Atrial fibrillation is the most common supraventricular arrhythmia in Western countries.
It affects predominantly older people, and, as life expectancy continues to improve and the average age of the population increases, an estimated 50 million patients are expected to be affected by 2060 across the United States and Europe .
Catheter ablation is an established treatment to achieve freedom from atrial fibrillation in a variety of (atrial fibrillation) patients .
Several randomized clinical trials have shown superiority of catheter ablation over antiarrhythmic drug (AAD) therapy to achieve sinus rhythm and to improve symptoms and quality of life [3,4].
After the seminal study by Haissaguerre et al. that showed triggers initiating atrial fibrillation in the pulmonary veins , pulmonary vein electrical isolation (PVI) has been performed in paroxysmal atrial fibrillation patients with satisfactory results. Meta-analyses have shown that isolation of the PVs is sufficient to achieve freedom from atrial fibrillation in the majority of the cases [6▪▪,7▪▪]. In patients with persistent and long-standing persistent (LSP) atrial fibrillation, results are not yet satisfactory [4,8,9▪▪]. In these patients, additional targets such as complex fractionated atrial electrograms (CFAEs) in addition to pulmonary veins and nonpulmonary vein triggers initiating atrial fibrillation became a necessary target for a successful ablation. These structures include the left atrial posterior wall, the coronary sinus, the left atrial septum, the left atrial appendage (LAA) and the superior vena cava (SVC) [9▪▪,10▪▪,11▪,12]. The prevalence and importance of nonpulmonary vein triggers progress over time as the arrhythmia does (Fig. 1).
This review will describe our ablation approach for the treatment of LSP atrial fibrillation.
LESSONS LEARNED FROM THE LITERATURE
Different techniques and outcome after a single procedure off AADs have been reported in the literature since 1990. The lessons learned are that:
- PVI confirmed by a circular mapping catheter is an essential step to be achieved during ablation of LSP atrial fibrillation. PVI alone has a poor success rate at 2 years’ follow-up, not exceeding 25%. Antrum isolation of the PVs or wider area of ablation around the PVs to achieve PVI (verified with a circular mapping catheter) is preferable to ostial ablation. Success rate does not increase much with repeated procedures if limited to PVI alone [9▪▪].
- CFAE ablation alone has a dismal success rate at long-term follow-up, ranging from 64% in a single study down to 24% at 18 months’ follow-up [9▪▪].
- CFAE ablation as an adjunct to PVI increases the outcome at long-term follow-up to 65%. The most common areas where CFAEs are localized are the posterior wall, the roof, the coronary sinus, the septum and the LAA. Right atrial CFAEs ablation does not seem to increase success. Redo procedures increase the success rate up to 80% [9▪▪].
- Stepwise procedures that include linear ablation in addition to PVI and CFAEs ablation do not seem superior to a stepwise procedure [9▪▪].
- Procedures that include isolation of the pulmonary vein antrum, the posterior wall, the roof, the coronary sinus, the left septum and the LAA with or without CFAEs seem to be the best strategy to achieve long-term success. Right atrial ablation does not seem to increase success except a few cases with severe sleep apnea or in women with thyroid problems. Redo procedures that include reisolation of the above-mentioned areas increase the success rate up to 88% at long-term follow-up [9▪▪].
- Termination during ablation does not seem to influence the long-term outcomes [11▪,12] (Figs 2 and 3).
OUR APPROACH: PREPROCEDURAL MANAGEMENT
All patients are required to undergo oral anticoagulation with warfarin for at least 1 month. After that, the patients are monitored and need to show 4–6 consecutive weeks of ‘therapeutic’ international normalized ratio (INR) with a target INR of 2–3 the day of the procedure. We do not discontinue warfarin before the procedure and we do not bridge with low-weight heparin.
Preprocedural transesophageal echocardiography (TEE) is not routinely performed unless patients cannot document (4–6 weeks) therapeutic INR or are subtherapeutic the day of the procedure.
As in the case of perforation warfarin needs to be reversed to interrupt bleeding, before procedures, all patients are type-matched and cross-matched so that fresh frozen plasma and red cells are available.
If the preprocedural INR is above 3.5, one or two units of fresh frozen plasma are utilized before the procedure to reduce the INR value.
