International Journal of Cardiology 179 (2015) 461–464
Contents lists available at ScienceDirect
International Journal of Cardiology journal homepage: www.elsevier.com/locate/ijcard
Editorial
Cardiovascular magnetic resonance imaging before catheter ablation for atrial fibrillation: Much more than left atrial and pulmonary venous anatomy Hirad Yarmohammadi, Chetan Shenoy ⁎ Cardiovascular Division, Department of Medicine, University of Minnesota Medical Center, Minneapolis, MN, USA
a r t i c l e
i n f o
Article history: Received 5 November 2014 Accepted 7 November 2014 Available online 11 November 2014 Keywords: Cardiovascular magnetic resonance imaging Atrial fibrillation Radiofrequency catheter ablation
Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in the United States and it affects between 2.7 million and 6.1 million American adults, and the number is expected to double over the next 25 years [1]. AF ablation including radiofrequency catheter ablation (RFCA) has become an increasingly safe and effective therapy and its use for patients with symptomatic paroxysmal AF refractory to at least one anti-arrhythmic medication is now a Class I recommendation [1]. Pulmonary vein (PV) isolation is the cornerstone for RFCA procedures and a detailed understanding of the left atrial (LA) and PV anatomy is crucial for successful RFCA. For this reason, pre-procedural cardiovascular magnetic resonance (CMR) or cardiac computed tomography (CCT) imaging has been increasingly used [2] and has now become integral to the procedure [3]. The use of pre-procedural CMR or CCT imaging is variable between countries and between individual centers. In a United States Medicare study of 11,525 patients over the age of 65 years, 50% underwent a pre-procedural CMR or CCT [2]. In a recent European survey of 78 centers in 20 countries, pre-procedural CMR or CCT was routinely performed in 42%, occasionally in 40%, and not at all in only 18% of the centers [4]. Pre-procedural imaging has been associated with several benefits for patients undergoing RFCA. CMR image integration with the ⁎ Corresponding author at: Cardiovascular Division, Department of Medicine, University of Minnesota Medical Center, 420 Delaware Street SE, MMC 508, Minneapolis, MN 55455, USA. E-mail address: [email protected] (C. Shenoy).
http://dx.doi.org/10.1016/j.ijcard.2014.11.085 0167-5273/© 2014 Elsevier Ireland Ltd. All rights reserved.
electroanatomical map has been shown to lower radiation exposure during the procedure [5]. Image integration of CCT images has been demonstrated to significantly improve the success of wide area circumferential ablation with confirmed isolation of PVs and result in a lower incidence of PV stenoses [6]. In a multicenter registry study, image integration of CMR or CCT images has also been shown to increase the efficacy of RFCA for paroxysmal AF in terms of lower recurrence [7]. Finally, the use of pre-procedural CMR or CCT has been associated with a significantly lower risk of stroke or transient ischemic attack [2]. In this issue of the Journal, Pontone et al. [8] report a retrospective comparison of CMR and CCT for characterization of LA anatomy prior to AF RFCA. They found that overall procedural characteristics and clinical outcomes were unaffected by the specific imaging modality used for image integration, but the dose of overall radiation was significantly lower in the CMR group as compared to the CCT group. This was essentially due to the lack of radiation with CMR imaging, since procedural radiation was not significantly different between the two groups. The investigators have to be congratulated for performing this first comparison between the two imaging modalities that are, in most institutions, interchangeably used in patients undergoing AF RFCA. A few limitations of the study are worth noting. Patients in the study were not randomized to receive CMR or CCT, and although propensity-score analysis was performed, residual confounding is inevitable. The blanking period was not included in the definition of recurrence of AF — typically, a three-month blanking period is employed after ablation when reporting efficacy outcomes during which time recurrences are not considered as treatment failure [9]. The investigators also did not include or account for the patients' rhythm during the CMR or CCT studies and its impact on LA size measurements. Nevertheless, this study is an important addition to the literature investigating the role of pre-procedural imaging in AF RFCA. While the investigators investigated recurrence of AF and radiation exposure as the outcomes of interest, there are several other important data that can be obtained from CMR in patients undergoing AF RFCA that were not included in the study. CMR with the SSFP cine sequence is capable of assessing the volume, phase characteristics, systolic and diastolic function of the LA [10]. Atrial volume measurements obtained from CMR were found to be very accurate in comparison with actual volumes assessed using cadaveric casts [11]. CMR has also been used to demonstrate a significant decrease in left atrial size after AF RFCA
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Editorial
Fig. 1. Recommended CMR protocol for assessment of patients prior to AF RFA. Outlined is our recommended protocol for clinical assessment of patients prior to AF RFA. The entire study can be performed in under an hour. SSFP — steady state free precession, HASTE — half-Fourier acquisition single-shot turbo spin-echo, ECG — electrocardiogram.
