Author + information
- Received February 14, 2007
- Revision received April 17, 2007
- Accepted May 14, 2007
- Published online August 28, 2007.
- Luigi Di Biase, MD1,
- Tamer S. Fahmy, MD,
- Dimpi Patel, MD,
- Rong Bai, MD,
- Kenneth Civello, MD,
- Oussama M. Wazni, MD,
- Mohamed Kanj, MD,
- Claude S. Elayi, MD,
- Chi Keong Ching, MD,
- Mohamed Khan, MD,
- Lucie Popova, MD2,
- Robert A. Schweikert, MD,
- Jennifer E. Cummings, MD,
- J. David Burkhardt, MD,
- David O. Martin, MD,
- Mandeep Bhargava, MD,
- Thomas Dresing, MD,
- Walid Saliba, MD,
- Mauricio Arruda, MD and
- Andrea Natale, MD⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Andrea Natale, Head, Section of Cardiac Electrophysiology and Pacing, Department of Cardiovascular Medicine, Cleveland Clinic, 9500 Euclid Avenue, Cleveland, Ohio 44195.
Objectives We aimed at assessing the feasibility and efficacy of remote magnetic navigation (MN) and ablation in patients with atrial fibrillation (AF).
Background This novel MN system could facilitate standardization of the procedures, reducing the importance of the operator skill.
Methods After becoming familiar with the system in 48 previous patients, 45 consecutive patients with AF were considered for ablation using the Niobe II remote magnetic system (Stereotaxis, St. Louis, Missouri) in a stepwise approach: circumferential pulmonary vein ablation (CPVA), pulmonary vein antrum isolation (PVAI), and, if failed, PVAI using the conventional approach. Remote navigation was done using the coordinate or the wand approach. Ablation end point was electrical disconnection of the pulmonary veins (PVs).
Results Using the coordinate approach, the target location was reached in only 60% of the sites, whereas by using the wand approach 100% of the sites could be reached. After step 2 ablation, only 1 PV in 4 patients (8%) could be electrically isolated. Charring on the ablation catheter tip was seen in 15 (33%) of the cases. In 23 patients, all PVs were isolated with the conventional thermocool catheter, and in 22 patients only the right PVs were isolated with the conventional catheter. After a mean follow-up period of 11 ± 2 months, recurrence was seen in 5 patients (22%) with complete PVAI and in 20 patients (90%) with incomplete PVAI.
Conclusions Remote navigation using a magnetic system is a feasible technique. With the present catheter technology, effective lesions cannot be achieved in most cases. This appears to impact the cure rate of AF patients.
Over the last decade, radiofrequency catheter ablation of atrial fibrillation (AF) has become an important and increasingly used therapy with good procedural success rates and long-term effectiveness. However, the procedure remains technically challenging, the results are difficult to replicate, and the operator’s experience could have a significant impact on the success rate. Mapping of the cardiac chambers is performed with relatively stiff deflectable catheters that are manually moved by the operator, standing at the table. The operator navigates the catheter to the desired position guided by fluoroscopy or other imaging systems. Recently, a novel magnetic navigation (MN) system was introduced that allows the use of a soft ablation catheter that can be guided and positioned by directional magnetic fields to the desired site (1–6). If feasible, remote MN could facilitate standardization of the procedures, reducing the dependency on the operator’s skill and allowing nonexperienced physicians to treat this arrhythmia. We report our initial experience with remote circumferential pulmonary vein ablation (CPVA) and pulmonary vein antrum isolation (PVAI) using MN in patients with symptomatic and drug-refractory AF.
After performing 48 procedures using MN, the following 45 consecutive patients were included in this study. The protocol was approved by the institutional review board and all patients gave a written informed consent.
The patient arrived to the Electrophysiology and Pacing lab in a fasting state. Access was obtained in the right femoral vein. An 11-F intracardiac echocardiography (ICE) catheter was placed into the right atrium. A 4-mm Carto RMT catheter (Biosense-Webster, Diamond Bar, California) was used in conjunction with the Niobe II magnetic navigation system (Sterotaxis, St. Louis, Missouri) and biplane flat panel fluoroscopy system (Axion Artis, Siemens, Erlangen, Germany).