This anticoagulation protocol is the result of our experience in a large series of patients in whom different anticoagulation strategies were compared. This approach significantly decreases the risk of bleeding and of thromboembolic complications when compared with warfarin discontinuation and bridging with low-molecular weight heparin [13▪▪,14▪▪,15].
Special consideration must be taken when patients arrive at the procedure with one of the newer anticoagulants. We have no data in regard to rivaroxaban and apixaban.
We recently published data on dabigatran, showing that if the procedure is performed without dabigatran discontinuation there is a risk for transient ischaemic attack (TIA)/stroke and higher bleeding [16▪▪].
Considering that no TIA/stroke was reported in a series of 2000 patients and that three embolic events happened in this relatively small series, it is our practice to discontinue dabigatran and to switch patients to Coumadin.
Four weeks after the procedure, patients are routinely switched back to dabigatran or rivaroxaban.
In addition to the embolic complications, it is important to note that no information is available on how to reverse dabigatran in case of perforation and bleeding.
This reinforces our decision not to perform an atrial fibrillation ablation procedure while patients are on dabigratan.
AADs, with the exception of amiodarone, are discontinued 3–5 days prior to the procedure [10▪▪]. Patients on amiodarone are asked to discontinue the drug 4–6 months prior to the ablation. In some cases, they are triaged to tikosin until 5 days before the procedure. This is done because AADs may suppress potential triggers for atrial fibrillation.
The Effect of Amiodarone on the Procedure Outcome in Long-standing Persistent Atrial Fibrillation Undergoing Pulmonary Vein Antral Isolation study (NCT01173809) is a randomized multicentre trial enrolling patients with long-standing persistent atrial fibrillation who are randomized to amiodarone discontinuation 4–6 months before the procedure versus no amiodarone discontinuation. Preliminary results [17▪▪] showed that after 6-month follow-up the success rate was significantly decreased in the group of patients undergoing ablation without amiodarone discontinuation. The results of this trial will be important in guiding decision on the use of amiodarone and other AADs before an atrial fibrillation ablation procedure.
Due to the improvement of the three-dimensional (3D) mapping system for the reconstruction of the atrial chambers, preoperative computed tomography (CT) scan or MRI of the left atrium and pulmonary veins is not routinely performed. They are considered in patients undergoing ‘redo’ procedures and/or with congenital heart disease.
In the case of repeat procedure, we want to exclude preprocedural pulmonary vein stenosis.
Postoperative CT or MRI scan is performed in all patients at 3 months’ follow-up to look for pulmonary vein stenosis [18,19].
OUR APPROACH: ANAESTHESIA PROTOCOL
All patients undergo ablation under general anaesthesia. Anaesthesia is initiated with propofol (2 mg/kg) and fentanyl (1–2 μg/kg), followed by a neuromuscular blocking agent (usually rocuronium 0.6–1 mg/kg) and by endotracheal intubation with intermittent positive pressure ventilation.
We prefer to avoid muscular paralysis to be able to detect diaphragmatic complications [20▪▪].
High-frequency jet ventilation (HFJV) has also been proposed as a valid alternative , although this technique is associated with a potential higher risk of complications and it requires specific skills .
An oesophageal probe is inserted in all patients to monitor oesophageal temperature during ablation in an area in close proximity to the oesophagus, such as the posterior wall. This probe is moved as the circular mapping catheter and the ablation catheter move to a different location [20▪▪]. It is the authors’ opinion that general anaesthesia adds several advantages, from both the physician's and the patient's side. From the physician's side, it improves catheter stability, allowing better lesion quality, and decreases procedural and fluoroscopy times. We have shown in a randomized trial on paroxysmal atrial fibrillation patients that pulmonary vein reconnection, fluoroscopy and procedural time are lower with general anaesthesia and that the success rate is higher [20▪▪]. From the patient's perspective, general anaesthesia improves the acceptance for such complex and long procedures and improves the patient's compliance to accept ‘redo procedures’. As, in LSP atrial fibrillation ablation, it is often necessary to perform more than a single procedure, patients’ compliance becomes extremely important. Although we have described an increased risk of oesophageal positive findings following atrial fibrillation ablation during general anaesthesia, we feel that the benefit of improved catheter stability is more important.
We do not routinely use any proton-pump inhibitor following the procedure, and, so far, we have not experienced any oesophageal complication .
