Atrial Arrhythmias After Single-Ring Isolation of the Posterior Left Atrium and Pulmonary Veins for Atrial Fibrillation Mechanisms and Management

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1 Atrial Arrhythmias After Single-Ring Isolation of the Posterior Left Atrium and Pulmonary Veins for Atrial Fibrillation Mechanisms and Management Toon Wei Lim, MBBS, FRACP; Choon Hiang Koay, BSc, RN; Rebecca McCall, BVCDes; Valerie A. See, BMedSci; David L. Ross, MBBS, FRACP; Stuart P. Thomas, PhD, FRACP Background Single-ring isolation of the posterior left atrium is feasible, but the incidence and mechanisms of postprocedural arrhythmias have not been described in detail. Methods and Results The first 100 consecutive patients ( years old, 80 male) who underwent single-ring isolation for atrial fibrillation (66 intermittent, 18 persistent, 16 long-standing persistent) were followed up for months. Recurrences were diagnosed by clinical symptoms and Holter monitoring. Patients with recurrences of sustained atrial arrhythmia 3 months after the procedure were offered a repeat procedure and were studied to determine the mechanisms of recurrence. Forty-six patients (46%) experienced sustained postprocedural atrial arrhythmias (35 had atrial fibrillation, and 34 had atrial flutter). Of these, 34 required a second procedure months after their initial procedure. Reconnection of the posterior left atrium was seen in all patients with atrial fibrillation. Atrial flutter was most commonly due to mitral isthmus-dependent macroreentry (n 8, cycle length ms) or macroreentry through 2 gaps in the ring of lesions (n 6, cycle length ms). Posterior left atrium reisolation was achieved at the second procedure in all patients. Atrial flutter was successfully ablated and rendered noninducible in all patients. Six months after their last procedure, the Kaplan-Meier estimate of freedom from recurrence for all 100 patients was 81 5%. Conclusions Atrial fibrillation and atrial flutter recurrence is common after single-ring isolation. Reconnection of the posterior left atrium and macroreentry are the common mechanisms. Repeat ablation results in satisfactory short-term outcomes. (Circ Arrhythmia Electrophysiol. 2008;1: ) Key Words: atrium fibrillation catheter ablation atrial flutter Circumferentially wide antral pulmonary vein isolation, with or without additional linear lesions, has been adopted by many centers for treatment of atrial fibrillation (AF). The reported success rates for ablation procedures range from 48% to 88% for paroxysmal AF and 0% to 79% for chronic AF after a single procedure. 1 6 Failure may be due to recurrence of AF and atrial tachycardia due to macroreentry or focal mechanisms. Left atrial reentrant tachycardia has been reported in 6% to 16% of patients with pulmonary vein isolation. 7,8 Clinical Perspective see p 126 Recently, we showed the feasibility of electrically isolating the pulmonary veins and posterior left atrium (PLA) with a single continuous ring of radiofrequency lesions. 9 Short-term clinical outcomes were similar to a historical control cohort of patients undergoing circumferential pulmonary vein isolation with a ring around each ipsilateral vein pair. We hypothesized that the new single-ring procedure may lead to different patterns of arrhythmia recurrence. The purpose of this study was to characterize arrhythmias occurring after the single-ring procedure and determine the longer term efficacy of catheter ablation for treatment of these procedures. Methods Patient Population Wide, electrically isolating percutaneous left atrial catheter ablation was performed in 100 consecutive consenting patients with intermittent (66), persistent (18), or long-standing persistent (16) AF to electrically isolate their pulmonary veins and PLA. No patients had prior left atrial ablation. Received January 29, 2008; accepted April 18, From the University of Sydney (T.W.L., D.L.R., S.P.T.); Department of Cardiology (T.W.L., C.H.K., V.A.S., D.L.R., S.P.T.), Westmead Hospital; and Westmead Private Hospital (R.M.), Sydney, Australia. Correspondence to Stuart P. Thomas, PhD, FRACP, Department of Cardiology, Westmead Hospital, Cnr Hawkesbury and Darcy Rds, Westmead, NSW 2145, Australia. stuartpt@yahoo.com 2008 American Heart Association, Inc. Circ Arrhythmia Electrophysiol is available at DOI: /CIRCEP