[12,13]. CMR can estimate LA phasic function, including the emptying function before and after AF RFCA. In a study of 33 patients, the LA emptying fraction was reduced by 14% after AF RFCA and the level of reduction correlated with the volume of LA scar [12]. In a recent study of 346 patients, the pre-procedural LA passive emptying fraction was found to be strongly associated with recurrence of AF after RFCA [14]. With an angiogram, CMR performs excellently for the evaluation of the number, size, and shape of the PVs [15]. Thus, CMR can accurately characterize the highly variant PV anatomy. CMR can not only identify PV stenosis — one of the well-known complications of RFCA — but also assess its anatomic as well as physiologic severity with outstanding accuracy prior to a repeat RFCA [15,16]. LA fibrosis can be also estimated by delayed enhancement CMR (DE CMR) and Peters et al. initially reported it in 23 patients who underwent AF RFCA [17]. Following this study, several efforts were made to quantify and categorize the extent of LA fibrosis. Mahnkopf et al. categorized the extent of LA fibrosis into 4 groups, dubbed as the Utah score (Utah I: ≤ 5% LA wall enhancement to Utah IV with ≥ 35% enhancement) [18]. Khurram et al. introduced another method of quantification of LA fibrosis using LA myocardial intensity signals divided by LA blood pool image intensity [19]. Beinart et al. demonstrated yet another method for the measurement of LA fibrosis using LA myocardial T1 relaxation times [20]. Concurrently, investigators, primarily from the University of Utah, described associations between extent of LA fibrosis and procedural outcomes. McGann et al. described lower recurrence of AF at 3 months with increased LA fibrosis post RFCA [21]. Segerson et al. demonstrated an association between post-procedural LA fibrosis in the septal and
posterior wall locations and lower recurrence of AF [22]. Studies from the University of Utah [23,24] were followed by a multicenter, prospective study of 272 patients demonstrating that pre-procedural LA fibrosis estimated by DE CMR was independently associated with likelihood of recurrent AF after RFCA [25]. Recently, DE CMR was also used to identify RFCA lesions and gaps that were utilized for ablation during a second procedure and this approach was found to shorten RF time during the second procedure [26]. CMR also allows for the stratification of stroke risk in AF patients. CMR has been used to identify different LA appendage morphologies (four rather amusing types have been described: cactus, chicken wing, windsock and cauliflower) [27]. The chicken wing morphology has been reported to have the lowest, and the cauliflower type has the highest risk for the incidence of stroke [27]. The identification of these LA appendage features might improve stroke risk stratification in patients with AF prior to RFCA. In addition, a strong association has been described between LA fibrosis detected by DE CMR and the risk of stroke identified by the CHADS2 score [28]. A few studies have investigated the role of CMR for the detection of LA appendage thrombus prior to AF RFCA. Ohyama et al. used noncontrast double- and triple-inversion recovery sequences for the evaluation of LA appendage thrombus in 50 subjects with chronic AF and a history of cardioembolic stroke, and demonstrated that CMR detected all 16 thrombi detected by transesophageal echocardiography (TEE) [29]. Mohrs et al. used perfusion CMR and a 3D DE CMR sequence with an inversion time of 200–350 ms for the detection of LA appendage thrombus in 25 patients (19 with thrombus documented by TEE) and found that CMR was inferior to TEE for the detection of LA appendage
Editorial
