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Magnetic navigation in adults with atrial isomerism (heterotaxy syndrome) and supraventricular arrhythmias

Irina Suman-Horduna, Sonya V. Babu-Narayan, Akiko Ueda, Lilian Mantziari, Marko Gujic, Procolo Marchese, Konstantinos Dimopoulos, Michael A. Gatzoulis, Michael L. Rigby, Siew Yen Ho, Sabine Ernst
DOI: http://dx.doi.org/10.1093/europace/eus384 877-885 First published online: 25 January 2013

Abstract

Aims We analysed the type and mechanism of supraventricular arrhythmias encountered in a series of symptomatic adults with atrial isomerism undergoing catheter ablation procedures.

Methods and results The study population included consecutive adults with atrial isomerism who had previously undergone surgical repair or palliation of the associated anomalies. Patients underwent electrophysiological study for symptomatic arrhythmia in our institution between 2010 and 2012 using magnetic navigation in conjunction with CARTO RMT and three-dimensional (3D) image integration. Eight patients (five females) with a median age of 33 years [interquartile range (IQR) 24–39] were studied. Access to the cardiac chambers of interest was obtained retrogradely via the aorta using remotely navigated magnetic catheters in six patients. Radiofrequency ablation successfully targeted twin atrioventricular (AV) nodal reentrant tachycardia in two patients, atrial fibrillation (AF) in three, focal atrial tachycardia (AT) mainly originating in the left-sided atrium in four patients, and macro-reentrant AT dependent on a right-sided inferior isthmus in three patients. The median fluoroscopy time was 3.0 min (IQR 2–11). After a median follow-up of 10 months (IQR 6–21), five of the ablated patients are free from arrhythmia; two patients experienced episodes of self-terminated AF and AT, respectively, within one month post-ablation; the remaining patient had only non-sustained AT during the electrophysiological study and was managed medically.

Conclusion Various supraventricular tachycardia mechanisms are possible in adults with heterotaxy syndrome, all potentially amenable to radiofrequency ablation. The use of remote magnetic navigation along with 3D mapping facilitated the procedures and resulted in a short radiation time.

  • Adult congenital heart disease
  • Atrial isomerism
  • Heterotaxy
  • Supraventricular arrhythmia
  • Remote magnetic navigation
  • Twin AV nodes

Introduction

Although atrial isomerism (heterotaxy syndrome) is rare,13 survival into adult life is more frequent as a result of improved surgical techniques and overall management. Adults are prone to develop arrhythmias because of the underlying anatomy, anomalies of conduction tissue, and scarring resulting from surgery.417 Catheter ablation of these arrhythmias can be difficult because of the complex morphology and difficulties in vascular access.

The terminology pertains to the presence of bilateral morphologically right or left atrial appendages, usually in the context of visceral heterotaxy, multiple anomalies of the systemic and pulmonary venous return, and various electrical conduction system disturbances.3,1316 Typically in left isomerism, there is azygos or hemi-azygos continuation of an interrupted inferior vena cava (IVC), with blood from the lower body draining into a left or right superior vena cava (SVC), and the hepatic venous drainage is directly to the left or right-sided atrium. Other frequently associated anomalies include abnormal atrioventricular connection, atrioventricular septal defect, pulmonary stenosis, or atresia,13,12 and abnormal ventriculo-arterial connections.

We analysed the type and mechanism of supraventricular arrhythmias encountered in a series of symptomatic adults with atrial isomerism undergoing catheter ablation procedures and illustrated how to overcome some of the potential difficulties faced by the electrophysiologist in this complex group of anomalies.

Methods

Patient population

The study population consisted of consecutive symptomatic patients with visceral heterotaxy/atrial isomerism who underwent an electrophysiological study in our institution between 2010 and 2012. Tachycardias were documented with the aid of 12-lead electrocardiograms (ECGs), Holter monitoring, or pacing interrogation in those with implanted devices. All patients had previously undergone surgical repair or palliation of the associated anomalies.

Patients with implanted devices underwent pre-ablation imaging studies using non-contrast cardiac magnetic resonance or computer tomographic scans. Informed consent was obtained prior to the study and all anti-arrhythmic medication was discontinued for at least five half-lives before the procedure. Each was performed under general anaesthesia using intravenous propofol.