2 Lim et al Atrial Arrhythmias After Single-Ring Isolation 121 Figure 1. Isolation of all 4 pulmonary veins using a single ring of ablation. The radiofrequency lesions are shown as red dots on the volume-rendered computerized tomographic image. A, Left posterior oblique view. B, Right anterior oblique cranial view. C, Posterior anterior view. D, Anterior posterior caudal view with clipping plane to display PLA: shows the ring of ablation encircling the posterior left atrial wall. LAA indicates left atrial appendage; LIPV, left inferior pulmonary vein (light blue dots); LSPV, left superior pulmonary vein (dark blue dots); RIPV right inferior pulmonary vein (light green dots); and RSPV, right superior pulmonary vein (dark green dots). Procedures Procedures were performed under midazolam and fentanyl sedation 10 or general anesthesia. Two transseptal punctures were performed, and long sheaths were used to position catheters in the left atrium. A fixed curve braided sheath was used to position a circular decapolar mapping catheter in the left atrium (Preface Multipurpose and Lasso catheter, Biosense Webster, Diamond Bar, Calif). A deflectable sheath (Ultimum Agilis, St Jude Medical, St Paul, Minn) was used to direct an open irrigated 7F, 3.5-mm tip, deflectable ablation catheter (Thermocool Navistar, Biosense Webster). Mapping and ablation were guided by 3D rendering of the left atrium created from a multislice computed tomographic image integrated with the electroanatomic mapping system to guide ablation catheter navigation (CARTO Merge, Biosense Webster). Ablation sites were displayed on the rendered atrial surface. Radiofrequency ablation power was limited to 40 W for the septum, left free wall, and roof of the left atrium. Power was reduced to 30 W during ablation of the PLA between the left and right inferior pulmonary veins and was further reduced to 25 W when ablation was required on the venous side of the ridge separating the left-sided veins and the left atrial appendage. Temperature was limited to 50 C. Ablation was continued at each site until the local electrogram was abolished (reduced to 0.05 mv) or for 30 seconds. Ablation was applied at 2- to 5-mm intervals. First Ablation Procedure: Single-Ring PLA Isolation The initial ablation procedure has been described in detail elsewhere. 9 In brief, a single ring of ablation passed up the anterolateral margin of the ridge separating the left-sided veins from the left atrial appendage (Figure 1A). At the top of this ridge, it passed along the superior margin of the left atrial appendage onto the left atrial roof (Figure 1B). The roof was traversed, and the line continued down the interatrial septum between the septal puncture site in the foramen ovale and the septal margin of the right pulmonary veins (Figure 1B). It then passed below the right inferior pulmonary vein and back across the PLA to below the left inferior pulmonary vein before ascending up the lateral left atrium anterior to the left inferior pulmonary vein to join the start of the line (Figure 1C). By placing the circular mapping catheter sequentially in each of the veins, it was possible, by comparison with a coronary sinus reference electrogram, to determine which vein was activated first. The activation sequence in the earliest vein indicated whether this activation was coming from the adjacent segment of the ring of lesions or the section of the posterior wall between the veins, where there was uncertainty. The mapping catheter was placed on the posterior wall, within the ring, and adjacent to the earliest vein to determine the order of activation. Where the posterior wall was activated first, point-by-point activation was used to identify the earliest breakthrough in the roof or inferior line. Successful ablation at the gap (earliest point of activation within the ring) isolated the entire region within the ring (pulmonary veins and posterior wall) if there was a single gap. When 1 gap was present, ablation of the gap caused delayed and altered activation within the ring. The mapping process was repeated until all the gaps were abolished. The PLA and pulmonary veins were considered isolated when 3 criteria were fulfilled: (1) electrical activity was dissociated or absent within the ring of ablation lesions during sinus rhythm, (2) pacing the PLA and pulmonary veins did not capture the rest of the atria, and (3) pacing from the coronary sinus catheter did not result in associated electrical activity in the PLA or pulmonary veins. A line of ablation was placed in the mitral isthmus joining the single ring of ablation near the left inferior pulmonary vein to the mitral annulus in 53 patients. The end point of ablation was bidirectional conduction block across the line. This was demonstrated by pacing the coronary sinus catheter both septal to and lateral to the ablation line. Conduction block across the line was defined as a delay 100 ms in conduction time and evidence of reversal of activation sequence distal to the line of ablation. 