thrombus [30]. More recently, Rathi et al. compared 97 patients with AF who underwent pre-procedure CMR prior to RFCA and demonstrated that DE CMR with inversion times of 200–600 ms detected the two cases of LA appendage thrombus that were detected by TEE [31]. Large studies using a DE CMR sequence with a fixed inversion time to selectively null thrombi (600 ms) [32] are warranted to establish that CMR can replace TEE for the routine evaluation of LA appendage thrombus prior to AF RFCA. Beyond the LA, characteristics of the left ventricle (LV) have also been shown to predict success of AF RFCA. Two recent studies have demonstrated that diffuse interstitial fibrosis in the LV estimated by extracellular volume quantification using T1 mapping techniques is associated with recurrence of AF after RFCA [33,34]. Interestingly, pericardial fat measured using CMR has also shown to be associated with the presence of AF, its severity and lower success rates after RFCA [35]. CMR also provides strong prognostic information in AF beyond the success of the RFCA. In 664 patients referred for AF RFCA, delayed enhancement in the LV myocardium was noted to be a frequent finding and a powerful independent predictor of mortality. For each 1% increase in the extent of delayed enhancement, there was 15% increase in risk of death [36]. Comprehensive CMR assessment prior to RF RFCA can be performed in less than an hour. Fig. 1 outlines our recommended protocol for such an evaluation. The excellent blood-to-myocardium contrast and a high signal-to-noise ratio of cine CMR imaging using a steady-state free precession (SSFP) pulse sequence allow for “gold-standard” assessment of LV function and volumes and precise evaluation of LA size and function. Morphologic imaging of the thorax by black blood and bright blood techniques provides the “lay of the land” and helps to set up 3D angiography of the PVs. ECG-gated, contrast-enhanced, 3D angiography allows multiplanar visualization of the PV anatomy and measurement of PV dimensions in true cross-sections. DE CMR using a fixed inversion time of 600 ms to selectively null thrombi and standard DE CMR with an inversion time of 300–40 ms to detect LV fibrosis are also routinely recommended. Velocity-encoded phase contrast imaging could be used in cases of known or suspected PV stenosis for assessment of the severity by peak velocity measurements. High-resolution 3D DE CMR could be performed if evaluation of LA fibrosis is desired. With the wealth of diagnostic, integrative and prognostic data that could be obtained without exposure to radiation, the choice of CMR as the preferred imaging modality for patients prior to AF RFCA should be a no-brainer.
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[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
Conflict of interest
[18]
The authors report no relationships that could be construed as a conflict of interest.
[19]
References
[20]
[1] C.T. January, L.S. Wann, J.S. Alpert, H. Calkins, J.C. Cleveland Jr., J.E. Cigarroa, et al., AHA/ACC/HRS guideline for the management of patients with atrial fibrillation: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines and the Heart Rhythm Society, Circulation 2014 (2014). [2] B.A. Steinberg, B.G. Hammill, J.P. Daubert, T.D. Bahnson, P.S. Douglas, L.G. Qualls, et al., Periprocedural imaging and outcomes after catheter ablation of atrial fibrillation, Heart 100 (2014) 1871–1877. [3] C. Blomstrom Lundqvist, A. Auricchio, J. Brugada, G. Boriani, J. Bremerich, J.A. Cabrera, et al., The use of imaging for electrophysiological and devices procedures: a report from the first European Heart Rhythm Association Policy Conference, jointly organized with the European Association of Cardiovascular Imaging (EACVI), the Council of Cardiovascular Imaging and the European Society of Cardiac Radiology, Europace 15 (2013) 927–936. [4] J. Chen, D.M. Todd, M. Hocini, T.B. Larsen, M.G. Bongiorni, C. Blomstrom-Lundqvist, et al., Current periprocedural management of ablation for atrial fibrillation in Europe: results of the European Heart Rhythm Association survey, Europace 16 (2014) 378–381. [5] D. Caponi, A. Corleto, M. Scaglione, A. Blandino, L. Biasco, Y. Cristoforetti, et al., Ablation of atrial fibrillation: does the addition of three-dimensional magnetic resonance imaging of the left atrium to electroanatomic mapping improve the clinical
