Europace Advance Access originally published online on February 21, 2006
Europace 2006 8(4):225-230; doi:10.1093/europace/euj026
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ELECTROPHYSIOLOGY
Magnetic navigation in AV nodal re-entrant tachycardia study: early results of ablation with one- and three-magnet catheters
Clinical Electrophysiology UnitDepartment of Cardiology, Thoraxcentre, Erasmus MC, Dr Molewaterplein 40, 3015 GD Rotterdam The Netherlands
Manuscript submitted 19 December 2004. Accepted after revision 8 September 2005.
* Corresponding author. E-mail address: a.thornton{at}erasmusmc.nl
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Abstract |
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Aims Steering soft, flexible catheters using an external magnetic field could have advantages for heart catheterization, especially for therapy of tachyarrhythmias. Our aims were to assess the feasibility of magnetic navigation to Koch's triangle and reliable ablation of atrioventricular nodal re-entry tachycardia (AVNRT) with a magnetic catheter.
Methods and results Consecutive patients with AVNRT were mapped and ablated with a magnetically enabled catheter (Helios I or II), with, respectively, one and three magnets at the tip. The catheter was remotely advanced with the CardiodriveTM system and orientated with the NavigantTM control system. After initial positioning with the external magnets, adjustment was made in 5° steps. Success rates, procedure, and fluoroscopy times were analysed, and compared with a local contemporary series of conventional AVNRT ablations. Magnetic navigation was feasible in all 20 patients. Targets were easily reached. Catheters remained stable in position during accelerated junctional rhythms. Ablation was successful in 18/20 procedures (90%). No significant complications occurred. Median patient fluoroscopy time was 12 min, median physician fluoroscopy time was 4 min. Fluoroscopy times tended to be shorter than that in the conventionally treated group. Procedure duration decreased significantly over time, median procedure time was similar to that in the conventional group.
Conclusion AVNRT can be successfully mapped and ablated using magnetic navigation. A learning curve was evident, unrelated to catheter type, but to increasing operator experience. Physician radiation times were one-third of patient times. No complications occurred. Procedure time is comparable with that of conventional ablation.
Key Words: Arrhythmias, AV nodal re-entrant tachycardia, Catheter ablation, Magnetic navigation, Stereotactic therapy
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Introduction |
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Although catheter design in electrophysiology has undoubtedly improved the handling of standard steerable diagnostic catheters, these are still fairly stiff, and have to be manoeuvred by means of a catheter handle, from outside the vascular system and the body. Standard catheters can be rotated manually and, by use of a pull-wire or wires, flexed and extended as necessary. Manipulation of these catheters to certain targets can be almost, or totally impossible, especially in complex anatomy. Application of manual force to ensure reliable contact with the myocardium is associated with a certain risk including perforation,1
especially in some locations, such as the left atrial appendage, or in structurally weaker tissue, such as aneurysms. Maintenance of a constant force can also be extremely difficult in a moving structure such as the heart. The ability to steer a soft, flexible catheter by the use of an external magnetic field may, therefore, have certain fundamental advantages. Studies have shown the ability of magnetic navigation to place catheters in difficult anatomical positions and to maintain good tissue contact throughout the cardiac and respiratory cycles, without additional risk of perforation.2,3 This new technology to map and ablate atrioventricular nodal re-entry tachycardia (AVNRT) has been used in only a relatively limited number of patients.3–5 In the largest series to date by Ernst et al.,4 only a single-magnet catheter was used. We report our initial experience in 20 consecutive cases of AVNRT, 11 with a single-magnet catheter and the other 9 with a similar three-magnet catheter. The goal was to study the feasibility of magnetic navigation to map a well-known region in the heart and to assess the possibility of reliable ablation of AVNRT in both catheter types.
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Methods |
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Patient population
Between February and August 2004, we enrolled a total of 20 consecutive patients with proven AVNRT at electrophysiological study. An additional two patients gave consent, but were found to have left-sided arrhythmias. No patient had a contraindication to the use of magnetic navigation (such as an implanted metallic device or claustrophobia). This study forms part of the safety and efficacy study approved by the Ethics Committee of the Erasmus Medical Centre. We compared our ablation data, results, procedure, and fluoroscopy times with those obtained in a contemporary group of 17 patients treated in 2004 for AVNRT, immediately before and after this study.
