Sakis Themistoclakis MD
Director
Atrial Fibrillation Center
E: [email protected]
Aldo Bonso MD
Director, EP labs
Antonio Raviele MD
Head Cardiovascular department
Cardiology Unit
Umberto I Hospital
Venice, Italy
Atrial fibrillation (AF) is the most common cardiac arrhythmia with a prevalence of 2–4% in the general population over 60 years old and an increasing incidence with age. Its presence causes a rise in morbidity and mortality rates due to the increase of embolic risk and loss of atrial function with a consequent decrease in cardiac performance. Very often AF is associated with disabling symptoms that can influence the quality of life significantly. This arrhythmia is also a significant social financial burden. In the US, AF causes more hospital admissions than any other arrhythmia, accounting for a nearly one million hospital days per year.
PV isolation
The clinical importance and financial impact of AF management drives the need for effective treatment. However, pharmacological treatment strategies directed at preventing the recurrence of AF are frequently ineffective or need to be discontinued due to adverse effects. In 1998, Haïssaguerre et al demonstrated that atrial ectopic beats within the pulmonary veins (PVs) were responsible for initiating AF in most patients.(1) For the last eight years the PVs have been increasingly implicated in the genesis of AF, and catheter ablation with PV isolation has been described as an effective curative treatment for AF. Although various techniques have been proposed to reach this goal, there is relative agreement about the end points to be achieved.
In fact, the vast majority of labs performing AF ablation are empirically isolating all four PVs because it was quickly recognised that any of the PVs can serve as a trigger and that isolating only a single PV could unmask triggers in others PVs. Furthermore, most groups are ablating around the “antrum” of the left and right PVs – defined as the funnel-shaped area proximal to the tube-like portion of the PVs. The ablation must be performed around the antra and not at the tubular ostium of the PVs, to encompass as much of the PV structure as possible, improving the efficacy of the procedure and reducing the risk of PV stenosis.(2)
Identifying the PV antrum
Despite efforts to better define the PV antrum and to perform the ablation outside the vein, this target may be difficult to identify with fluoroscopy alone because of the complex 3D relationship of the PVs and the left atrium (LA), and may require integration of data from a variety of tools and technologies. In fact, it has been observed that anatomic variants, such as common ostium of pulmonary veins, supernumerary veins and ostial branching, are very common and detectable with computerised tomography (CT) or magnetic resonance imaging (MRI) in a large percentage of patients.(3,4) In particular, the presence of common left PV is near ubiquitous, and it has been detected in more than 90% of cases.(4) Wood et al compared the PV ostial anatomy defined with MRI, CT, transoesophageal echocardiography, angiography, or fluoroscopy and intracardiac echocardiography (ICE) in 24 patients with AF having radiofrequency catheter ablation. They observed that CT identified the greatest number of PV ostia, followed by ICE. Moreover, the angiography overestimates and the transoesophageal echocardiography underestimates ostial diameters compared with CT or ICE.(5)
During the ablation procedure, the cardiac chambers can be visualised in real time only with angiography, fluoroscopy or ICE – and this last imaging technique seems to be more accurate than the first two in identifying venous structures. Marrouche et al assessed the correlation between PV ostium defined angiographically and the ICE-defined PV ostium in only 15% of PVs, while in 85% of PVs ICE showed that the placement of the circular mapping catheter based on angiography was inaccurate and distal to the true LA–PV junction (5±3mm within the PV).(6)
Furthermore, Schwartzman et al observed that there was complete concordance between CT and phased-array ICE in discerning common and supernumerary veins, as well as ostial branches.(7) ICE imaging allows for a high degree of accuracy with the significant advantage that it is in real time (see Figure 1). However, the images acquired by ICE are 2D, and this technique in clinical practice can be limited in some cases by poor image quality due to unusual patient anatomy (eg, septal hypertrophy) or by the operator expertise. Pappone et al proposed the use of a nonfluoroscopic navigation system (Carto) for the reconstruction in real time of the LA and for the definition of PV osta. It is important to emphasise that the Carto is an electroanatomic mapping system and requires an imaging system to acquire the points in the LA and PVs generally represented by fluoroscopy.