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outcome? A randomized comparison of Carto-Merge vs. Carto-XP three-dimensional mapping ablation in patients with paroxysmal and persistent atrial fibrillation, Europace 12 (2010) 1098–1104. M. Martinek, H.J. Nesser, J. Aichinger, G. Boehm, H. Purerfellner, Impact of integration of multislice computed tomography imaging into three-dimensional electroanatomic mapping on clinical outcomes, safety, and efficacy using radiofrequency ablation for atrial fibrillation, Pacing Clin. Electrophysiol. 30 (2007) 1215–1223. E. Bertaglia, P.D. Bella, C. Tondo, A. Proclemer, N. Bottoni, R. De Ponti, et al., Image integration increases efficacy of paroxysmal atrial fibrillation catheter ablation: results from the CartoMerge Italian Registry, Europace 11 (2009) 1004–1010. G. Pontone, D. Andreini, E. Bertella, M. Petullà, E. Russo, E. Innocenti, et al., Comparison of cardiac computed tomography versus cardiac magnetic resonance for characterization of left atrium anatomy before radiofrequency catheter ablation of atrial fibrillation, Int. J. Cardiol. (2014). H. Calkins, K.H. Kuck, R. Cappato, J. Brugada, A.J. Camm, S.A. Chen, et al., HRS/EHRA/ECAS expert consensus statement on catheter and surgical ablation of atrial fibrillation: recommendations for patient selection, procedural techniques, patient management and follow-up, definitions, endpoints, and research trial design: 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), a registered branch of the European Society of Cardiology (ESC) and the European Cardiac Arrhythmia Society (ECAS); and in collaboration with the American College of Cardiology (ACC), American Heart Association (AHA), the Asia Pacific Heart Rhythm Society (APHRS), and the Society of Thoracic Surgeons (STS). Endorsed by the governing bodies of the American College of Cardiology Foundation, the American Heart Association, the European Cardiac Arrhythmia Society, the European Heart Rhythm Association, the Society of Thoracic Surgeons, the Asia Pacific Heart Rhythm Society, and the Heart Rhythm Society, Heart Rhythm 9 (632–96) (2012) e21. W.Y. Tseng, T.Y. Liao, J.L. Wang, Normal systolic and diastolic functions of the left ventricle and left atrium by cine magnetic resonance imaging, J. Cardiovasc. Magn. Reson. 4 (2002) 443–457. V.M. Jarvinen, M.M. Kupari, P.E. Hekali, V.P. Poutanen, Right atrial MR imaging studies of cadaveric atrial casts and comparison with right and left atrial volumes and function in healthy subjects, Radiology 191 (1994) 137–142. J.V. Wylie Jr., D.C. Peters, V. Essebag, W.J. Manning, M.E. Josephson, T.H. Hauser, Left atrial function and scar after catheter ablation of atrial fibrillation, Heart Rhythm 5 (2008) 656–662. I.E. Hof, B.K. Velthuis, S.M. Chaldoupi, F.H. Wittkampf, V.J. van Driel, J.F. van der Heijden, et al., Pulmonary vein antrum isolation leads to a significant decrease of left atrial size, Europace 13 (2011) 371–375. J.A. Dodson, T.G. Neilan, R.V. Shah, H. Farhad, R. Blankstein, M. Steigner, et al., Left atrial passive emptying function determined by cardiac magnetic resonance predicts atrial fibrillation recurrence after pulmonary vein isolation, Circ. Cardiovasc. Imaging 7 (2014) 586–592. R. Kato, L. Lickfett, G. Meininger, T. Dickfeld, R. Wu, G. Juang, et al., Pulmonary vein anatomy in patients undergoing catheter ablation of atrial fibrillation: lessons learned by use of magnetic resonance imaging, Circulation 107 (2003) 2004–2010. T. Dill, T. Neumann, O. Ekinci, C. Breidenbach, A. John, A. Erdogan, et al., Pulmonary vein diameter reduction after radiofrequency catheter ablation for paroxysmal atrial fibrillation evaluated by contrast-enhanced three-dimensional magnetic resonance imaging, Circulation 107 (2003) 