Later in the text, we will describe the use of high dosage of isoproterenol for 15–20 min to disclose nonpulmonary vein triggers and to look for pulmonary vein reconnection. It is important to mention that, during isoproterenol administration, phenylephrine is necessary to maintain a stable blood pressure [20▪▪].
As the procedures are performed without Coumadin discontinuation, one may argue that it is important to have an arterial line. We, instead, do not obtain any arterial access to avoid vascular complication while on therapeutic Coumadin. Blood pressure is monitored with noninvasive instrumentation, and the presence of effusion is constantly monitored with intracardiac echocardiography (ICE).
OUR APPROACH: CATHETER INSTRUMENTATION
Four venous accesses via a modified Seldinger technique are obtained: two accesses in the right femoral vein, one in the left femoral vein and one in the right internal jugular vein [2,10▪▪].
To avoid access complication due to anticoagulation, the right internal jugular vein is accessed after advancing a mapping catheter or a guidewire in the SVC first and in the jugular vein later under fluoroscopic guidance. Once the catheter is in the vein, the access is performed under fluoroscopic guidance with the needle perpendicular to the mapping catheter previously advanced.
An alternative option to minimize bleeding complication while obtaining access in patients with ‘therapeutic’ INR is the use of ultrasound (Fig. 4a–d).
Through the right internal jugular vein, a 20-pole catheter is advanced. The distal 10 poles are placed in the coronary sinus and the proximal 10 poles along the high right atrium and/or crista terminalis depending on the patient's right atrial size.
An 11-Fr venous sheath is placed in the left femoral vein, by which a 10-Fr phased-array ultrasound catheter (AcuNav; Acuson, Mountain View, California, USA) is placed in the right atrium.
The intracardiac echo catheter (ICE) is positioned in the mid-right atrium at the level of the fossa ovalis and far from the septum.
The right femoral accesses are utilized to instrument the left atrium via a double transseptal puncture.
We utilize a LAMP 90° sheath (8.5 Fr, St Jude Medical, Minnesota, USA) for the first transseptal and a SLO 50° sheath (8.5 Fr, St Jude Medical) for the second transseptal.
As part of our anticoagulation protocol, a bolus of unfractioned heparin (10 000 U in men and 8000 U in women) is administered before transseptal [13▪▪]. In fact, we have seen that in 8–9% of the cases a soft thrombus forms on the right sheath quickly  (Fig. 4e). The use of ICE to obtain transseptal access allowed us to visualize and quantify this phenomenon and convinced us to administer a heparin bolus before transseptal. During the procedures, we maintain an activated clotting time (ACT) above 300 s. With a single heparin bolus, it is unlikely that in patients with ‘therapeutic INR’ the ACT goes below 300 s [13▪▪,15]. In these rare cases, additional heparin bolus is given. Our sheaths are continuously infused with heparinized physiological saline and a lot of attention is devoted to avoid air embolism.
In cases of thickened or hypertrophied interatrial septum resistant to transseptal puncture, and often during redo procedures, radiofrequency energy is applied to the distal end of the needle during septum tenting. We utilize an electrocautery in the ‘cut’ modality, with a power of 30 Watts and the coagulation modality set to zero. After the left atrium is accessed, a circular mapping catheter (Biosense Webster, Diamond Bar, California, USA) is positioned at the level of the left superior pulmonary vein. It is the authors’ opinion that catheter manoeuvrability depends on the way the left atrium is accessed. The use of ICE allows the operator to choose a more posterior access to the left atrium than a fluoroscopic-guided transseptal (Fig. 4f and g).
OUR APPROACH: ABLATION PROTOCOL
The circular mapping catheter is positioned under fluoroscopic guidance confirmed by ICE at the antrum of each pulmonary vein. Pulmonary vein potentials recorded by the circular mapping catheter are the target for ablation until abolition of all pulmonary vein antrum electrograms is achieved. The left pulmonary veins are usually isolated first and then the ablation encompasses the posterior wall contained within the left and right pulmonary veins, and consequently the right pulmonary veins are isolated. The circular mapping catheter is dragged around each vein and the posterior wall to achieve the endpoint. Entry block is verified when no pulmonary vein potentials can be recorded along the antrum or inside the vein by the circular mapping catheter.
When present, dissociated firing of the pulmonary vein from the left atrium confirms exit block as well.