11 If an epicardial connection was suspected, ablation was also performed in the coronary sinus (25 W). A cavotricuspid isthmus line of ablation was also performed in 90 patients. Repeat Procedure Patients in sinus rhythm at the start of the procedure were systematically checked for electrical isolation of the pulmonary veins and the PLA as described earlier. Patients in AF at the start of the procedure were electrically cardioverted to restore sinus rhythm. The previously isolated region within the ring was mapped to identify where the earliest activation was entering the PLA or pulmonary veins as described above. These areas were ablated, and if ablation resulted in isolation of the PLA or changed the site of earliest activation to another part of the ring, the ablation sites were considered a gap. Further gaps were mapped and closed with additional ablation to reestablish electrical isolation of the PLA and the pulmonary veins. Patients underwent rapid atrial pacing to induce atrial flutter after completion of the ring of lesions. Any further atrial flutters induced were characterized and ablated. Patients in atrial flutter at the start of the repeat procedure were studied by using activation mapping and entrainment to locate sites suitable for ablation. Entrainment was performed in both atria to determine whether the tachycardia was macroreentrant or focal and to define whether the atrial flutter was right or left sided in origin. Electroanatomic activation mapping was then performed, guided by the entrainment mapping to confirm the mechanism of the arrhythmia. If needed, mitral isthmus ablations were performed as described for the first procedure. The previous ring of lesions was checked for gaps that were ablated as described earlier after successful ablation and restoration of sinus rhythm. Postprocedural Care and Long-Term Follow-Up Patients were not routinely administered antiarrhythmic medications after pulmonary vein isolation. Patients with symptoms suggestive of

3 122 Circ Arrhythmia Electrophysiol June 2008 recurrence were investigated by electrocardiography or Holter monitoring. A 3-month blanking period was applied to exclude early recurrences that spontaneously resolved. Repeat procedures were offered to patients experiencing a recurrence of atrial arrhythmia 3 months after their first procedure. Patients who remained free of symptomatic recurrences had 7-day Holter monitoring to look for previously undetected atrial tachyarrhythmias at 6 months to identify asymptomatic arrhythmias. Analysis Continuous variables are expressed as mean SD. Kaplan-Meier analysis was used to determine the likelihood of atrial arrhythmia recurrence. The log-rank test and Cox regression analysis were used to determine whether it was significantly affected by any patient or procedural variables. P 0.05 was considered statistically significant. The study was approved by the Sydney West Area Health Service Human Ethics Committee, and all participants provided written informed consent. The authors had full access to the data and take responsibility for the integrity of the data. All authors have read and agree to the manuscript as written. Results Patients Between February 2006 and September 2007, 100 patients underwent single-ring pulmonary vein isolation and were followed up for months. Their mean age was years, and 80 were male. AF had been present for years, and these patients had failed treatment with antiarrhythmic medications. Twenty-three patients had concomitant structural heart disease. Mean left atrial size on 2D echocardiography in the parasternal long-axis view was 42 7 mm. The PLA and pulmonary veins were successfully isolated in 96 (96%) patients. Recurrence After First Procedure Recurrent atrial arrhythmia that was present 3 months after the first procedure occurred in 46 (46%) patients. Thirty-five (35%) patients had recurrence of AF, and 34 (34%) patients had recurrence of regular atrial tachycardia