Perceiving the Imperceptible in Atrial Macro-Reentry

HomeCirculation: Arrhythmia and ElectrophysiologyVol. 10, No. 1Perceiving the Imperceptible in Atrial Macro-Reentry Free AccessEditorialPDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissions ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toFree AccessEditorialPDF/EPUBPerceiving the Imperceptible in Atrial Macro-ReentryUltrahigh Resolution Mapping to Characterize the Critical Isthmus Bhupesh Pathik, MBBS and Jonathan M. Kalman, MBBS, PhD Bhupesh PathikBhupesh Pathik From the Department of Cardiology, Royal Melbourne Hospital & Faculty of Medicine, Dentistry, and Health Sciences, University of Melbourne, Australia. and Jonathan M. KalmanJonathan M. Kalman From the Department of Cardiology, Royal Melbourne Hospital & Faculty of Medicine, Dentistry, and Health Sciences, University of Melbourne, Australia. Originally published30 Dec 2016https://doi.org/10.1161/CIRCEP.116.004850Circulation: Arrhythmia and Electrophysiology. 2017;10:e004850Mapping macro-reentrant atrial tachycardias occurring late after prior atrial fibrillation ablation or atrial surgery requires identification of the critical isthmus necessary for maintaining the circuit.1 The current most widely accepted approach is a combination of entrainment and 3-dimensional (3D) electroanatomic mapping with insights into atrial anatomy. 3D electroanatomic mapping systems have been shown to be important in delineating complex circuits, particularly in the setting of abnormal atrial anatomy, multiple circuits, and regions of scar.2–6 However, entrainment mapping has remained central to determining which parts of the atria are directly involved in the flutter circuit versus those that are activated passively.7,8See Article by Latcu et alThe recent advent of an ultrahigh resolution 3D electroanatomic mapping system (Rhythmia, Boston Scientific) provides a level of detail not previously possible9 and may lead to a rethinking of the standard approaches to arrhythmia management. The Rhythmia mapping system allows real-time automated electrogram annotation and the rapid acquisition of thousands of activation points using the IntellaMap Orion (Boston Scientific) mapping catheter, which has 64 flat microelectrodes (0.8 mm diameter) with a 2.5 mm interelectrode distance (center-to-center) arranged in a basket configuration of 8 splines. Multiple feasibility studies from different centers have shown this mapping system to be safe and accurate in the 3D electroanatomic mapping of both atrial and ventricular arrhythmias.10,11In the current issue of the Journal, Latcu et al12 evaluated the utility of the Rhythmia mapping system in 33 patients with scar-related atrial tachycardias either after atrial fibrillation ablation or after surgical mitral valve repair. They observed 26 macro-reentrant atrial tachycardias, 4 micro-reentrant atrial tachycardias, and 1 focal atrial tachycardia in their study population. The majority of macro-reentrant atrial tachycardias were peri-mitral flutter or roof dependent.An important observation of this study has been the significantly lower background noise using the IntellaMap Orion mapping catheter compared with the decapolar catheter and surface ECG (0.011 mV versus 0.016 mV and 0.02 mV, respectively). Using this feature, the authors were able to lower the confidence mask threshold (or dense scar threshold) to just above the background noise level. This allowed differentiation of low amplitude signals from what were truly areas of dense scar. Using this technique, 12% of the left atrial surface area had bipolar voltages below the dense scar threshold, no doubt reflecting a patient population having undergone considerable prior ablation or prior atrial surgery and with advanced remodeling. Additionally, 58% of the left atrial surface was found to have bipolar voltages <0.5 mV. It is unclear whether this finding is solely a manifestation of extensive atrial low voltage in scarred and remodeled atria (using existing definitions) or is also a reflection that normal bipolar voltages in the atria have never been elucidated. The cutoffs of <0.05 mV as scar and <0.5 mV as low voltage zone are largely arbitrary, derived from the antenna-like bipole of an ablation catheter and based on animal work. They do not necessarily reflect the voltage measured by tight flat bipoles nor do they take into account both individual and regional voltage variation. The advent of ultrahigh resolution atrial mapping necessitates a thorough reconsideration of normal atrial voltage and conduction ranges. The use of confidence mask thresholding in the current study is an important and insightful contribution to the use of this mapping technology. The authors have provided cogent examples where its use facilitated the identification of a complete circuit and the critical channel region that might otherwise have been misclassified as dense scar. Indeed, 50% of patients demonstrated bipolar electrogram amplitude within the critical isthmus of <0.05 mV, yet above the level of background noise.The study also provides important insights into the electrophysiological properties of the critical isthmus when mapped with a high-resolution 3D system. The critical isthmus was identified solely using the propagation map as a site of wavefront narrowing and conduction slowing (0.27 m/s within the critical isthmus compared with upstream or downstream; both 1.0 m/s). Similarly, the bipolar voltage at the level of the critical isthmus (0.08 mV) was significantly lower than that upstream or downstream (0.62 and 0.80, respectively).These findings may again reflect the considerable prior ablation/atrial surgery and advanced remodeling in these patients because in many atrial circuits, the critical isthmus may not always be characterized by these features. For example, in peri-mitral flutter, the narrowest point of the circuit (the so-called mitral isthmus) may be >3 cm in width13 through healthy, normal voltage tissue, and extensive ablation is often required.14 The same is true for cavo-tricuspid-dependent atrial flutter and some cases of roof-dependent flutter as examples. In these cases, linear ablation through normal voltage and normally conducting tissue may be required.In the current study, to demonstrate the mapping fidelity of