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CGCI systems for arrhythmia ablations

Louisa Malcolme-Lawes MB ChB MRCP Nicholas S Peters MD FRCP FHRS Prapa Kanagaratnam PhD MRCP D Wyn Davies MD FRCP FHRS
27 July, 2012  
Pipin Kojodjojo PhD MRCP
Louisa Malcolme-Lawes MB ChB MRCP
Nicholas S Peters MD FRCP FHRS
Prapa Kanagaratnam PhD MRCP
D Wyn Davies MD FRCP FHRS
St Mary’s Hospital,
Imperial College Healthcare NHS Trust, London, UK

Over the past decade, catheter ablation of cardiac arrhythmias has developed rapidly into a highly effective treatment strategy for drug-refractory arrhythmias. The transition has been from a process of simple focal ablation of supraventricular tachycardias to ablation of more complex arrhythmias such as atrial fibrillation (AF) and ventricular tachycardia (VT). These complex arrhythmias generally arise in patients with more adverse cardiovascular risk profiles and quite often in sicker patients with more diseased hearts. Thus effective catheter modification of these complex substrates is greatly facilitated by the use of three-dimensional mapping systems to guide anatomically and electrophysiologically based lesion placement. Traditionally, operators are required to become highly skilled in manual catheter manipulation, which is no mean feat, given that the learning curve for manual catheter manipulation and ablation is shallow. Furthermore, inter-patient variability in anatomy adds significantly to the technical challenge of the already lengthy procedures. Even in highly experienced centres, complex AF and VT ablations require long procedural and X-ray exposure times. Long-term success rates for complex arrhythmia ablations can vary significantly between centres and remain between 50% and 85% for the first procedure, with a high risk of requiring a repeat procedure due to arrhythmia recurrence. Recovery of partially ablated myocardium, inability to create permanent transmural lesions  and the formation of gaps between non-overlapping adjacent lesions are thought to be partly responsible for arrhythmia recurrence. To counteract this, catheter guidance control and imaging (CGCI) systems have been developed to allow for optimal catheter stability and reproducible catheter movements and to minimise inter-operator technical variability with the aim of achieving contiguous and transmural lesion delivery that should translate into more clinically effective ablation procedures.

Available systems
Four commercially available catheter guidance control systems have been developed to enhance catheter manipulation and perform minimally invasive ablation within the heart. Two are robotic navigation systems (RNSs): Hansen Sensei (Mountain View, CA, USA); and Catheter Robotics Amigo system (Mount Olive, NJ, USA). The others are magnetic navigation systems (MNSs): Stereotaxis (St Louis, MO, USA); and Magnetecs (Inglewood, CA, USA). In this article, we review each of these systems in turn with specific emphasis on their clinical applications and effectiveness. 

Hansen Sensei electromechanical robotic navigation system
The Hansen Sensei electromechanical robotic navigation system is capable of remotely steering a guide catheter to enable precise positioning and manipulation of any type of electrophysiological catheter within the heart for mapping and ablation. In brief, the system comprises three linked components: the physician’s workstation (Sensei™ robotic control system), remote catheter manipulator (RCM) and steerable guide catheter (Artisan™ Sheath) (Figure 1). The steerable guide catheter comprises an outer (14F) and inner (10.5F) steerable sheath through which any commercially approved 8.5F or less ablation catheter can be placed. The outer guide can be inserted, withdrawn and can bend up to 90 degrees, whereas the inner guide is controlled by the three-dimensional joystick and can be directed anywhere within the toroidal workspace. 
The Artisan sheath maintains the catheter position by the tensile strength of four pullwires so that the shape adopted by the sheath is uniquely suited to where the catheter is being positioned. This is in contrast to the manual approach, in which the operator has to dynamically apply torque and flexion to prevent the catheter displacing from the point of interest. The Sensei™ robotic control system also incorporates a pressure sensor (Intellisense™), which calculates the contact force at the tip of the catheter using the differential resistance when continuously dithering  the catheter in and out of the Artisan sheath. This tissue contact pressure enhances the validation of tissue contact and also allows identification of a pressure curve for optimal lesion production.

The physician’s workstation comprises three screens that can display any selected data including fluoroscopy, intracardiac echocardiography, electroanatomic mapping systems (Carto Biosense Webster, NavX St Jude Medical), rotational angiographic imaging (Philips ElectroNav, etc) and electrogram display systems (Bard Lab System Pro, etc) (Figure 2). 
The freestanding physician’s workstation and remote catheter manipulation system mounted on the patient table can be moved between laboratories and do not require any floor reinforcement such as that required for magnetic remote navigation systems. 

The intended benefit of robotic catheter manipulation is that the catheter position is maintained once the physician has released the three-dimensional joystick providing increased stability throughout the duration of radiofrequency (RF) delivery. In addition, the Intellisense system can confirm catheter contact during ablation. Several studies have been performed in animals to investigate whether the theoretical benefits of robotic ablation are translated into improved measurable parameters of lesion quality. In a study comparing robotic (Sensei) and manual ablation in porcine atria, robotic ablation reduced local electrogram amplitude (a marker of successful ablation) to a greater degree than manual ablation(1) (49±2.6% vs 29±4.5% signal reduction after one minute; p=0.0002). Macroscopically, the robotic lesions were more consistently transmural compared to the manual lesions, with no evidence of charring or perforation.