suggesting atrial flutter or focal atrial tachycardia. Repeat Procedure A repeat procedure was performed on 34 patients with arrhythmia recurrence months after the initial procedure. The mean total procedure time was minutes, and mean total ablation time was seconds. Electrophysiological Study Findings At the start of the procedure, 21 (62%) patients were in sinus rhythm, 1 (3%) was in AF, and 12 (35%) patients were in organized atrial tachycardia. Mapping revealed that 30 (88%) of 34 patients had breaches in the single ring of lesions that led to resumption of electrical activity in the previously isolated PLA, and 22 patients had 1 gap (mean gaps per patient). Atrial Fibrillation In total, 15 (44%) of the 34 patients who underwent a repeat procedure had AF recurrence after their first procedure. They all had gaps in their previous ring of isolating lesions resulting in associated electrical activity in the PLA and Figure 2. Schematic representation of breaches in ring of lesions isolating the PLA. The numbers in brackets indicate the number of patients who had breaches in each region. The sites were reablated to achieve isolation of the PLA and pulmonary veins. LAA indicates left atrial appendage; LIPV, adjacent to left inferior pulmonary vein; LSPV, adjacent to left superior pulmonary vein; RIPV, adjacent to right inferior pulmonary vein; and RSPV, adjacent to right superior pulmonary vein. pulmonary veins. The gaps in the ring lesions were most commonly seen in the roof and along the ridge between the left-sided veins and the ostium of the left atrial appendage (Figure 2). Conversely, none of the 4 patients who still had intact isolation of the PLA had recurrence of AF after their first procedure (see later). Ablation of gaps in the ring was undertaken in the 30 patients who had reconnection of the PLA. At the end of the procedure, there was successful isolation of the PLA and pulmonary veins in all these patients. Regular Atrial Tachyarrhythmias Observed at Repeat Procedure Organized atrial tachycardia was present in 12 (35%) patients at the start of the procedure and was inducible with rapid atrial pacing or by catheter manipulation in another 6 (18%) patients during the procedure. All of these 18 patients had clinically documented organized atrial tachycardia after their first procedure. Mapping of the atrial tachycardias showed that 16 of them had organized left atrial tachycardias, while the other 2 had typical cavotricuspid isthmus flutter. Atrial flutter was not induced in 9 of 25 patients who had documented organized atrial tachycardia after their first procedure. These 9 patients had gaps in the ring of lesions resulting in reconnection of the PLA and pulmonary veins. These gaps were closed at the repeat procedure. Five patients had 1 form of organized left atrial tachycardia. Two patients had both mitral annular reentrant tachycardia and ring breachdependent atrial flutter. Two other patients with ring breach atrial flutter and a patient with mitral annular flutter also had other forms of atrial flutter that were not sustained enough to be mapped. These atrial flutters had cycle lengths that were different from the flutters that were mapped in these patients, and their mechanisms were unclear. Mitral Annular Macroreentrant Atrial Flutter Mitral annular atrial flutter was found in 8 (24%) patients (Figure 3) with cycle lengths of ms. Among the 34 patients who had a repeat procedure, 11 had an electrically intact mitral isthmus line created at the first procedure, 10 patients had an incomplete mitral isthmus line, and 13 patients did not have any mitral isthmus ablation. Ablation within the coronary sinus was not performed in any patients

4 Lim et al Atrial Arrhythmias After Single-Ring Isolation 123 Figure 3. Anticlockwise mitral annular atrial flutter. Atrial flutter recurred in this patient 13 months after the initial procedure. An isochronal activation map was superimposed over a volumerendered computerized tomographic image. The broken line represents the location of the ring of ablation lesions from the previous procedure, and the white arrows indicate the direction of activation propagation. Tachycardia cycle length was 250 ms. A, Right anterior oblique view: The atrial flutter circuit propagates left to right over the anterior left atrium. B, Left anterior oblique view: The circuit propagates around the mitral annulus and is right to left inferiorly. C, Left posterior