To simplify the anatomical description and to avoid confusion, for the azygos venous system, a left-sided vein was described as ‘hemi-azygos’, while on the right we used ‘azygos’. Remote-controlled mapping and ablation were undertaken using the magnetic navigation system (Niobe II, Stereotaxis Inc.) in conjunction with the three-dimensional (3D) electroanatomical mapping system CARTO (Biosense Webster); a detailed description has been published previously.1820 A multipolar diagnostic catheter was positioned in the lateral tunnel in patients who had undergone surgical palliation using total cavo-pulmonary connection (TCPC)17 or in the hemiazygos/azygos in patients with interrupted IVC, to record left atrial far-field signals and serve as a timing reference for the electro-anatomical mapping system. In addition, a quadripolar catheter was placed retrogradely in the ventricle. The irrigated-tip ablation catheter (Navistar RMT Thermocool, Biosense Webster) was placed in the atria antegradely when possible or via a retrograde arterial approach, and manipulation was performed entirely by remote control.

Statistical analysis

Values were expressed as median with first to third quartiles range. Due to the small number of patients, no comparative statistical analyses were performed.

Results

The study group consisted of eight patients with a median age of 33 years [interquartile range (IQR) 24–39] and included five females (Table 1). One patient had right (RAI) and seven had left atrial isomerism (LAI). Associated cardiac anomalies are detailed in Table 1. Peri-procedural parameters are illustrated in Table 2.

View this table:
Table 1

Clinical characteristics

PatientGenderAgeType of atrial isomerismAssociated anomalies and surgical variants
#1F21RAIBiventricular AV connection
Double outlet right ventricle
Bilateral SVC
Left IVC
Hepatic veins to common atrium
AVSD
Dextrocardia
TCPC with incorporation of hepatic veins
#2F27LAIBiventricular AV connection
Double-outlet right ventricle
Hemi-azygos continuation of IVC to left SVC
Absent right SVC
Hepatic veins to left-sided atrium
AVSD
Left Glenn shunt
#3M19LAIBiventricular AV connection
Concordant ventriculo-arterial connection
Hemiazygos continuation of IVC to left SVC
Absent right SVC
Hepatic venous drainage to right-sided atrium
Left SVC baffled to right-sided atrium
#4M31LAIBiventricular AV connection
Concordant ventriculo-arterial connection
Bilateral SVC
Azygos vein to right SVC
Right SVC to coronary sinus
Left SVC to left-sided atrium
Ventricular septal defect
Dextrocardia
RV–PA homograft conduit
#5F46LAIBiventricular AV connection
Concordant ventriculo-arterial connection
Bilateral IVCs and SVCs
AVSD
Septation of the common atrium
#6F37LAIBiventricular AV connection
Concordant ventriculo-arterial connection
Hemiazygos continuation to left SVC
Left SVC to coronary sinus
Absent right SVC
Hepatic veins to right-sided atrium
AVSD
AVSD repair
Surgical epicardial biatrial Maze
Excision of the left-sided atrial appendage
#7F41LAIBiventricular AV connection
Double outlet right ventricle
Hemiazygos continuation to left SVC
Bilateral SVC
Common AV valve
Ventricular septal defect
TCPC (bilateral cavopulmonary shunt; hepatic veins via intra-atrial lateral tunnel to right pulmonary artery)
#8M36LAIUniventricular AV connection
Double outlet right ventricle
Hypoplastic left ventricle
Pulmonary stenosis
Hemiazygos continuation to left SVC
Bilateral SVC
Hepatic veins to left-sided atrium
Large ASD
Ventricular septal defect
Bidirectional Glenn shunts
  • AV, atrioventricular; AVSD, atrioventricular septal defect; IVC, inferior vena cava; LAI, left atrial isomerism; RAI, right atrial isomerism; SVC, superior vena cava; TCPC, total cavopulmonary connection.

View this table:
Table 2

Electrophysiological diagnoses, peri-procedural parameters, and follow-up duration

PatientType of tachycardiaCycle length (ms)Procedure duration (min)Radiation time (min)Radiation exposure (cGym2)Follow-up (months)
#1Twin AV nodal tachycardia3504402.5197.128
#2Twin AV nodal tachycardia4703143.516618
#3Macroreentrant AT2802232.4130.510
#4Focal AT270/250/250/2201451.622.18
#5AF33014.696024
Macroreentrant AT28035420.31453.8
#6AF2803.1182.36
Focal AT230/246
#7AF2303.3169.15
Focal AT240/260
#8Macroreentrant AT375/4204508.28653
Focal AT560/310
Median (IQR)275 (248–362)314 (227–397)3.0 (2–11)182 (148–912)10 (6–21)
  • AF, atrial fibrillation; AT, atrial tachycardia; IQR, interquartile range.