The electrical isolation of the pulmonary veins is extended to the entire posterior wall down to the coronary sinus and to the left side of the septum. Ablation of CFAEs in the left atrium and in the coronary sinus is also performed if fragmented potentials are found (Fig. 5). If, after extensive ablation of all anatomical structures known as potential triggers for atrial fibrillation, termination cannot be achieved, cardioversion is performed [10▪▪]. Procedural termination of atrial fibrillation is not sought as an endpoint at our institution [11▪]. In a prospective study on 306 patients with longstanding persistent atrial fibrillation undergoing the first procedure of pulmonary vein antrum isolation (PVAI) along with CFAEs ablation, six (2%) patients converted directly to sinus rhythm during ablation, while 172 (56%) organized into atrial tachycardia, which was mapped and ablated, and 128 (42%) patients remained in atrial fibrillation and were cardioverted at the end of the procedure. After a mean follow-up of 25 ± 6.9 months, 69% of patients remained in sinus rhythm without significant differences between those who had procedural termination/organization of atrial fibrillation and those who remained in atrial fibrillation and received cardioversion [11▪].
In addition, we believe that atrial fibrillation termination does not influence the outcome at follow-up, while it increases procedural time and possibly complication rate [11▪].
After stable sinus rhythm is achieved either during ablation or after cardioversion, in all patients, isoproterenol up to 30 μg/min for 15–20 min is given to disclose any nonpulmonary vein triggers or tachycardia and to look for acute pulmonary vein reconnection [10▪▪].
Radiofrequency energy is delivered with a maximum temperature setting of 41°C and a power of 40 up to 45 W with a flow rate of 30 ml/min.
Before 2011, we utilized a 3.5-mm irrigated tip catheter (THERMOCOOL Catheter; Biosense Webster).
Since 2012, the THERMOCOOL SF Catheter with Surround Flow technology is utilized (Biosense Webster). This catheter allows more effective uniform cooling of the entire catheter tip, with half of the volume load to the patient, when compared with the Thermocool catheter.
When ablating the posterior wall, the power is decreased to 35 W.
Energy delivery is discontinued when the oesophageal temperature probe has a fast increase and/or reaches 39°C. The oesophagus position and its contact with the posterior wall and the ablation catheter can be visualized with ICE (Fig. 4g).
We limit each radiofrequency application to 20 s/site and then move to a proximal or different location.
Mapping during isoproterenol is performed, positioning the circular mapping catheter at the ostium of the LAA and the ablation catheter in the right superior pulmonary vein (Fig. 6a), allowing us to quickly identify the site of origin of any significant ectopic atrial activity and/or tachycardia, by comparing the activation sequence of the sinus beat with that of the ectopic beat (Fig. 6b, c).
In this case, we see triggers from either the coronary sinus or the LAA, and isolation of these structures is the ablation endpoint (Fig. 7b–d). After ablation, catheters and sheaths are pulled back in the right atrium. The circular mapping catheter is positioned at the junction between the SVC and the high right atrium under ICE guidance, and ablation to isolate the SVC is performed. We usually begin targeting potentials at the septal aspect of the SVC. High-voltage pacing (at least 30 mA) is used to check for phrenic nerve stimulation prior to ablating the lateral portion of the SVC [25,26].
At the end of the procedure, systemic anticoagulation with heparin is partially reversed with protamine guided by the ACT, and the sheaths are removed when the ACT is 250 s or less.
OUR APPROACH: OTHER TARGETS
As briefly stated above, in patients with LSP atrial fibrillation, in adjunct to PVAI, the ablation is extended to the posterior wall, the coronary sinus, the left side of the septum and additional sites showing CFAEs in the left atrium [10▪▪,27].
Isolation of the coronary sinus is usually performed first along the left atrium endocardial aspect and after within the coronary sinus (Fig. 7a). Endocardially, the ablation usually starts from the distal coronary sinus, progressing to the septal area anterior to the right pulmonary veins.
To achieve a better contact, it is often necessary to advance the long sheath or to loop the catheter at the level of the coronary sinus.
Continuous monitoring of the PR interval is warranted along the low septum, as leftward extensions of the atrioventricular (AV) node could be damaged, resulting in PR prolongation and, rarely in AV block.