oblique view: Activation then goes inferior to superior through the left mitral isthmus. Brown spheres in this region represent ablation lesions that terminated the atrial flutter. Delay over the left mitral isthmus was 170 ms after restoration of sinus rhythm. D, Posterior anterior view: The PLA and regions around the pulmonary veins were still isolated from the first procedure and were electrically silent. This is represented by gray areas in C and D. Abbreviations as shown in Figure 1. at the first procedure. Reconnection across an electrically intact mitral isthmus line created at the first procedure was found in 5 of 11 patients, and all of these patients had mitral annular atrial flutter at the repeat procedure. None of 6 patients who still had an intact mitral isthmus line had mitral annular flutter. One of 10 patients who had an incomplete mitral isthmus line and 2 of 13 patients who did not have a mitral isthmus line ablated at the first procedure had mitral annular atrial flutter at the second procedure. During the repeat procedure, an electrically insulating mitral isthmus was successfully completed in all 8 patients who had mitral annular flutter. Coronary sinus ablation was required in 2 of these patients to achieve bidirectional block across the line of ablation. Gap-Related Macroreentrant Atrial Flutter Left atrial flutter sustained by reentry through at least 2 gaps in the ring was observed in 6 (18%) patients (Figure 4) with cycle lengths of ms. A gap in the ring was present in either the septum (n 3) or the roof (n 3) in all patients, together with another gap elsewhere in the ring. Activation in these cases entered the ring through 1 gap and exited to the remainder of the left atrium through a second breach in the Figure 4. Atrial flutter sustained by 2 gaps in the ring of ablation lesions. Atrial flutter recurred in this patient 1 month after the initial procedure and persisted. An isochronal activation map was superimposed over a volume-rendered computerized tomographic image. The broken line represents the location of the ring of ablation lesions from the previous procedure, and the white arrows indicate the direction of activation propagation. Tachycardia cycle length was 275 ms. A, Left posterior oblique view: The atrial flutter circuit entered through a gap in the lateral part of the ring and passed between the left-sided pulmonary veins into the PLA before exiting through another gap in the ring near the right inferior pulmonary vein. Brown spheres near the left pulmonary veins represent ablation lesions that terminated the atrial flutter but did not isolate the PLA. B, Right lateral view: After termination of the atrial flutter, further ablation (red spheres) near the right inferior pulmonary vein isolated the left atrium. The yellow sphere marks the ablation site that resulted in reisolation. C, Superior view: Activation over the roof of the left atrium was from right to left. Hence the flutter circuit was a figure-8 reentry using the region of the PLA within the single ring as the common pathway. Abbreviations as shown in Figure 1. ring. Ablation of these gaps terminated gap-related macroreentrant atrial flutter in all 6 patients. Macroreentrant Right Atrial Flutter Typical right atrial flutter, which was anticlockwise around the tricuspid annulus and dependent on the cavotricuspid isthmus, was observed in 2 (6%) patients. Of the 34 patients who had a repeat procedure, cavotricuspid isthmus ablation was performed at the previous procedure in 30 (88%) patients and was unsuccessful in only 1 patient. This patient had right atrial flutter, whereas the other had reconnection across a previously ablated cavotricuspid isthmus. Seven other patients had reconnection across their previously ablated cavotricuspid isthmus, but they did not have inducible right atrial flutter. Right atrial flutter could not be induced in any of the 21 patients who still had an electrically intact cavotricuspid line or in the 4 patients who did not have any cavotricuspid ablation at the previous procedure. Irregular Conduction of AF Through a Ring Gap A single patient had a different form of atrial tachycardia characterized by irregular activation of the atria. There were