In all but two patients (#3 and #5), the magnetic mapping catheter was advanced retrogradely in the atrial chambers via an arterial approach. In patient #3 (Table 1), the systemic venous return was re-routed by baffling the left SVC to the right-sided atrium, but a residual connection of the left SVC to left-sided atrium was also present which allowed us to reach the cardiac chambers of interest via a venous access. In patient #5 (Table 1), we gained access to the left-sided atrium via a transseptal puncture; however, this was performed at a posterior and superior site, at the junction between the patch insertion and a small posterosuperior rim of a remnant interatrial septum. To enhance our ability to manipulate the catheters, we elected to map the enlarged left-sided atrium remotely using magnetic navigation.

Evidence of twin atrioventricular nodes

Twin atrioventricular nodal reentrant tachycardia: patients #1 and #2

Patients #1 and #2 underwent electrophysiological studies and radiofrequency ablation for recurrent, haemodynamically poorly tolerated narrow QRS complex tachycardia (Figure 1). The anatomical background is detailed in Table 1 and visually depicted in Figure 2.

Figure 1

(A) Baseline 12-lead ECG in patient #1. The left part of the tracing shows sinus rhythm with normal PR interval and narrow QRS complex with inferior axis. At the end of the tracing, one junctional beat with narrow QRS and indeterminate axis is captured. (B) Clinical tachycardia in patient #1: narrow complex QRS tachycardia with QRS morphology similar to that recorded during junctional rhythm and indeterminate axis; the P′-wave is positive in inferior leads. (C) Baseline ECG in patient #2. Two different QRS complexes are spontaneously captured for different atrial rates. A superior faster supraventricular rhythm is conducted with an inferiorly directed QRS, whereas a slower inferior rhythm likely captures the nearby posterior nodal tissue leading to a superiorly directed narrow QRS complex. (D) Clinical tachycardia in patient #2. The QRS complex is narrow and axis is inferior. P′-wave is negative. Paper speed is 25 mm/s.

Figure 2

(A and B) Three-dimensional reconstruction of a non-contrast magnetic resonance scan in patient #1 with antero-posterior view (A) and postero-anterior view (B). Note dextrocardia. Aorta (AO), displayed in red, is placed anteriorly to the pulmonary artery. The TCPC is shown in purple. The right ventricle (RV) is displayed in yellow and the left ventricle (LV) in light blue, while atrial chambers are light-yellow coloured. (C) A right anterior oblique (RAO) fluoroscopic projection in patient #1 is provided. A decapolar catheter (A multi) is advanced in the lateral tunnel to serve as atrial timing reference while both the magnetic navigation catheter (Map) and the quadripolar diagnostic catheter are advanced retrogradely across the aortic valve and positioned in the atrial chamber and the RV, respectively. (D and E) Three-dimensional reconstruction of a non-contrast magnetic resonance scan in patient #2 with antero-posterior view (D) and postero-anterior view (E). For visual depiction, the subaortic RV is displayed in dark blue and the LV in purple. The left-sided atrial chamber is shown in green and the right in yellow. The aorta (Ao), displayed in red, is anterior and right sided. The pulmonary artery (PA) is blue-coloured and has been anastomosed to the left SVC (LSVC), shown in grey (Glen shunt). In the posterior view, the azygos vein (Azy) is displayed in purple, in continuation to the left SVC in grey, and the hepatic veins shown in light-yellow drain into the left-sided atrial chamber (LA) (green).

Two different narrow QRS complexes (Figure 1) were captured in both patients, each preceded by its own His potential (Figure 3). Sequential mapping performed around the common AV junction during spontaneous rhythm, atrial pacing, or short bursts of tachycardia allowed us to identify two different AV nodes interconnected by a sling of conduction tissue (Figure 3); the sling was located by high-frequency potentials preceding the local ventricular potential.