Remote magnetic navigation and ablation system
The remote MN system consists of 2 focused-field permanent magnets of a neodymium-iron-boron compound that are computer controlled inside a fixing housing and located on either side of the body (1,7). In the “navigate” position they create a relatively uniform magnetic field (0.08 T) of approximately 15 cm inside the chest of the patients. The mapping and ablation catheter are equipped with a small permanent magnet positioned at the tip that aligns itself with the direction of the externally controlled magnetic field to enable it to be steered effectively. By changing the orientation of the outer magnets relative to each other, the orientation of the magnetic field changes and thereby leads to deflection of the catheter. All magnetic field vectors can be stored and, if necessary, reapplied for automatic navigation of the magnetic catheter. In addition, a computer-controlled catheter advancer system (Cardiodrive unit, Stereotaxis) is used to allow truly remote catheter navigation without the need for manual manipulation. The video workstation (Navigant II, Stereotaxis) in conjunction with the Cardiodrive unit allows precise orientation of the catheter by 1-mm or larger increments and by 1-mm or larger steps in advancement or retraction. The system is controlled by coordinate or wand remote control of the ablation catheter from inside the control room. The wand remote control of the catheter allows a single hand control: the orientation of the tip of the catheter is manipulated by the joystick and the advancement or withdrawal can be done by the roller wheel on the side of the wand. The coordinate control of the catheter is a 2-hand remote control: with the joystick it is possible to advance or withdraw the catheter, and the mouse controls the orientation. The cursor is represented as an arrow (vectors) with a circle. A vector consists of a line with a circle at the starting point, and an arrow at the ending point of the vector. The circle contains a dot (representing the arrow tip) if the vector in the 3-dimensional space is pointing out of the screen and a cross (representing the crossed feathers of an arrow) if it is going away from the user into the screen. Vectors are indications of the direction of the magnetic field, as projected onto the respective X-ray image plane (Fig. 1).A vector is drawn to indicate the position from which the field strength should be set and the direction the magnetic field needs to point to orient the distal tip of the magnetic catheter or guidewire.
Using this system, radiofrequency (RF) ablation was performed with the 4-mm solid-tip magnetic ablation catheter in a temperature-controlled mode. The initial RF delivery settings were 55°C with a maximum power of 50 W for 60 s with the use of a Stocker RF generator (Biosense-Webster). However, after charring was observed in a few cases, the RF delivery settings were changed to 55°C, 40 W, for 45 s.
Left atrial mapping and ablation
Left atrial (LA) electroanatomic mapping was performed in 45 patients by using an integrated Carto RMT system (Biosense-Webster) together with the Niobe II remote magnetic system, and LA maps were created using the automatic software tools of the system.
Ablation in the LA followed a stepwise strategy:
Circumferential pulmonary vein ablation of the right and left pulmonary veins (PVs) using the 4-mm solid-tip magnetic ablation catheter and the Niobe II remote magnetic system was performed. A line was designed circumferentially on the constructed electroanatomic map. This line was then sent to the Navigant software and the ablation catheter was then navigated along the course of the designed line, using either the wand (28 patients) or the coordinate approach of the Cardiodrive (17 patients) by direct or targeted navigation.
The accuracy of reaching a target location was evaluated by the ability to exactly follow the intended lesion design. Ablation was applied at each point, after which the ablation catheter was navigated to the following sites along the line. The technique of circumferential ablation in LA has been described elsewhere (4). After ablation, the circular mapping catheter was placed in all PVs to confirm electrical disconnection between the PVs and LA. Pacing from LA or inside PV was used to verify entrance or exit block of the PV, respectively. If the PVs were not isolated, Step 2 ablation was applied.