5 124 Circ Arrhythmia Electrophysiol June 2008 Figure 5. Irregular conduction through a ring gap. This patient had a recurrence 6 days after his first procedure that persisted. A, Surface electrocardiogram demonstrated monomorphic P waves with irregular P P intervals (arrows). At the repeat procedure, a circular mapping catheter in the PLA showed fibrillation. This was conducting out through a gap in his ring of lesions near the right inferior pulmonary vein at irregular intervals. B, Electrogram recording of PLA (circular mapping catheter) and coronary sinus catheter. Ablation at the site of the gap in the ring of lesions resulted in isolation of the fibrillating PLA and the restoration of sinus rhythm in the rest of the heart as seen in the coronary sinus catheter electrogram. monomorphic P waves on the surface electrocardiogram, with an isoelectric period separating each complex. Endocardial recordings demonstrated disorganized irregular activation of the myocardium within the ring of ablation lesions (PLA and pulmonary veins) that was conducting irregularly out through a single gap in the ring of ablation lesions (Figure 5). Other Atrial Tachycardia Mechanisms in Patients With Intact Rings Four patients were found to still have electrically isolated posterior left atria and pulmonary veins, despite clinical recurrence of atrial tachyarrhythmias. In these patients, AF was not observed during follow-up after their first procedure or during the repeat procedure. These patients only had macroreentrant or focal tachycardias. Two patients had mitral annular atrial flutter. One patient had long complex fractionated signals near the base of the left atrial appendage with centrifugal activation of the left atrium from this site to the rest of the atria. This pattern was thought to indicate localized reentry in the left atrial roof outside the ring of ablation lesions. The last patient had an apparently focal atrial tachycardia arising from the ridge between the left atrial appendage and the left superior pulmonary vein. Activation from this region spread centrifugally to the rest of the left atrium outside the ring of lesions. The mechanism of this tachycardia was not clear, and local reentry could not be excluded. Follow-Up After Second Procedure No patients had atrial tachyarrhythmias before discharge. There were no significant complications associated with the procedures. After a mean follow-up period of months, 27 of 34 (79%) patients who underwent a redo procedure remained free of atrial arrhythmias. On the basis of follow-up since their last procedure, the Kaplan-Meier estimate of atrial arrhythmia-free survival at 6 months after the procedure was 81 5%. Discussion The main finding of this study is that approximately half of the patients who underwent single-ring PLA isolation had recurrence of an atrial arrhythmia, but a repeat procedure was usually successful in reestablishing sinus rhythm. Single-ring isolation of all pulmonary veins was followed by recurrence of atrial flutter in a high proportion of patients. However, these flutters had one of a small number of mechanisms and could usually be identified and treated effectively. We found at these repeat procedures that the reconnection of the previously isolated PLA and pulmonary veins was seen in a large majority of patients and that ablating any gaps in the ring of lesions led to reisolation of these structures. Recurrent macroreentrant atrial flutter was associated with discontinuity in the ring or previous ablation lines in the mitral isthmus or cavotricuspid isthmus. By reclosing these gaps, atrial flutter was rendered noninducible, and short-term follow-up showed that recurrence after a second procedure was uncommon. Recurrent AF After Left Atrial Ablation and the Role of PLA In previous studies of catheter ablation for AF, recurrence of atrial arrhythmias after a single percutaneous catheter ablation procedure, mostly AF, occurred in 12% to 85% of cases and was more common in patients with persistent or permanent AF rather than intermittent AF. 2 6 Electrical reconnection in these studies was usually observed in patients who had recurrence of AF after these procedures and was thought to be the mechanism in most patients. 