Coronary sinus isolation is then completed epicardially [28▪▪]. Ablation within the coronary sinus is performed at 30–35 W. The endpoint is the complete abolition of all coronary sinus potentials.
We believe that continuous movement of the ablation catheter along the coronary sinus, limiting the ablation to 20 s per site, minimizes complications.
Several studies and meta-analyses have shown the importance of extending ablation to structures other than the pulmonary vein antra in patients with LSP atrial fibrillation [6▪▪,7▪▪,9▪▪].
LSP atrial fibrillation is characterized by significant changes in atrial substrate, and it is the authors’ opinion that nonpulmonary vein sites play a significant role in triggering and maintaining the arrhythmia, supporting the dismal success reported in these patients when PVAI alone is performed .
Although initially the ablation of the LAA as an additional trigger source for atrial fibrillation was limited to redo cases, we recently presented data of a multicentre prospective study in which patients were divided to PVI along with CFAEs, posterior wall and left atrial septum versus the same lesion sets along with the empirical isolation of the LAA.
The study showed that empirical LAA isolation improved the long-term success rate of catheter ablation of LSP atrial fibrillation. Interestingly, the Kaplan–Meier curves separated after 6 months follow-up, indicating that longer follow-up is important when evaluating the role of the LAA isolation [29▪▪].
Previously, in a series of 987 patients (71% nonparoxysmal atrial fibrillation) undergoing redo ablation [10▪▪], we reported that 27% of patients with demonstrated pulmonary vein isolation showed firing from the LAA at baseline or after administration of isoproterenol, and that in 8.7% of the patients the LAA was the only source of arrhythmia. We showed that complete isolation and not focal ablation was the best strategy to improve outcomes at follow-up [10▪▪].
The technique we utilize for LAA isolation is similar to the one utilized for PVAI, although more ablation time is required to achieve LAA isolation when compared with isolation of pulmonary veins.
About 30 min of radiofrequency energy is necessary to achieve isolation. Importantly, as the LAA has a thin wall, caution should be taken when isolating this structure to avoid high-contact pressure (Fig. 7b–e).
As for the pulmonary veins, we place the circular mapping catheter at the ostium of the LAA guided by ICE, and we deliver radiofrequency applications targeting the earliest electrograms activation on the circular mapping catheter. It is important to realize that, when in atrial fibrillation, it is hard to identify the earliest activation on the circular mapping catheter, thus resulting in ineffective isolation. On the contrary, in sinus rhythm, the earliest activation is easily recognized and the LAA isolation is relatively easier.
For those who are concerned about the mechanical function of the LAA after isolation and the long-term need for anticoagulation, our experience has shown a preserved contractility by TEE 6 months after the LAA isolation in about 40–45% of patients.
This may be possible due to LAA reconnection or to a passive contraction of the LAA following left ventricular contraction. As in some redo cases with LAA not reconnected LAA mechanical function was within the normal range at TEE, we believe that isolation of the LAA does not completely alter its contractility. For patients with reduced LAA contractility, we believe that long-term anticoagulation is necessary.
For patients who prefer long-term anticoagulation discontinuation, an appendage closure device could be considered.
As data so far are mostly limited to our experience, randomized trials are necessary to verify the clinical relevance of LAA isolation in patients with LSP and its consequences with respect to potential complications.
The Effect of Empirical Left Atrial Appendage Isolation on Long-term Procedure Outcome in Patients With Persistent or Long-standing Persistent Atrial Fibrillation Undergoing Catheter Ablation study (NCT01362738), a randomized controlled trial comparing the ablation outcome in patients with LSP atrial fibrillation undergoing empirical LAA isolation along with our standard approach versus our standard approach without LAA isolation, will probably answer this issue.
OUR APPROACH: POSTPROCEDURAL MANAGEMENT AND FOLLOW-UP
After the procedure, heparin is partially reversed with protamine (usually 30–50 mg).
All patients receive a single dose of aspirin (325 mg) and continue their warfarin dosage regimens to maintain a target INR of 2–3. All patients undergo an echocardiogram to rule out pericardial effusion 6 h following the procedure [13▪▪]. We noticed that the use of an open-irrigated tip catheter in patients with LSP, due to the extensive tissue burned, may result in fluid overload and/or pulmonary congestion, as up to 5 l of fluid can be administered. Therefore, it is our habit to monitor the fluid intake during each procedure. Between 40 and 60 mg of furosemide is routinely used immediately after the procedure, with an additional 80 mg of intravenous furosemide and 40 mg of potassium the morning following the procedure and an oral prescription of furosemide 80 mg twice a day for 2 or 3 days after discharge. In addition, patients are asked to monitor their body weight and to immediately contact the assigned atrial fibrillation nurse if the body weight does not reach the preprocedure value.