845–850. D.C. Peters, J.V. Wylie, T.H. Hauser, K.V. Kissinger, R.M. Botnar, V. Essebag, et al., Detection of pulmonary vein and left atrial scar after catheter ablation with threedimensional navigator-gated delayed enhancement MR imaging: initial experience, Radiology 243 (2007) 690–695. C. Mahnkopf, T.J. Badger, N.S. Burgon, M. Daccarett, T.S. Haslam, C.T. Badger, et al., Evaluation of the left atrial substrate in patients with lone atrial fibrillation using delayed-enhanced MRI: implications for disease progression and response to catheter ablation, Heart Rhythm 7 (2010) 1475–1481. I.M. Khurram, R. Beinart, V. Zipunnikov, J. Dewire, H. Yarmohammadi, T. Sasaki, et al., Magnetic resonance image intensity ratio, a normalized measure to enable interpatient comparability of left atrial fibrosis, Heart Rhythm 11 (2014) 85–92. R. Beinart, I.M. Khurram, S. Liu, H. Yarmohammadi, H.R. Halperin, D.A. Bluemke, et al., Cardiac magnetic resonance T1 mapping of left atrial myocardium, Heart Rhythm 10 (2013) 1325–1331. C.J. McGann, E.G. Kholmovski, R.S. Oakes, J.J. Blauer, M. Daccarett, N. Segerson, et al., New magnetic resonance imaging-based method for defining the extent of left atrial wall injury after the ablation of atrial fibrillation, J. Am. Coll. Cardiol. 52 (2008) 1263–1271. N.M. Segerson, M. Daccarett, T.J. Badger, A. Shabaan, N. Akoum, E.N. Fish, et al., Magnetic resonance imaging-confirmed ablative debulking of the left atrial posterior wall and septum for treatment of persistent atrial fibrillation: rationale and initial experience, J. Cardiovasc. Electrophysiol. 21 (2010) 126–132. R.S. Oakes, T.J. Badger, E.G. Kholmovski, N. Akoum, N.S. Burgon, E.N. Fish, et al., Detection and quantification of left atrial structural remodeling with delayedenhancement magnetic resonance imaging in patients with atrial fibrillation, Circulation 119 (2009) 1758–1767. N. Akoum, M. Daccarett, C. McGann, N. Segerson, G. Vergara, S. Kuppahally, et al., Atrial fibrosis helps select the appropriate patient and strategy in catheter ablation of atrial fibrillation: a DE-MRI guided approach, J. Cardiovasc. Electrophysiol. 22 (2011) 16–22. N.F. Marrouche, D. Wilber, G. Hindricks, P. Jais, N. Akoum, F. Marchlinski, et al., Association of atrial tissue fibrosis identified by delayed enhancement MRI and atrial fibrillation catheter ablation: the DECAAF study, JAMA 311 (2014) 498–506.
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[26] F. Bisbal, E. Guiu, P. Cabanas-Grandio, A. Berruezo, S. Prat-Gonzalez, B. Vidal, et al., CMR-guided approach to localize and ablate gaps in repeat AF ablation procedure, J. Am. Coll. Cardiol. Img. 7 (2014) 653–663. [27] L. Di Biase, P. Santangeli, M. Anselmino, P. Mohanty, I. Salvetti, S. Gili, et al., Does the left atrial appendage morphology correlate with the risk of stroke in patients with atrial fibrillation? Results from a multicenter study, J. Am. Coll. Cardiol. 60 (2012) 531–538. [28] M. Daccarett, T.J. Badger, N. Akoum, N.S. Burgon, C. Mahnkopf, G. Vergara, et al., Association of left atrial fibrosis detected by delayed-enhancement magnetic resonance imaging and the risk of stroke in patients with atrial fibrillation, J. Am. Coll. Cardiol. 57 (2011) 831–838. [29] H. Ohyama, N. Hosomi, T. Takahashi, K. Mizushige, K. Osaka, M. Kohno, et al., Comparison of magnetic resonance imaging and transesophageal echocardiography in detection of thrombus in the left atrial appendage, Stroke 34 (2003) 2436–2439. [30] O.K. Mohrs, B. Nowak, S.E. Petersen, M. Welsner, C. Rubel, A. Magedanz, et al., Thrombus detection in the left atrial appendage using contrast-enhanced MRI: a pilot study, AJR Am. J. Roentgenol. 186 (2006) 198–205. [31] V.K. Rathi, S.T. Reddy, S. Anreddy, W. Belden, J.A. Yamrozik, R.B. Williams, et al., Contrast-enhanced CMR is equally effective as TEE in the evaluation of left atrial appendage thrombus in patients with atrial fibrillation undergoing pulmonary vein isolation procedure, Heart Rhythm 10 (2013) 1021–1027.