5,6,12,13 The rate of AF recurrence in our study (35%) was nearer the lower end of the range reported in previous studies, for a cohort with a third of patients who have persistent or long-standing persistent AF. Reconnection of the previously isolated PLA and pulmonary veins was seen in all patients with recurrent AF in the present series. Moreover, AF did not recur in any of the 4 patients whose pulmonary veins and PLA remained isolated. This finding suggested that reconnection of the PLA was an important factor in AF recurrence. The PLA has been implicated in animal studies in the maintenance of AF. 14,15 Mapping studies in patients with chronic AF have since demonstrated rapid repetitive electrical activity in the PLA. 16,17 The PLA has also been shown to be a common site for the initiation and maintenance of AF. 18,19 Electrically isolating this part of the left atrium may thus prevent the initiation and perpetuation of AF. Recurrent Atrial Flutter After Left Atrial Ablation Left atrial flutter has been reported rarely after segmental pulmonary vein isolation. Gerstenfeld et al 20 found that organized left atrial tachyarrhythmia recurred in 2.9% of patients after this procedure, whereas AF recurred in 20%. At repeat ablation, only 1 in 10 patients had macroreentrant atrial flutter. The incidence of left atrial flutters with pulmonary vein antral isolation techniques was higher than that seen with segmental pulmonary vein isolation at 11% to 14%. 11,21

6 Lim et al Atrial Arrhythmias After Single-Ring Isolation 125 Recurrent macroreentrant atrial arrhythmias were associated with gaps in ablation lines. 21,22 Despite this, the addition of linear ablations in the roof or left mitral isthmus to this procedure has been shown to reduce the recurrence of atrial arrhythmias overall. 11,21,23 Wide antral left atrial ablation with anatomic end points aiming for substrate modification but without electrical isolation is also associated with postablation left atrial flutter that is persistent in 4% to 16%. 7,8,24 In patients who underwent a repeat procedure, mitral isthmus-dependent left atrial flutter was the most common mechanism. 8,25,26 Gaps in previous ablation lines were thought to be responsible for creating these reentry circuits. 25,26 The rate of atrial flutter recurrence we observed in our study was high compared with these previous studies at 34%. We found that mitral isthmus atrial flutters were frequently seen, as found in previous studies. We also observed that most of the recurrences were in patients who had undergone mitral isthmus ablation in their last procedure but who had reconnection across the line of ablation. In comparison, only 2 of 13 patients who did not have previous mitral isthmus ablation had mitral annular atrial flutter. It is possible that slow conduction across a recovered gap in the line predisposes to sustained atrial flutter. Further study will be required to elucidate the role of mitral isthmus ablation in the context of single-ring isolation of the PLA. A second common mechanism of atrial flutter in this series was reentry through 2 gaps in the single ring of lesions (Figure 4). Mechanistically, this form of atrial flutter requires at least 2 gaps in the ring to be sustained. Where the flutter is dependent on 2 gaps, ablation of one of these gaps rendered atrial flutter noninducible, but the PLA was not isolated unless both gaps were closed by reablation. These macroreentrant flutters are due to recovery of conduction across the ring of ablation lesions as these patients all had an electrically isolating ring of lesions at the end of their previous procedure. Gaps in the ring resulting in electrical reconnection were most commonly seen in the roof line and also along the ridge between the left atrial appendage and the left pulmonary veins. This is not unexpected as we have previously noted that these were the regions that were most difficult to isolate in the initial procedure. 9 Conclusions Recurrence of atrial arrhythmias after single-ring isolation of the PLA and pulmonary veins is usually associated with reconnection across the ring of ablation lesions, resulting in electrical activity in the PLA and pulmonary veins or reconnection across previous ablation lines in the mitral isthmus. Left atrial flutter after this procedure is common, often due to macroreentry involving discontinuity in linear lesions. These recurrent atrial tachyarrhythmias are amenable to repeat ablation, and the short-term outcomes are satisfactory. Source of Funding This research is funded by the Department of Cardiology, Westmead Hospital. Disclosures Drs Thomas and Ross are the principal investigators for a National Health and Medical Research Council project grant titled Circumvenous ablation for treatment of atrial fibrillation. 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