Electrophysiological study
Patients were studied in the fasting, post-absorptive state under light sedation using intravenous boluses of diazepam and fentanyl as necessary. A standard conventional, diagnostic, electrophysiological study was performed using three transvenous catheters, placed from the subclavian and femoral veins in the coronary sinus, right ventricular apex, and the His bundle region. Standard techniques were used to measure conduction properties and to induce tachycardia. In all patients, typical AVNRT was induced either before or after the use of isoprenaline. The presence of dual AV-nodal conduction was assessed, and accessory pathways were excluded. Once the diagnosis was confirmed, an additional femoral access sheath was used to manually advance a magnetic navigation ablation catheter with a 4 mm tip to the right atrium. Catheters used for ablation were the 8 Fr Helios I, with a single magnet of 1.8 mm at the tip, or Helios II, with three 1.8 mm magnets at the tip and the distal shaft (Stereotaxis Inc., St Louis, MO, USA). The choice was determined by availability. Electrograms were recorded using a standard recording system (Sensis, Siemens, Erlangen, Germany).
Magnetic navigation
The Niobe magnetic navigation system (Stereotaxis Inc.) is combined with a monoplane fluoroscopy system (AXIOM Artis, Siemens). The Niobe system consists of two permanent magnets situated on either side of the patient, which are computer-controlled via a workstation (Navigant, Stereotaxis Inc.) to allow for changes in the orientation of a stable magnetic field within the chest of the patient. A combined field strength of 0.08 T is produced in navigation mode. As navigation is best performed with a fixed table position, this must be optimized prior to the start of magnetic navigation. When the magnets are positioned next to the patient, only limited angulation of the C-arm is possible (
28° in the right and left anterior oblique angulations). Remote catheter advancement and retraction from the control room was performed using a catheter advancer system, CardiodriveTM (Stereotaxis Inc.), positioned on the high anterior thigh (Fig. 1). Remote control of the fluoroscopy system is also performed from the control room. The ablation catheters, with a single or multiple magnets within the distal tip segment, align themselves with the field produced by the external magnets, allowing effective catheter orientation. After the magnets are brought in next to the patient, the physician is free to leave the room and performs the rest of the procedure from the control room. A single nurse remains with the sedated patient to monitor vital signs and administer drugs when required.
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Mapping and ablation
Mapping of the low right atrial septum in the region of the coronary sinus os (Koch's triangle) was performed combining an anatomical approach with an attempt to obtain electrograms suggestive of a slow pathway potential. Radiofrequency ablation was delivered via the 4 mm tip of the Helios I or II catheter using temperature-controlled ablation from an EP Shuttle RF generator (Stockert GmbH, Freiburg, Germany) with settings of 55°C, 60 s and power titrated from 15 to 50 W to obtain the set temperature. Radiofrequency applications were stopped after 20 s if no accelerated junctional rhythm was seen. These applications were included in the total number of applications. The generator was also operated remotely from the control room allowing the electrophysiologist to manage all aspects of the procedure.
Endpoint for ablation
The endpoint for ablation was non-inducibility of tachycardia using standard electrophysiological manoeuvres and evidence of slow pathway modification or ablation in those where this was possible. Isoprenaline was given after all apparent successful applications. Manoeuvres were repeated after a waiting period of 30 min. Procedure times were defined from the time the patient was put on the table until removal of the sheaths, 30 min after the last application, and include the time for the standard diagnostic electrophysiological study. Fluoroscopy times were measured for the patient and separately for the physician.
Statistical analysis
All continuous variables are expressed as mean±standard deviation. When appropriate, median values are reported and non-parametric tests were used.
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Results |
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Patient data
The patient group consisted of 4 males and 16 females, with a mean age of 54±12 years. Apart from inducible typical AVNRT, a jump in the AH interval of >50 ms was detected in 16 of the 20 patients. At baseline, isoprenaline was necessary for reproducible induction in 4/20 patients. There were no significant differences in clinical data between the study group and the conventional treatment group, which consisted of 17 patients, who were treated with a standard approach (using either standard cryotherapy or radiofrequency catheters).
Remote navigation and mapping using the magnetic navigation system
We were able to obtain a satisfactory position using the advancer and magnetic navigation in all 20 patients (Fig. 2). This implied that the initial target was situated in the low septum, at the level of the coronary sinus os. Modification of the position was based on electrogram criteria such as the AV relationship and the presence of potentials. Adjustment was made magnetically in 5° steps, laterally and vertically.