Pulmonary vein antrum isolation using the Niobe II remote magnetic system (electroanatomic approach) and the integrated Carto RMT system was performed. The circular catheter was placed at the antrum of the targeted PV, guided by ICE. The ablation catheter was then navigated to the targeted sites. Ablation was applied at the targeted site until the potential amplitude decreased by 80% or for at least 45 s. The ablation catheter was then moved to the next targeted site. The circular catheter was moved by the second operator along the antrum, and the ablation catheter was navigated to all locations. The accuracy of reaching a target location was evaluated by the ability to reach the different poles of the circular catheter. This was confirmed by ICE, fluoroscopic imaging, and the noise artifact observed during ablation. The technique of PVAI has been described elsewhere (8). After all sites were ablated, PV isolation was checked again with the circular catheter. If PV disconnection failed, step 3 ablation was applied in all PVs in the first 23 patients and in the right PVs in the remaining 22 patients.
Importantly, once we moved to open irrigation, we performed the PVAI at a more proximal location of the antrum.
Pulmonary vein antrum isolation using the conventional approach under ICE and fluoroscopy guidance was performed. For this step, the magnetic system was removed and ablation was performed using a 3.5-mm-tip open-irrigation catheter (Biosense-Webster) following the conventional protocol described elsewhere (8).
The end point of the study was complete PV isolation (entrance and exit block verified by the circular catheter). The procedure was terminated when the end point was achieved. As mentioned above, manual ablation was performed at a more proximal location than the one targeted during step 2.
All patients were monitored in the hospital overnight. On discharge, patients were given an arrhythmia transmitter for documentation of any arrhythmias. All patients underwent screening computerized tomography (CT) scans before ablation and 3 months after the procedure. Additional CT scans were obtained at 6 months and 12 months if PV narrowing was detected. Patients were also followed up in the outpatient clinic at 6 months and 9 months and as needed.
Continuous data are presented as mean ± SD. One-way analysis of variance for repeated measures with post hoc analysis using Bonferroni comparison was used to compare the ablation and fluoroscopy times among the different methods (steps 1 to 3) used. The difference between the wand and the coordinate has been analyzed by Student ttest. Fisher exact test was used to analyze AF recurrences after step 3 ablation. A p value of <0.05 was considered to be statistically significant.
A total of 45 patients with symptomatic drug-refractory AF were included in the study. Patient demographics are presented in Table 1.
Ablation duration and fluoroscopy times
The total procedural duration was 175.6 ± 32.7 min for patients who had the left and right PVs conventionally isolated and 129.8 ± 46.2 min when only the right veins were ablated conventionally. There were no statistical differences in the ablation times using either the coordinate or the wand approach. The ablation times of the conventional PVAI approach was significantly less than any other method using MN (45.7 ± 5.3 min, 42.2 ± 5.2 min, 43.1 ± 2.8 min, and 41.2 ± 2.3 vs. 37.1 ± 3.4 min; p < 0.001) (Table 2).On the other hand, the fluoroscopy time using the conventional approach was significantly longer than using only the wand approach in either MN method (17.5 ± 2.4 min vs. 8.9 ± 1.9 min or 9.3 ± 2.2 min; p < 0.001) (Table 2). There were no differences between ablation and fluoroscopy times between steps 1 and 2.
During steps 1 and 2 using the coordinate approach, the target location was reached by the catheter in only 64% and 60% (steps 1 and 2, respectively) of the selected sites, whereas by using the wand approach all sites could be reached (100%) (Table 3).
Remote catheter ablation
With the magnetic catheter at the target site, RF energy was delivered from the control room. Electrical disconnection could be attained in only 4 veins in 4 different patients (8%) after step 2 ablation protocols with the wand approach. In the remaining 41 patients (92%), no evidence of disconnection was seen in any vein (Fig. 2).Step 3 was applied to all patients to achieve electrical isolation of the PVs. In the first 23 patients, all PVs were successfully disconnected by conventional ablation, and in the remaining 22 patients only the right PVs were isolated conventionally after attempts of isolation of the left PVs were performed with MN.
No procedural complications were observed during the procedures. Catheter instability with respiration was evident in 4 patients (8%). Charring on the ablation catheter tip was seen in 15 (33%) of the 45 cases (Figs. 3 and 4).⇓⇓
After a mean follow-up period of 11 ± 2 months, 25 patients (55%) had recurrence. Out of the 22 patients who had only right PV isolation by the conventional approach, 20 patients (90%) had recurrence of AF, whereas only 5 patients (22%) out of the 23 patients who had complete isolation with the conventional approach had recurrence (p < 0.001) (Fig. 5).
