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PERIOPERATIVE CROSS-SECTIONAL IMAGING |
1 Mallinckrodt Institute of Radiology, Washington, University School of Medicine, St Louis, Missouri
Atrial fibrillation is the most common arrhythmia and is present in up to 5% of patients aged 65 years and over (1). Percutaneous catheter ablation, a method of transcatheter RF ablation or cryoablation of arrythmogenic foci, has recently developed into a commonly performed treatment for atrial fibrillation. First described in 1994 (2), percutaneous catheter ablation for atrial fibrillation has rapidly become a minimally invasive alternative to the maze procedure (the surgical method of interrupting electrical pathways by creation of left atrial scar tissue) and a more successful alternative to cardioversion. Thus, it is anticipated that the radiologist in practice will encounter requests to assess pulmonary venous anatomy prior to the procedure or be called on to perform CT or MR imaging to assess a patient for possible postprocedure complications.
In the preceding two articles, Ghaye et al (3) and Lacomis et al (4) provide comprehensive reviews on the role of the radiologist in percutaneous catheter ablation for atrial fibrillation. Lacomis et al (4) focus in detail on RFCA, whereas Ghaye et al (3) also discuss cryoablation. Both provide the radiologist with the tools required to assist the clinician in preprocedure planning and follow-up.
Of principal importance in preprocedure planning for percutaneous catheter ablation is accurate identification of the ostia of the main PVs. This is especially helpful to the referring cardiologist, as he or she will need to selectively cannulate and electrically map each vessel ostium to decide where to perform ablation. As indicated by Ghaye et al (3), model anatomy is found in only 70% of cases, with the remaining 30% of individuals having pulmonary venous anatomy variation related to under- or overincorporation of the common PV into the left atrium during embryologic development. Since percutaneous catheter ablation is performed under fluoroscopy, Lacomis et al (4) discuss the length of the procedure and prolonged radiation exposure as one of the disadvantages to percutaneous catheter ablation. Detailed knowledge about a specific patient’s pulmonary venous anatomy prior to the procedure may shorten the length of the procedure and decrease the fluoroscopy time that would otherwise be required to identify each vessel. In addition, a "road map" may provide a more favorable outcome, preventing additional smaller ostia from being missed as potential sources of arrhythmia. Also of importance is the size of the ostia. This is true not only for selection of catheter size but also for preprocedure identification of accessory veins with small ostia, which are more likely to develop stenoses.
Radiologists need to be aware of postablation complications. They need to know of these complications in order to identify them when specifically asked to perform CT or MR imaging after ablation; however, they also need to be aware of postablation complications as possible causes of nonspecific signs, symptoms, and radiologic findings when the history provided is less clear. In this way, the complication will not be missed. The most common complication of percutaneous catheter ablation is PV stenosis. In one study, 44% of patients with PV stenosis after catheter ablation presented with shortness of breath, cough, or hemoptysis (5). Pulmonary stenoses may develop up to 8 months after percutaneous catheter ablation, with patients presenting to their internist or to the emergency department some months after the procedure. In one study, nonspecific respiratory symptoms along with imaging findings initially led to erroneous radiologic diagnoses of pneumonia, pulmonary embolism, and lung cancer (5). As mentioned by Ghaye et al (3), chest radiography may demonstrate focal pulmonary edema in both PV stenosis and thrombosis, and this could be interpreted as pneumonitis. Also, ventilation-perfusion scanning in both entities may demonstrate a ventilation-perfusion mismatch, mimicking pulmonary embolism. Thus, it is important for the radiologist to consider complications of percutaneous catheter ablation in order to seek the appropriate clinical history when interpreting the images.
The newest technologies, multi–detector row contrast-enhanced spiral CT and fast MR angiography, allow noninvasive, high-resolution, 3D imaging of the PVs and left atrium. There are few noninvasive alternatives to these two techniques. Transesophageal echocardiography can be used but is moderately operator dependent.