this high-resolution system, the authors eschewed other traditional mapping tools, including entrainment. The results obtained by this leading atrial mapping group are indeed impressive, but it remains to be seen whether they can always be reproduced with such precision. We have observed cases where the initial maps were misleading. Complex propagation patterns with inactive circuits and blind loops can create the impression of circuits which are not actually present. In the current study, the authors described dead end pathways in 34% of cases, and in 10 cases potential circuits were noted in addition to the apparent active circuit. In 2 cases, once the active circuits were ablated, these once potential circuits became active. In this study, to discriminate between active and blind-loop circuits, wavefront propagation was visualized by following a 10 ms window of activation, which was slowly advanced along the timescale. However, even with this technique, a blind circuit can seem as active when it accounts for >90% of the cycle length. Furthermore, the technique critically relies on visual interpretation of the circuit by the electrophysiologists and supporting technicians. The variability of such visual interpretation of the propagation maps is unclear.In this context, we continue to find that simple entrainment remains a powerful tool for rapid determination of arrhythmia mechanism and when trying to determine where to focus mapping detail. Entrainment in the circuit of just 2 sites 2 cm apart immediately confirms macro-reentry and identifies a region relevant to the tachycardia. Similarly, orthodromic capture of upstream electrograms on a single multipolar catheter confirms macro-reentry and identifies the pacing site as near the circuit.15Entrainment may also be deployed to rapidly detect the change to another tachycardia circuit during ablation of the initially defined circuit. Cycle length change may be minor, the ECG frequently insensitive, and unless high-density intracardiac mapping is located near the circuit (usually not used in the left atrial), a change in tachycardia may not be evident. Entrainment at 1 or 2 sites in the circuit and adjacent to a critical isthmus will rapidly indicate whether this is the same circuit or changed after initial ablation. In a recent study of atrial tachycardias post–atrial fibrillation ablation, <1% of stable atrial tachycardias were altered or terminated with overdriving pacing.16 Indeed, the authors also affirm the use of entrainment outside of this clinical study ("…entrainment should be systematically used in the setting of sustained organised tachycardias"). On the other hand, entrainment mapping also has potential limitations. For example, decremental conduction during pacing increases the postpacing interval, which may lead to reentrant sites being incorrectly labeled as outside the circuit. In scarred and remodeled atria, sites with a long postpacing interval may occur immediately adjacent to sites critical to the circuit and may, therefore, be misleading. For mapping complex atrial circuits, it remains important to use all available tools.The current study using the Rhythmia mapping system takes us into territory rarely explored. Reliable recording of atrial signal amplitudes <0.05 mV and approaching the levels of background noise allowed definition of a critical isthmus, which might otherwise have been characterized as scar. In doing so, the study has provided important new insights into the mechanisms of complex atrial arrhythmia circuits in patients with prior atrial ablation or atrial surgery.DisclosuresDr Kalman serves on the advisory board of Boston Scientific and Biosense Webster. On behalf of Dr Kalman, The Royal Melbourne Hospital has received unrestricted research and fellowship support from Boston Scientific, Biosense Webster, Medtronic Inc, and St. Jude Medical.FootnotesThe opinions expressed in this article are not necessarily those of the editors or of the American Heart Association.Correspondence to Jonathan Kalman, MBBS, PhD, Department of Cardiology, Royal Melbourne Hospital, Grattan St, Parkville, Victoria 3052, Australia. E-mail [email protected]References1. Patel AM, d'Avila A, Neuzil P, Kim SJ, Mela T, Singh JP, Ruskin JN, Reddy VY.Atrial tachycardia after ablation of persistent atrial fibrillation: identification of the critical isthmus with a combination of multielectrode activation mapping and targeted entrainment mapping.Circ Arrhythm Electrophysiol. 2008; 1:14–22. doi: 10.1161/CIRCEP.107.748160.LinkGoogle Scholar2. 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Barbhaiya CR, Kumar S, Ng J, Nagashima K, Choi EK, Enriquez A, Chinitz J, Epstein LM, Tedrow UB, John RM, Stevenson WG, Michaud GF.Avoiding tachycardia alteration or termination during attempted entrainment mapping of atrial tachycardia related to atrial fibrillation ablation.Heart Rhythm. 2015; 12:32–35. doi: 10.1016/j.hrthm.2014.09.002.CrossrefMedlineGoogle Scholar Previous Back to top Next FiguresReferencesRelatedDetailsCited By Mascia G, Sartori P and Porto I (2020) Effective and fast characterization of a complex atrial flutter in patient with previous atrial surgery and ablation procedure: The importance of new ultra–high‐density mapping tools, Clinical Case Reports, 10.1002/ccr3.3440, 8:12, (3457-3461), Online publication date: 1-Dec-2020. Jiang J, Yang Y, Liu C, Ma Y, Wang L, He J, Tang A, Hess P, Kerlan J, Feng C and Lan D (2019) Overdrive pacing mapping: An alternative approach used in scar associated localized atrial tachycardia, Journal of Cardiovascular Electrophysiology, 10.1111/jce.14200, 30:12, (2668-2677), Online publication date: 1-Dec-2019. Scaglione M, Caponi D, Di Donna P and Battaglia A (2019) Ultra-high definition mapping to choose the optimal ablation strategy in atrial macro-re-entrant tachycardia, EP Europace, 10.1093/europace/euz149, 21:Supplement_3, (iii13-iii14), Online publication date: 1-Aug-2019. January 2017Vol 10, Issue 1 Advertisement Article InformationMetrics © 2016 American Heart Association, Inc.https://doi.org/10.1161/CIRCEP.116.004850PMID: 28039283 Originally publishedDecember 30, 2016 Keywordsmappingentrainmentatrial flutter3D mappingEditorialscritical isthmusatrial tachycardiaPDF download Advertisement SubjectsArrhythmiasElectrophysiology