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Ablation data
Ablation was acutely successful in 18/20 patients. Junctional rhythm was seen in all patients. During this rhythm, the catheter remained entirely stable, as assessed on fluoroscopy and assessment of the intracardiac electrograms (Fig. 3). In 16 patients, clear dual AV nodal physiology, as shown by a jump in AH interval, was present at some time during the procedure. After ablation, no further slow pathway conduction could be demonstrated in 11 of 15 (73%) successful cases, whereas only modification of the slow pathway was obtained in the other four patients in whom this could be completely assessed. There was no difference on the basis of catheter type.
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In the two failed patients, 22 and 15 application attempts were made, respectively; also there was no difference in the incidence of junctional rhythm. In one of these patients, in whom the area of interest appeared close to the region of the His bundle and in whom RF application was associated with fast accelerated junctional rhythm, a transient, minor delay in AV conduction was observed. The transient AV delay was thought to be due to the initial catheter position and not to catheter displacement. Cryocatheter ablation in the same position was associated with slow pathway ablation without impairment of AV conduction. In the other, with a very large coronary sinus os, cryotherapy failed.
The median number of RF applications in the successful cases was 6 (range 1–22). The total RF ablation time was a median of 240 s (range 60–633 s). There was no significant difference between the single- and three-magnet catheter approach.
Procedure data
The mean procedure time (including the waiting time) was 167±46 min (median 163 min) with a patient fluoroscopy time of 17±12 min (median 12 min). Of the total fluoroscopy duration, the physician was exposed only during introduction of the sheaths and positioning of the catheters, which averaged 5±2 min (median 4 min). The evolution of the procedure and fluoroscopy times is given in Fig. 4. The procedure time decreased significantly in a linear fashion (R2=0.2541). There was no significant difference in procedure and fluoroscopy time between those patients in whom the single-magnet catheter or the three-magnet catheter was used.
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Comparison with the control group
Table 1 shows a comparison of ablation data, results, procedure, and fluoroscopy times between the magnetically treated group as a whole, and broken down into one- and three-magnet groups, and the non-magnetically treated patients. The control group had the same ablation settings as those used in the magnetically treated group. No significant differences were observed between magnet and conventional groups or between the one- and three-magnet groups.
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Follow-up results
No complications related to the advancer or the magnetic system occurred. Neither AV block nor distal conduction disturbances were observed. AH and HV intervals remained comparable. One complication, a pectoral haematoma, was a sequel of the subclavian puncture and did not require intervention. During a follow-up of 90 days, there have been no recurrences of AVNRT in the 19 (18 radiofrequency and 1 with additional cryotherapy ablation) successful procedures.
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Discussion |
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Although AVNRT can be successfully ablated with success rates of the order of 95% and with a 1–2% risk of complete AV block,1
,6 there are other catheter-based procedures which are much more difficult because of anatomical and technical issues. Patients with corrected congenital heart disease and patients requiring left atrial procedures for atrial fibrillation therapy are good examples. The possibility of using floppy catheters with new technology in association with magnetic navigation and remote ablation is therefore appealing.
We have shown that using this technology we can achieve success rates equalling those with more conventional technology without significant complications and with reasonable patient radiation exposure.
Procedure time
It is important to note that our procedure times are not from initial sheath placement, but from time on table to removal of sheaths; this adds approximately 15–20 min. Procedure time also includes the time for the initial diagnostic study and an observation period of 30 min after the last application.
This explains the difference with a prior study using magnetic navigation performed in the same substrate.4 However, the procedure time was comparable with that in our control group. The time to set up the advancer was short, and it would have been interesting to log the duration of different parts of the procedure to have a better understanding of the time effects of this new technology.
Fluoroscopy time
Of great significance is the fact that in a procedure with relatively little radiation the physician radiation time was less than one-third of that of the patient. Although individual patient radiation exposure is not increased in comparison with other trials, it is evident that physician dosage is significantly decreased. In more complex procedures with longer exposure times, this will be of even more significance.
Part of the somewhat long radiation time relates to initial unease about the stability of the catheter during ablation and junctional rhythm. The time did not decrease as might be expected, because of the second learning curve due to the transition to three-magnet catheters. We wished to check on the stability of these catheters as well.
Catheter stability
It must be noted that catheter position was extremely stable when compared with standard catheters, even when patients tended to take deep breaths, and during accelerated junctional rhythm.