Since 2012, we have switched the ablation catheter from the standard thermocool open-irrigated catheter to the surround flow catheter. This catheter is able to apply the same energy as the standard thermocool, but utilizing half as much fluid. We found a statistically significant difference in the fluid intake at the end of the procedure and a median of 1.5 versus 4 l (P < 0.005).
All patients are strictly monitored prior to discharge after a single overnight stay using symptom assessment, neurological examinations and puncture site checks. Patients are usually discharged on their previously ineffective AADs, with the exception of amiodarone, which is never restarted during the blanking period (12 weeks), and after 8 weeks AADs are discontinued.
In cases of recurrences after the blanking period, patients are first given their previously ineffective AADs and then considered for a redo procedure [10▪▪].
The follow-up is performed at 3, 6, 9 and 12 months after the procedure and every 6 months thereafter, with a 12-lead ECG, and a 7-day Holter monitoring. Patients are given an event recorder for 5 months after ablation and are asked to transmit their rhythm every time they have symptoms compatible with arrhythmias and at least twice a week even if asymptomatic. Any episode of atrial fibrillation/atrial tachycardia longer than 30 s is considered as a recurrence.
Dedicated nurses are in charge of the preprocedure education and the postprocedure follow-up.
In the case of symptoms or complications, patients are asked to contact their assigned nurse and to seek medical attention. In regard to the anticoagulation management in the postprocedural period, we follow a strict protocol. For at least 6 months following the procedure, patients are asked to maintain their anticoagulation with Coumadin, monitoring their INR. After 6 months, oral anticoagulation is discontinued, regardless of the Congestive heart failure, Hypertension, Age >75, Diabetes mellitus and prior Stroke or transient ischaemic attack (CHADS2) score, if patients do not experience any recurrence of atrial tachyarrhythmias, severe pulmonary vein stenosis (pulmonary vein narrowing >70%) and severe left atrial mechanical dysfunction, as assessed by transthoracic and TEE .
Patients with a CHADS2 score at least 1 experiencing early recurrence of atrial fibrillation are maintained on warfarin. In these patients, warfarin is discontinued if there is no atrial fibrillation recurrence in the last 3 months without AADs, and aspirin 81–325 mg is started. In the case of new atrial fibrillation recurrence after warfarin discontinuation in patients with a CHADS2 score at least 1, oral anticoagulation is restarted.
Patients who have undergone LAA isolation are assessed with a TEE 6 months after the procedure to evaluate the left atrium contractility, as discussed previously in the article [10▪▪].
If the left atrium contractility is preserved and the flow velocity in the left atrium appendage is normal (> 0.4 m/s), warfarin is discontinued; otherwise, patients are maintained on warfarin.
In patients with LSP atrial fibrillation, pulmonary vein antrum and posterior wall isolation, although insufficient, represents an essential step to achieve success. In these subsets of patients, identification of triggers outside the pulmonary vein antrum using high dosage of isoproterenol challenge is important to achieve long-term cure.
Termination of atrial fibrillation during ablation does not seem to influence the long-term outcome (Fig. 8).
Conflicts of interest
Dr Di Biase is a consultant for Biosense Webster and Hansen Medical.
Dr Natale has received consultant fees or honoraria from Biosense Webster, Boston Scientific, Medtronic, Biotronik and LifeWatch.
Dr Santangeli has no conflicts of interest.
REFERENCES AND RECOMMENDED READING
Papers of particular interest, published within the annual period of review, have been highlighted as:
- ▪ of special interest
- ▪▪ of outstanding interest
Additional references related to this topic can also be found in the Current World Literature section in this issue (p. 81).
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This study summarizes the importance of extensive ablation to increase freedom from atrial fibrillation in patients with nonparoxysmal atrial fibrillation.
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The first study showing the importance of LAA isolation in addition to standards procedures for long-term freedom from atrial fibrillation, especially in nonparoxysmal atrial fibrillation patients.
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