Some cardiologists perform percutaneous catheter ablation without an initial imaging examination. The arguments against a preprocedure imaging study mainly arise from those who believe that a significant number of arrhythmogenic foci in atrial fibrillation are not isolated to the PVs. Other individuals believe that since there is no optimal treatment for PV stenosis, preprocedure imaging is not warranted. Nevertheless, greater numbers of cardiologists are choosing to perform CT, MR imaging, or both to provide a road map and a baseline. Proponents of pre–percutaneous catheter ablation CT at our institution have argued that, besides providing information about pulmonary venous anatomy, the images can serve as a reference for comparison at follow-up should symptoms later arise. One reason for this is that some normal PVs have regions of narrowing. A preprocedure imaging study allows one to determine whether the pulmonary venous narrowing identified is normal for that patient or iatrogenic.
Both Ghaye et al (3) and Lacomis et al (4) thoroughly discuss imaging techniques, with Lacomis et al providing additional information on methods of image postprocessing. These quick and noninvasive methods, which are now so readily available, along with the appropriate knowledge as provided by these two sets of authors, place the radiologist in an ideal situation to assist with pre–percutaneous ablation planning and follow-up in patients with atrial fibrillation.
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Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania
The authors thank Dr Woodard for her insight and comments. As we imaged increased numbers of atrial fibrillation patients, we were initially surprised by the large number of anatomic variations in pulmonary venous anatomy. We agree that knowledge of these variations, in particular the presence of accessory veins that can drain a lobe or segment of a lobe or even cross fissures from one lobe to an adjacent lobe, are important in planning RFCA and understanding the appearance of complications when they occur. Knowledge of these specific anatomic variations is certainly important to accurately evaluate post-RFCA complications, since the area of radiologic abnormality should correlate with the anatomic location of the lung tissue drained by the stenosed vein.
Since the symptoms of post-RFCA pulmonary venous stenosis are so common and nonspecific, screening for a history of RFCA and the importance of providing radiologists with this necessary clinical information prior to imaging cannot be overstated. RFCA can be technically challenging even for the most experienced electrophysiologists. As the number of less experienced electrophysiologists performing ablation procedures increases, the probability of radiologists encountering a chest imaging abnormality that may represent an RFCA complication will also likely increase. It is necessary for radiologists to understand RFCA and its complications to avoid misinterpreting the imaging findings in these patients.
However, in our opinion, the major role of pre-RFCA CT in atrial fibrillation patients is not only to define and understand the anatomy for cases of post-RFCA complications but also to define the anatomic road map for the procedure itself. Radiologists need to know how RFCA is performed at their own institutions since electrophysiologists perform these procedures differently. In addition, as RFCA continues to evolve, radiologists need to be aware of the changing ablation techniques and how these changes may affect the specific imaging requirements of their electrophysiologists.
Radiologists are now becoming more involved in cardiac imaging. The success of radiology participation is dependent on two factors: communication and cooperation between the referring cardiologists and the radiologists. This requires education of not only radiologists and radiology residents but also our technologists (particularly CT and MR imaging technologists) so that they understand the cardiac anatomy and physiology pertinent to the cardiac diseases that we may be asked to image.
We have now imaged approximately 200 pre-RFCA patients and have seen both the imaging and postprocessing techniques improve as the scanners and workstations have evolved. (Fig 13 in our article is a beautiful example of the anatomic quality of images we’re capable of generating with electrocardiographic gating and a 16–detector row scanner.) Our electrophysiologists rely on us for performing these examinations and postprocessing the images on the same day, so that they will have completed image sets for the procedure the next morning.
More important, we have developed a close working relationship with our electrophysiologists, even a partnership in some respects, and our electrophysiology department has been one of the chest imaging division’s strongest advocates to other cardiologists for use of cardiac CT and MR imaging. This is affording our division new opportunities for other collaborative cardiac imaging. The success of our imaging will require continued communication, cooperation, and education among radiologists and cardiologists to ensure that optimal cardiovascular imaging will be achieved.
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