One-magnet vs. three-magnet catheters
A clear learning curve is evident. There was no apparent benefit in this ablation situation with a three-magnet when compared with a single-magnet catheter. This may reflect partly the learning curve, but probably the relative ease with which a catheter can be placed in the slow pathway region.
The orientation of the three-magnet catheter appears somewhat different. With all three magnets orientated in the field, the distal part of the catheter is straighter than with the single-magnet catheter. In one patient, this did seem to cause some instability of the catheter position as the straightening caused the more proximal portion to buttress against the IVC–RA junction. The catheter tended to move backwards and forwards over the annulus during respiration. This was not a noticeable problem in the other patients.
It is possible that in more complex anatomy and more challenging positions the three-magnet catheter may perform better than the one-magnet version.
Advantages in AVNRT
The slow pathway region is easy to reach and mappable with the magnetic navigation system. The success rate is in the range expected with conventional technology. There was no incidence of permanent AH or PR prolongation. We had no evidence of any fast pathway modification. The high ratio of slow pathway ablation vs. modification compares favourably with that seen with cryoablation7 and is the inverse of that seen in the only other published magnetically ablated series.4
Future concepts
Combining standard catheters with systems such as LocaLisa and CARTO resulted in shorter radiation times in prior studies.8,9 It is expected that with the incorporation of catheter registration technology or echocardiography10 into this system, or using these technologies in parallel, procedure and radiation times will further decrease.
Combining this technology with other new technologies, such as cryotherapy and the other technologies mentioned previously, may significantly improve outcome, as well as reduce the number of applications, and associated collateral tissue damage.11
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Conclusions |
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AVNRT can be successfully mapped and ablated using magnetic navigation with comparable risks and benefits as obtained when using standard catheters. A learning curve was evident, not related to the catheter type, but to increasing experience. Physician radiation times were only one-third of patient radiation times and are therefore significantly reduced. No complications occurred. Our results show that magnetic navigation in Koch's triangle is feasible, easy to learn, and can assist in successful ablation.
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Acknowledgements |
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We would like to thank Ms P. Kennedy and Mr H. Meulenbrug (Stereotaxis Inc.) and the group in Allgemeines Krankenhaus, St Georg, Hamburg (Sabine Ernst and Karl-Heinz Kuck) for support while we started the project in Rotterdam. We received an institutional grant from Stereotaxis Inc.
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References |
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[1] Calkins H, Yong P, Miller JM, et al. Catheter ablation of accessory pathways, atrioventricular nodal reentrant tachycardia, and the atrioventricular junction: final results of a prospective, multicenter clinical trial. The Atakr Multicenter Investigators Group. Circulation 1999; 99: 262–70.
[2] Faddis MN, Blume W, Finney J, et al. Novel, magnetically guided catheter for endocardial mapping and radiofrequency catheter ablation. Circulation 2002; 106: 2980–5.
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[5] Ernst S, Ouyang F, Linder C, et al. Modulation of the slow pathway in the presence of a persistent left superior caval vein using the novel magnetic navigation system Niobe. Europace 2004; 6: 10–14.
[6] Hindricks G. The Multicentre European Radiofrequency Survey (MERFS): complications of radiofrequency catheter ablation of arrhythmias. The Multicentre European Radiofrequency Survey (MERFS) investigators of the Working Group on Arrhythmias of the European Society of Cardiology. Eur Heart J 1993; 14: 1644–53.
[7] Skanes AC, Dubuc M, Klein GJ, et al. Cryothermal ablation of the slow pathway for the elimination of atrioventricular nodal reentrant tachycardia. Circulation 2000; 102: 2856–60.
[8] Kopelman HA, Prater SP, Tondato F, et al. Slow pathway catheter ablation of atrioventricular nodal re-entrant tachycardia guided by electroanatomical mapping: a randomized comparison to the conventional approach. Europace 2003; 5: 171–4.
[9] Sporton SC, Earley MJ, Nathan AW, et al. Electroanatomic versus fluoroscopic mapping for catheter ablation procedures: a prospective randomized study. J Cardiovasc Electrophysiol 2004; 15: 310–5.[CrossRef][ISI][Medline]
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[11] Kimman GP, Theuns DAMJ, Szili-Torok T, et al. CRAVT: a prospective, randomized study comparing transvenous cryothermal and radiofrequency ablation in atrioventricular nodal re-entrant tachycardia. Eur Heart J 2004; 25: 2232–7.
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