (HRS) Task Force on Catheter and Surgical Ablation of Atrial Fibrillation Developed in partnership with the European Heart Rhythm Association (EHRA) and the European Cardiac Arrhythmia Society (ECAS); in collaboration with the American College of Cardiology (ACC), American Heart Association (AHA), and the Society of Thoracic Surgeons (STS). Endorsed and Approved by the governing bodies of the American College of Cardiology, the American Heart Association, the European Cardiac Arrhythmia Society, the European Heart Rhythm Association, the Society of Thoracic Surgeons, and the Heart Rhythm Society.
Europace, June 1, 2007; 9(6): 335 - 379.
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