Figure 3

A right lateral (RL) view in patient #2 is illustrated. The Mönckeberg sling labelled by white dots is bordered superiorly and inferiorly by the two AV nodes, coloured in yellow. The right ventricle (red) is made more transparent to facilitate the visualization of the ventricular component of the AVSD. The Mönckeberg sling borders the ventricular component of the AVSD as shown. Top and bottom inserts: the mapping catheter, placed at the superior and inferior aspects, respectively, of the common AV junction during sinus (top) and junctional (bottom) rhythms, records a small His-bundle potential (arrowed).

The mechanism of the tachycardias was reentrant with a circuit established between two distinct AV nodes. In each case, at least one AV node exhibited bidirectional conduction properties (antero-superior AV node in patient #1 and postero-inferior AV node in patient #2) and represented the antegrade limb of the circuit, whereas the remaining AV node represented the retrograde limb. The antegrade properties of the two AV nodes were very similar in both patients and for stability reasons, the postero-inferior AV node was targeted with radiofrequency current, completely abating conduction across the targeted AV node.

Evidence of twin atrioventricular nodes without twin atrioventricular nodal reentrant tachycardia: patient #3

In patient #3 with LAI (Table 1), retrograde conduction was absent both at baseline and during isoprenaline infusion. Two completely separate electrical systems were identified on each side of the septum; the right AV node connected to the right branch resulting in a left bundle branch block QRS morphology and the left AV node connected to the left branch causing a right bundle branch block QRS morphology (Figure 4). Each QRS complex was preceded by its own His deflection during spontaneous supraventricular rhythms.

Figure 4

(A) Baseline ECG in patient #3. The rhythm was driven by a junctional rhythm with a cycle length of 1250 ms dissociated from an atrial escape/slow sinus rhythm with a cycle length of 1700 ms. In lead V1, QRS has a complete left bundle branch block morphology. When the AV node and ventricular refractoriness allows it, occasional antegrade capture of timed atrial/sinus beats occur, conducted with the same QRS morphology (25 mm/s). (B) During atrial pacing at a fast constant cycle length of 330 ms when 1 : 1 AV node conduction across the right AV node (left bundle branch block morphology) is lost, 1 : 1 AV node conduction across the left AV node with long AV conduction time occurs, leading to a QRS complex with right bundle branch block morphology (50 mm/s). (C) A left lateral view (LL) of a reconstructed 3D scan in patient #3 is provided in the middle panel. The left SVC (LSVC, light purple) is connected to the right-sided atrium (RA, in light blue) via the inter-atrial baffle (green) and via a residual connection to the left-sided atrium (labelled LA, in beige). The subaortic (purple) and subpulmonary (dark green) ventricles are rendered more transparent such that the two separated electrical systems labelled by yellow dots are visible on each side of the septum. Examples of right and left His-bundle potentials recorded by the mapping catheter (Map), preceding the two different QRS complexes during spontaneous junctional rhythms are displayed on each side of the picture (arrowed). Ad and Ap represent the distal and proximal poles of the diagnostic catheter placed in the right-sided atrium. Map d, Map p and Map u are the distal, proximal, and unipolar recordings of the magnetic mapping catheter. Paper speed is 50 mm/s.

Macroreentrant atrial tachycardia: patients #3, #5, and #8

In patients #3 and #8, the index tachycardia was a right inferior isthmus-dependent macroreentry, which was mapped and successfully ablated in the right-sided atrium by deploying an ablation line across the right inferior isthmus bordered by the tricuspid annulus anteriorly and the confluence of the hepatic veins posteriorly. Patient #5 presented with paroxysmal palpitations 3 months after ablation for persistent atrial fibrillation (AF) and the pacemaker interrogation revealed paroxysms of regular supraventricular tachycardia with a cycle length of 280 ms (Table 2); the latter was proven to be a right inferior isthmus-dependent tachycardia which was successfully ablated conventionally by deploying a line of block between the tricuspid annulus and the right-sided IVC. Re-isolation of the right pulmonary veins using remote magnetic navigation was subsequently performed during the same procedure, as detailed below.

Atrial fibrillation and focal atrial tachycardia: patients #4–8

In patient #4 (Table 1), only non-sustained atrial tachycardias (ATs) with variable cycle length suggesting a focal mechanism were induced; no ablation was performed therefore, and he was managed medically. The mean cycle lengths of the four different ATs induced are detailed in Table 2.

Three female patients with LAI underwent electrophysiological study and radiofrequency ablation for persistent (patient #5) and paroxysmal AF (patient #6 and #7; Table 1).