[[HHE06_fig1_Ca4]]
In fact, the PV ostia were defined by fluoroscopic visualisation of the catheter tip entering the cardiac shadow with spontaneous impedance decrease and the appearance of atrial potential. The chamber geometry was reconstructed in real time by interpolation of the acquired points. However, as the same authors described, the definition of the true PV ostia may also be difficult with Carto, and they do not rule out that a common ostium and separate PV branch could be missed in some patients. In their population a common ostium and a separate PV branching were only found in 1–3% of patients, respectively.(8) Therefore, with this technique the anatomic variants of the PVs seem to be underestimated considering their significantly lower percentage compared with the literature data. The additional use of the impedance monitoring does not guarantee a catheter tip position outside the PVs. It has been observed that the impedance difference of 1cm within the PVs and the PV ostia was subtle and not significant. Therefore, a defined cut-off value for defining the PV–LA transition could not be identified.(9)
In our opinion, it is clear that, regardless of the strategy for ablation, the need for accurate, 3D and preferably real-time visualisation of the atrial anatomy during the ablation procedure is important. The last evolution of Carto is heading in the right direction. It incorporates the new imaging integration software, Carto Merge (Biosense Webster), which can accept images from either MRI or CT modalities. The ability to integrate 3D CT into a mapping system and therefore visualise and direct the catheter location offers the potential for greater accuracy of lesion placement around the antra of the PVs. The CT or MR images are imported into the Carto system, and the structure of interest is isolated with a segmentation process of the cardiac images that allows for the separation of the LA and PVs from the surrounding cardiac structures. The LA and PVs are then exported into the Carto system, and this previously acquired image is merged with the Carto map acquired in real time (Figure 2).(10)
[[HHE06_fig2_Ca4]]
This registration process is crucial for accurate image integration. The goal is to have a perfect alignment of the 3D image and the real-time map. The CT or MR images can only be useful during the ablation if they can be accurately overlaid with the electroanatomic mapping, so that real-time catheter positions can be shown in precise relation to the 3D surface. Currently, the registration of the CT/MRI image has been performed using landmark and surface mapping under fluoroscopic guide. The landmark registration involves the 3D orientation of the imported image on X, Y and Z axes. The points are acquired on the Carto system under fluoroscopic guidance, and the corresponding points on the CT are selected and labelled.
When at least three locations have been identified, the system matches the three points on the CT as closely as possible to the three corresponding Carto points. The surface registration aims to further refine the match between the CT and the true LA geometry by creating an electroanatomical map of the LA with points acquired at the left atrial walls, the left atrial appendage, PV ostia and mitral annulus. However, this 3D image creation and the level of precision in the image integration have been evaluated by the software statistical analysis, and it has not been validated with other different real-time imaging techniques. To overcome this problem we have performed the landmark registration under fluoroscopic and ICE guidance. In particular, the points are acquired on the Carto system at each LA–PV junction, accurately identified by ICE, obtaining the ICE view of the catheter position at the lower and upper corner of LA–PV junction and then tagged at the corresponding location on the CT (Figure 2). The correlation between the position of the ablation catheter on 3D CT imaging and its real-time location detected by ICE and fluoroscopy has been compared. When performed this way, the landmark registration allows an accurate match of the electroanatomical map and CT imaging with the corresponding catheter position on CT and on ICE during the ablation catheter manipulations in the LA (Figures 3 and 4). The surface registration acquired by fluoroscopy may result in a misalignment of the electroanatomical map and the CT image, increasing the error of the landmark registration and yielding the position of the catheter on CT inconsistent with that detected by ICE.
[[HHE06_fig3_Ca5]]
[[HHE06_fig4_Ca6]]
Conclusion
Accurate definition of the PV antra is crucial for the AF ablation procedure and may result in increased success and reduced complications. The integration of Carto Merge with ICE allows the accurate definition of the PV antra and simplifies the navigation of the ablation catheter for the PV antrum electrical disconnection. In the future, this image integration could reduce the learning curve for the AF ablation and make this procedure less operator-dependent.
References
- Haïssaguerre M, Jais P, Shah DC, et al. Spontaneous initiation of atrial fibrillation by ectopic beats originating in the pulmonary veins.N Engl J Med 1988;339:659-66.
- Verma A, Marrouche NF, Natale A. Pulmonary vein antrum isolation:intracardiac echocardiography-guided technique. J Cardiovasc Electrophysiol 2004;15:1335-40.
- Jongbloed MR, Dirksen MS, Bax JJ, et al. Atrial fibrillation: multi-detector row CT of pulmonary vein anatomy prior to radiofrequency catheter ablation – initial experience. Radiology 2005;234:702-9.
- Ahmed J, Sohal S, Malchano ZJ, et al. Three-dimensional analysis of pulmonary venous ostial and antral anatomy: implications for balloon catheter-based pulmonary vein isolation. J Cardiovasc Electrophysiol 2006;17:251-5.
- Wood MA, Wittkamp M, Henry D, et al. A comparison of pulmonary vein ostial anatomy by computerized tomography,echocardiography, and venography in patients with atrial fibrillation having radiofrequency catheter ablation. Am J Cardiol 2004;93:49-53.
- Marrouche NF, Martin DO, Wazni O, et al. Phased-array intracardiac echocardiography monitoring during pulmonary vein isolation in patients with atrial fibrillation. Impact on outcome and complications.Circulation 2003;107:2710-6.
- Schwartzman D, Lacomis J, Wigginton WG. Characterization of left atrium and distal pulmonary vein morphology using multidimensional computed tomography. J Am Coll Cardiol 2003;41:1349-57.
- Pappone C, Oreto G, Rosanio S, et al. Atrial electroanatomic remodelling after circumferential radiofrequency pulmonary vein ablation: efficacy of an anatomic approach in a large cohort of patients with atrial fibrillation.Circulation 2001;104:2539-44.
- Vaseghi M, Cesario DA, Valderrabano M, et al. Impedance monitoring during catheter ablation of atrial fibrillation. Heart Rhythm 2005;2:914-20.
- Dong J, Dickfeld T, Dalal D, et al. Initial experience in the use of integrated electroanatomic mapping with three-dimensional MR/CT images to guide catheter ablation of atrial fibrillation. J Cardiovasc Electrophysiol 2006;17:459-66.