[32] J.W. Weinsaft, H.W. Kim, A.L. Crowley, I. Klem, C. Shenoy, L. Van Assche, et al., LV thrombus detection by routine echocardiography: insights into performance characteristics using delayed enhancement CMR, J. Am. Coll. Cardiol. Img. 4 (2011) 702–712. [33] A.J. McLellan, L.H. Ling, S. Azzopardi, A.H. Ellims, L.M. Iles, M.A. Sellenger, et al., Diffuse ventricular fibrosis measured by t1 mapping on cardiac MRI predicts success of catheter ablation for atrial fibrillation, Circ. Arrhythmia Electrophysiol. 7 (2014) 834–840. [34] T.G. Neilan, F.P. Mongeon, R.V. Shah, O. Coelho-Filho, S.A. Abbasi, J.A. Dodson, et al., Myocardial extracellular volume expansion and the risk of recurrent atrial fibrillation after pulmonary vein isolation, J. Am. Coll. Cardiol. Img. 7 (2014) 1–11. [35] C.X. Wong, H.S. Abed, P. Molaee, A.J. Nelson, A.G. Brooks, G. Sharma, et al., Pericardial fat is associated with atrial fibrillation severity and ablation outcome, J. Am. Coll. Cardiol. 57 (2011) 1745–1751. [36] T.G. Neilan, R.V. Shah, S.A. Abbasi, H. Farhad, J.D. Groarke, J.A. Dodson, et al., The incidence, pattern, and prognostic value of left ventricular myocardial scar by late gadolinium enhancement in patients with atrial fibrillation, J. Am. Coll. Cardiol. 62 (2013) 2205–2214.
Contents lists available at ScienceDirect
International Journal of Cardiology journal homepage: www.elsevier.com/locate/ijcard
Editorial
Cardiovascular magnetic resonance imaging before catheter ablation for atrial fibrillation: Much more than left atrial and pulmonary venous anatomy Hirad Yarmohammadi, Chetan Shenoy ⁎ Cardiovascular Division, Department of Medicine, University of Minnesota Medical Center, Minneapolis, MN, USA
a r t i c l e
i n f o
Article history: Received 5 November 2014 Accepted 7 November 2014 Available online 11 November 2014 Keywords: Cardiovascular magnetic resonance imaging Atrial fibrillation Radiofrequency catheter ablation
Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in the United States and it affects between 2.7 million and 6.1 million American adults, and the number is expected to double over the next 25 years [1]. AF ablation including radiofrequency catheter ablation (RFCA) has become an increasingly safe and effective therapy and its use for patients with symptomatic paroxysmal AF refractory to at least one anti-arrhythmic medication is now a Class I recommendation [1]. Pulmonary vein (PV) isolation is the cornerstone for RFCA procedures and a detailed understanding of the left atrial (LA) and PV anatomy is crucial for successful RFCA. For this reason, pre-procedural cardiovascular magnetic resonance (CMR) or cardiac computed tomography (CCT) imaging has been increasingly used [2] and has now become integral to the procedure [3]. The use of pre-procedural CMR or CCT imaging is variable between countries and between individual centers. In a United States Medicare study of 11,525 patients over the age of 65 years, 50% underwent a pre-procedural CMR or CCT [2]. In a recent European survey of 78 centers in 20 countries, pre-procedural CMR or CCT was routinely performed in 42%, occasionally in 40%, and not at all in only 18% of the centers [4]. Pre-procedural imaging has been associated with several benefits for patients undergoing RFCA. CMR image integration with the ⁎ Corresponding author at: Cardiovascular Division, Department of Medicine, University of Minnesota Medical Center, 420 Delaware Street SE, MMC 508, Minneapolis, MN 55455, USA. E-mail address: [email protected] (C. Shenoy).
http://dx.doi.org/10.1016/j.ijcard.2014.11.085 0167-5273/© 2014 Elsevier Ireland Ltd. All rights reserved.