In patient #5, ipsilateral pulmonary vein isolation as confirmed by a circumferential mapping catheter was achieved using sequential irrigated-tip radiofrequency applications; this resulted in the termination of AF during the isolation of the left veins. A second ablation procedure was performed 3 months later for paroxysmal right isthmus-dependent macroreentrant tachycardia; on the same occasion, re-isolation of the septal veins and closure of a gap at the anterior aspect of the line around the lateral pulmonary veins was equally performed using the remote magnetic navigation system.

Isolation of the common ostium of the four veins was also performed by deploying a box-shaped ablation line in patients #6 and #7 (Table 1), after having obtained access to the left-sided atrium retrogradely, via a femoral arterial puncture. The AF converted during ablation into stable focal ATs with cycle lengths of 230 and 240 ms, respectively, in both patients, which were mapped and successfully ablated at the posterior aspect of the remnant stump of the left-sided atrial appendage in patient #6, and at the ostium of the right superior pulmonary vein in patient #7. Subsequently induced AT with a cycle length of 246 ms in patient #6 was mapped and ablated at the anterior aspect of the mitral annulus.

In patient #8, two focal ATs induced by atrial pacing after ablation of index tachycardia were mapped and successfully ablated at the posterior wall and the roof of the left atrium, respectively. Sustained AF was also induced by atrial pacing during the electrophysiological study requiring direct current cardioversion. In the absence of documented AF, we did not consider performing pulmonary vein isolation in this particular patient.

No major complications were recorded; one patient (#6) developed groin haematoma immediately post-procedure which necessitated manual compression and subsequent thrombin injection of a pseudo-aneurism of the femoral artery, consequence of the arterial puncture for retrograde arterial access. Complete resolution was noted on Doppler ultrasound follow-up at 1 month.

Follow-up

After a median of 10 months of follow-up (IQR 6-21), all patients but two (#3 and #6) are symptom free and no significant arrhythmia burden was noted during routine Holter monitoring, 12-lead ECG or pacemaker interrogation. Patients #3 and #6 experienced two paroxysmal episodes of AT/AF, respectively, within the first month post-procedure.

Discussion

In our study on adult patients with atrial isomerism, the types of supraventricular tachycardias encountered were twin AV ‘nodal’ reentrant tachycardias, atrial macroreentry, focal tachycardia, and AF; they were related to the underlying cardiac condition, previous surgery, and longstanding haemodynamic burden. Radiofrequency ablation was feasible in all arrhythmia substrates, using advanced technologies including remote magnetic navigation and 3D image integration.

Types of supraventricular arrhythmias

The present report describes two ‘mirror’ cases of twin AV nodal reentrant tachycardia with circuits established between paired or ‘twin’ AV nodes, to our knowledge, the first to be investigated and treated remotely using the magnetic navigation system and 3D electro-anatomical mapping.

Duplication of the electrical conduction system is commonly seen in these patients. The presence of paired sinus nodes and twin AV nodes3,1316 connected by a sling of conduction tissue21 is common with RAI. In patients with LAI, however, the sinus node is usually hypoplastic, absent, or abnormally positioned,13 with subsequent cardiac rhythm driven by an atrial or junctional escape rhythm; the AV node can be solitary or duplicated.

With complete AV septal defect, the AV node is usually abnormally located in a postero-inferior position, but additional nodal tissues may be found when there are abnormalities of looping and connection.14 The presence of a dual AV node system connected by a sling of conduction tissue on a background of an AV septal defect, sets the conditions for developing reentry between the two nodes, each playing the role of either antegrade or retrograde limb of the circuit. We have also been able to track the Mönckeberg sling21 for the first time by following the Purkinje-like potentials along the margins of the ventricular component of the AV septal defect.

Interestingly, in the third patient, the two AV nodes were completely separated and located on either sides of the interatrial septum, each connected to its own electrical distal system, leading to two different wide QRS complexes, of left and right bundle branch block morphology, respectively. The absence of the necessary anatomical substrate, such as an AV septal defect, and the absence of the retrograde conduction prevented the occurrence of twin AV nodal reentrant tachycardia in this particular patient.