We report our initial experience with navigation and catheter ablation of AF using a magnetic remote control system. Our results indicate that remote MN in the LA is feasible.
Precise navigation to the target site and catheter stability are prerequisites for effective RF applications (6,9,10). The present results suggest that the use of the wand approach facilitates navigation to the desired locations.
Of interest, the rate of PVs that remained electrically connected was 100% after step 1 and 92% after step 2. It is likely that the 4-mm-tip catheter is not capable of creating adequate lesions for AF ablation. Finally, we observed a remarkable recurrence rate of AF in patients whom left pulmonary isolation was not completed with conventional manual ablation. This suggests that until the 8-mm or open-irrigation catheters become available, the MN system should not be used for AF ablation procedures targeting the antrum.
Comparison of remote ablation with conventional PVAI
Circumferential electroanatomic PV ablation and PVAI are both complex procedures requiring a rather extensive learning curve that could limit their wide application in clinical practice. In this respect, results are difficult to replicate and often are operator dependent. Early results suggest that remote MN is indeed a simple and safe approach for AF ablation and does not require a long learning curve. Similar to the results of Pappone et al. (7), we found that with the magnetic system it is feasible to maneuver the mapping/ablation catheter. However, in contrast to Pappone et al. (7), we found the ablation/mapping times using the MN were significantly longer than those using the conventional method. However, our fluoroscopy times were lower with MN than with the conventional method, especially when using the wand approach, and similarly the percentage of fluoroscopy required through the whole procedure was in all MN methods significantly lower than in the conventional method (7). Importantly, in contrast to Pappone et al. (7), who could reach electrical disconnection by >90% reduction (or <0.1 mV) of the local electrogram (EGM) within 10 to 15 s of RF application, we found that with both the CPVA and the PVAI guided by the circular mapping, it was not possible to obtain complete electrical disconnection of the PVs with the 4-mm tip. Although reduction of local EGMs was seen at or after 30 s, we could not achieve electrical isolation. It is possible that reduction of the local EGMs does not reflect transmurality of the lesion. In this respect, the present series suggests that lack of isolation is associated with an increased risk of recurrence.
No acute complications occurred during MN and ablation, confirming previous experimental and clinical studies on the safety of the catheter (7). However, larger studies are required to evaluate whether remote navigation is associated with fewer complications than standard CPVA and PVAI.
In the present series, catheter instability with respiration was evident in a small number of patients. This could be resolved by increasing the strength of the magnetic field.
Of concern was the relatively frequent occurrence of charring on the ablation catheter tip. Throughout the study, charring was closely monitored and immediately detected by ICE imaging and/or a sudden drop of the delivered power during energy application in temperature control mode. Although no embolic complications were seen in the present series, charring at the catheter tip could be a source of embolism. On the other hand, we were able to closely monitor charring formation with ICE imaging and by a sudden reduction of the delivered power in the temperature control mode used during ablation. The increased contact and better stability might have been responsible for the prevalence of charring. This finding confirms the inadequacy of the 4-mm tip for AF ablation and that an irrigated catheter is needed.
The present experience is limited to a relatively small number of patients. However, this series is as large as the only other existent report in the literature. In addition, patients were included in the study only after using the system in 48 previous patients.
Remote navigation using a magnetic system is a feasible technique. The wand approach may allow better catheter navigation to predetermined locations. Circumferential ablation does not appear to achieve isolation in any of the PV antra. Inability to isolate the PV antrum was associated with a significant recurrence rate. Whether the lack of isolation depends exclusively on the type of ablation catheter used for the procedure requires further testing.
- Abbreviations and Acronyms
- atrial fibrillation
- circumferential pulmonary vein ablation
- intracardiac echocardiography
- left atrium/atrial
- magnetic navigation
- pulmonary vein
- pulmonary vein antrum isolation
- Received February 14, 2007.
- Revision received April 17, 2007.
- Accepted May 14, 2007.
- American College of Cardiology Foundation
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