electroanatomical map has been shown to lower radiation exposure during the procedure [5]. Image integration of CCT images has been demonstrated to significantly improve the success of wide area circumferential ablation with confirmed isolation of PVs and result in a lower incidence of PV stenoses [6]. In a multicenter registry study, image integration of CMR or CCT images has also been shown to increase the efficacy of RFCA for paroxysmal AF in terms of lower recurrence [7]. Finally, the use of pre-procedural CMR or CCT has been associated with a significantly lower risk of stroke or transient ischemic attack [2]. In this issue of the Journal, Pontone et al. [8] report a retrospective comparison of CMR and CCT for characterization of LA anatomy prior to AF RFCA. They found that overall procedural characteristics and clinical outcomes were unaffected by the specific imaging modality used for image integration, but the dose of overall radiation was significantly lower in the CMR group as compared to the CCT group. This was essentially due to the lack of radiation with CMR imaging, since procedural radiation was not significantly different between the two groups. The investigators have to be congratulated for performing this first comparison between the two imaging modalities that are, in most institutions, interchangeably used in patients undergoing AF RFCA. A few limitations of the study are worth noting. Patients in the study were not randomized to receive CMR or CCT, and although propensity-score analysis was performed, residual confounding is inevitable. The blanking period was not included in the definition of recurrence of AF — typically, a three-month blanking period is employed after ablation when reporting efficacy outcomes during which time recurrences are not considered as treatment failure [9]. The investigators also did not include or account for the patients' rhythm during the CMR or CCT studies and its impact on LA size measurements. Nevertheless, this study is an important addition to the literature investigating the role of pre-procedural imaging in AF RFCA. While the investigators investigated recurrence of AF and radiation exposure as the outcomes of interest, there are several other important data that can be obtained from CMR in patients undergoing AF RFCA that were not included in the study. CMR with the SSFP cine sequence is capable of assessing the volume, phase characteristics, systolic and diastolic function of the LA [10]. Atrial volume measurements obtained from CMR were found to be very accurate in comparison with actual volumes assessed using cadaveric casts [11]. CMR has also been used to demonstrate a significant decrease in left atrial size after AF RFCA
462
Editorial
Fig. 1. Recommended CMR protocol for assessment of patients prior to AF RFA. Outlined is our recommended protocol for clinical assessment of patients prior to AF RFA. The entire study can be performed in under an hour. SSFP — steady state free precession, HASTE — half-Fourier acquisition single-shot turbo spin-echo, ECG — electrocardiogram.
[12,13]. CMR can estimate LA phasic function, including the emptying function before and after AF RFCA. In a study of 33 patients, the LA emptying fraction was reduced by 14% after AF RFCA and the level of reduction correlated with the volume of LA scar [12]. In a recent study of 346 patients, the pre-procedural LA passive emptying fraction was found to be strongly associated with recurrence of AF after RFCA [14]. With an angiogram, CMR performs excellently for the evaluation of the number, size, and shape of the PVs [15]. Thus, CMR can accurately characterize the highly variant PV anatomy. CMR can not only identify PV stenosis — one of the well-known complications of RFCA — but also assess its anatomic as well as physiologic severity with outstanding accuracy prior to a repeat RFCA [15,16]. LA fibrosis can be also estimated by delayed enhancement CMR (DE CMR) and Peters et al. initially reported it in 23 patients who underwent AF RFCA [17]. Following this study, several efforts were made to quantify and categorize the extent of LA fibrosis. Mahnkopf et al. categorized the extent of LA fibrosis into 4 groups, dubbed as the Utah score (Utah I: ≤ 5% LA wall enhancement to Utah IV with ≥ 35% enhancement) [18]. Khurram et al. introduced another method of quantification of LA fibrosis using LA myocardial intensity signals divided by LA blood pool image intensity [19]. Beinart et al. demonstrated yet another method for the measurement of LA fibrosis using LA myocardial T1 relaxation times [20]. Concurrently, investigators, primarily from the University of Utah, described associations between extent of LA fibrosis and procedural outcomes. McGann et al. described lower recurrence of AF at 3 months with increased LA fibrosis post RFCA [21]. Segerson et al. demonstrated an association between post-procedural LA fibrosis in the septal and