In our series of adult patients with atrial isomerism, several other types of supraventricular tachycardias were demonstrated, all potentially amenable to ablation. Data in the literature regarding the actual incidence of these types of arrhythmia in adults are scarce. With RAI, previous studies have shown a propensity for supraventricular tachycardias varying between 1810 and 26%,6,7 whereas in patients with LAI, up to 54% of the patients may develop bradyarrhythmia by the age of 6,22 while the incidence of supraventricular arrhythmias may be as high as 23%.23

Interestingly, our patients with AF had LAI and were the oldest of the group. Similarly, in a recent retrospective series of 83 patients with LAI,23 supraventricular arrhythmias occurred in 23% of the patients, and AF was documented in nine. Older patients in the cohort and those with sinus node dysfunction were more likely to develop arrhythmias on follow-up. However, from our data in a limited number of patients, definitive conclusion with regard to any relationship with age or appendage morphology remains uncertain.

In our series, index ATs were macroreentrant peri-triscuspid tachycardias dependent on the right inferior isthmus, whereas focal tachycardias were subsequently induced during the electrophysiological study with various sites of origin identified, mainly in the left-sided atrium, such as pulmonary veins ostia, appendage, mitral annulus, or roof.

Benefits of magnetic navigation

One of the major challenges in these complex cases is the difficulty in entering the relevant cardiac chambers.

In the presence of TCPCs, access to the right-sided part of the heart is restricted unless a transbaffle puncture is performed or there is a baffle leak. We have recently shown that in patients with intracardiac baffles, the retrograde arterial approach using magnetic navigation is a valid option with a high chance of success, low risk, and limited fluoroscopy exposure.20 Alternative solutions in these cases might include, depending on the operator's preferences and experience and/or the available equipment, the retrograde arterial route20 or percutaneous puncture of the hepatic,24 internal jugular, or subclavian veins.25 In the first two cases described in this report, we elected to use the retrograde approach in conjunction with magnetic navigation which allowed us to improve the manoeuvrability and the anatomical reach of the mapping catheter.

In cases with prior surgical septation of the atria, access to the left-sided chambers can be achieved by transseptal puncture through the patch, puncture across the residual rim of the interatrial septum if present, or retrograde arterial access using remotely navigated ablation catheters.26 In the case described here (#5), due to an inability to cross the two dacron layers of the interatrial septum, we punctured the remnant native septum which led us into a superior and posterior angle restricting the manipulation of a conventional catheter in the left-sided atrium and carrying a risk of cardiac perforation. Again, use of the magnetic navigation system facilitated reaching all sites within the left-sided atrium with minimal risk of perforation.

In situations where the entire venous return is to the coronary sinus via a persistent left or right SVC, such as present in patient #6, the antegrade or retrograde access to the left-sided atrium is very difficult using conventional catheters. Remote navigation of the soft magnetic catheters retrogradely allows one to reach the left-sided atrium in these particularly challenging cases without the need for transhepatic catheterization.

Limitations

This is a single-centre experience that illustrates several ways of overcoming some of the difficulties encountered during the electrophysiological procedures in adults with heterotaxy syndrome. However, the limited number of patients and the extreme heterogeneity of the underlying anatomy and surgical procedures do not allow us to extrapolate the electrophysiological findings to a larger population.

Conclusion

A variety of supraventricular arrhythmia substrates are found in adults with atrial isomerism/heterotaxy syndrome. Unusual tachycardia mechanisms can be sometimes identified, such as twin AV nodal reentrant tachycardias. Successful radiofrequency ablation depends on the complete understanding of the cardiac morphology and the surgical procedures performed, as well as the specific properties of the electrical conduction system. The use of advanced imaging techniques with remote magnetic navigation facilitated the procedures and resulted in a short radiation time.

Funding

The study was supported by the NIHR Cardiovascular Biomedical Research Unit at the Royal Brompton and Harefield NHS Foundation Trust and Imperial College London. This report is independent research by the National Institute for Health Research Biomedical Research Unit Funding Scheme. The views expressed in this publication are those of the authors and not necessarily those of the NHS, the National Institute for Health Research or the Department of Health. S.B.-N. is supported by the British Heart Foundation. A.U. is supported by a fellowship from Fukuda-Denshi and St Jude Medical. L.M. is supported by grants from the European Heart Rhythm Association and the Hellenic Cardiological Society.

Conflict of interest: S.E. is a consultant for Stereotaxis Inc. and Biosense Webster. The other authors have no conflicts of interest to declare.

References

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