posterior wall locations and lower recurrence of AF [22]. Studies from the University of Utah [23,24] were followed by a multicenter, prospective study of 272 patients demonstrating that pre-procedural LA fibrosis estimated by DE CMR was independently associated with likelihood of recurrent AF after RFCA [25]. Recently, DE CMR was also used to identify RFCA lesions and gaps that were utilized for ablation during a second procedure and this approach was found to shorten RF time during the second procedure [26]. CMR also allows for the stratification of stroke risk in AF patients. CMR has been used to identify different LA appendage morphologies (four rather amusing types have been described: cactus, chicken wing, windsock and cauliflower) [27]. The chicken wing morphology has been reported to have the lowest, and the cauliflower type has the highest risk for the incidence of stroke [27]. The identification of these LA appendage features might improve stroke risk stratification in patients with AF prior to RFCA. In addition, a strong association has been described between LA fibrosis detected by DE CMR and the risk of stroke identified by the CHADS2 score [28]. A few studies have investigated the role of CMR for the detection of LA appendage thrombus prior to AF RFCA. Ohyama et al. used noncontrast double- and triple-inversion recovery sequences for the evaluation of LA appendage thrombus in 50 subjects with chronic AF and a history of cardioembolic stroke, and demonstrated that CMR detected all 16 thrombi detected by transesophageal echocardiography (TEE) [29]. Mohrs et al. used perfusion CMR and a 3D DE CMR sequence with an inversion time of 200–350 ms for the detection of LA appendage thrombus in 25 patients (19 with thrombus documented by TEE) and found that CMR was inferior to TEE for the detection of LA appendage
Editorial
thrombus [30]. More recently, Rathi et al. compared 97 patients with AF who underwent pre-procedure CMR prior to RFCA and demonstrated that DE CMR with inversion times of 200–600 ms detected the two cases of LA appendage thrombus that were detected by TEE [31]. Large studies using a DE CMR sequence with a fixed inversion time to selectively null thrombi (600 ms) [32] are warranted to establish that CMR can replace TEE for the routine evaluation of LA appendage thrombus prior to AF RFCA. Beyond the LA, characteristics of the left ventricle (LV) have also been shown to predict success of AF RFCA. Two recent studies have demonstrated that diffuse interstitial fibrosis in the LV estimated by extracellular volume quantification using T1 mapping techniques is associated with recurrence of AF after RFCA [33,34]. Interestingly, pericardial fat measured using CMR has also shown to be associated with the presence of AF, its severity and lower success rates after RFCA [35]. CMR also provides strong prognostic information in AF beyond the success of the RFCA. In 664 patients referred for AF RFCA, delayed enhancement in the LV myocardium was noted to be a frequent finding and a powerful independent predictor of mortality. For each 1% increase in the extent of delayed enhancement, there was 15% increase in risk of death [36]. Comprehensive CMR assessment prior to RF RFCA can be performed in less than an hour. Fig. 1 outlines our recommended protocol for such an evaluation. The excellent blood-to-myocardium contrast and a high signal-to-noise ratio of cine CMR imaging using a steady-state free precession (SSFP) pulse sequence allow for “gold-standard” assessment of LV function and volumes and precise evaluation of LA size and function. Morphologic imaging of the thorax by black blood and bright blood techniques provides the “lay of the land” and helps to set up 3D angiography of the PVs. ECG-gated, contrast-enhanced, 3D angiography allows multiplanar visualization of the PV anatomy and measurement of PV dimensions in true cross-sections. DE CMR using a fixed inversion time of 600 ms to selectively null thrombi and standard DE CMR with an inversion time of 300–40 ms to detect LV fibrosis are also routinely recommended. Velocity-encoded phase contrast imaging could be used in cases of known or suspected PV stenosis for assessment of the severity by peak velocity measurements. High-resolution 3D DE CMR could be performed if evaluation of LA fibrosis is desired. With the wealth of diagnostic, integrative and prognostic data that could be obtained without exposure to radiation, the choice of CMR as the preferred imaging modality for patients prior to AF RFCA should be a no-brainer.
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Conflict of interest
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The authors report no relationships that could be construed as a conflict of interest.
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