Several studies demonstrating the feasibility of using the Sensei to perform catheter ablation of a wide range of arrhythmias – including atrial flutter, left and right VT and, in particular, AF – have been conducted. The largest comparative study to date was performed by Di Biase and colleagues, with a total of 390 patients with symptomatic AF undergoing either robotic (Hansen) or manual catheter ablation.(2) Alternate procedures were performed with either robotic or manual ablation (193 and 197 patients respectively) by two operators, each performing a similar number of robotic and manual cases. The success rate for robotic ablation was 85% (164 patients) while for manual, it was 81% (159 patients) after 14.1±1.3 months of follow-up. The difference was not statistically significant (p=0.264). Three patients in the Hansen Robotic group and two patients in the manual group developed complications. 

In the robotic group, two patients had a tamponade requiring pericardiocentesis – one of which occurred during the manual transeptal puncture prior to any robotic ablation. In the manual group, one patient had a tamponade and one had a groin haematoma. The complication rate was not statistically different between the groups (p=0.683). Fluoroscopic time was significantly lower for robotic ablation (48.9±24.6 minutes) than for manual ablation (58.4±20.1 minutes) (p<0.001). Mean fluoroscopy time was statistically reduced after the first 50 procedures (61.8±23.2 minutes for the first 50 case vs 44.5±23.6 minutes for the subsequent cases; p<0.001), suggesting that fluoroscopy time can be reduced with increasing expertise. 

These findings were corroborated by Steven et al who randomised 60 AF patients to identify a difference in fluoroscopy times between robotic and manual ablation.(3) The robotic ablation was performed using the instinctive navigation software (CoHesionTM, Hansen Medical) that intuitively integrates the three-dimensional mapping system with the catheter steering console. Electrical disconnection of the pulmonary veins was achieved with Hansen RNS in all patients. The use of RNS significantly lowered the overall fluoroscopy time (9±3.4 vs 22±6.5 minutes; p<0.001). The difference was most marked in the ablation part of the procedure, with the fluoroscopy time prior to ablation being similar between the two modalities. The overall procedure duration tended to be longer using Hansen software, although this was not statistically significant (156±44 vs 134±12 minutes for robotic and manual respectively; p=0.099). Clinical success rates were similar between robotic and manual ablation at six-month follow up (73% vs 77% respectively; p=0.345). 

When first working with RNS, procedural times are longer, which may be the result of increased caution while performing ablation with the unfamiliar increased stiffness of the artisan catheter. However, with increased numbers of robotic procedures, setup time, fluoroscopy time and overall procedure time have reduced significantly. Similarly, in the majority of reported experiences, the occurrence of complications was greater during the learning curve for each centre and operator, and the incidence of complications is greatly reduced in operators performing more than 30 cases per year. 

Amigo robotic system
The Amigo remote catheter system is an alternative RNS technology, which was developed by Catheter Robotics, Inc. Similar to the Hansen Sensei system, conventional, commercially available mapping catheters are mounted onto a robotic arm that is controlled remotely (Figure 3). An intuitive handle enables the user to move the catheter with three standard degrees of movement: insertion and withdrawal, rotation and tip deflection. The similarity between these movements and conventional catheter manipulation allows for a short learning curve. Performing the ablation procedure in the control room – away from the fluoroscopy system – allows radiation exposure to operators to be reduced. Unlike the Hansen system, no additional workstation is needed, reducing installation complexity. However, such a set-up does not allow for integration with three-dimensional mapping systems to allow for instinctive catheter manipulation. The first human use of the system was in April 2010 at Glenfield Hospital, Leicester, UK for atrial flutter ablation. Further studies are awaited to confirm its safety and efficacy. 

The Niobe Stereotaxis system
The Niobe Stereotaxis system uses a steerable magnetic field that guides a soft-tipped magnetic catheter within the chambers of the heart for mapping and ablation. This system incorporates two computer-controlled permanent magnets, each weighing 1.8 tons, on either side of the patient’s body (Figure 4). A uniform magnetic field (0.08T) is generated to which the three smaller magnets – located within the ablation catheter – align in parallel allowing precise navigation of the catheter tip using a vector driven system. Computer-guided mechanical movements of the magnets alter magnetic-field orientations. Magnetic-field vectors can be stored and design lines drawn on the virtual anatomy to facilitate repetitive and semi-automated catheter manipulation. A computer-controlled catheter advancer system (Cardiodrive, Stereotaxis Inc) and a video workstation (Navigant, 2.1, Stereotaxis Inc) are required to assist precise catheter manipulation within the heart. The constant application of the magnetic field during ablation ensures that the catheter tip is in contact with the myocardium throughout the cardiac cycle, which improves energy delivery. The soft catheter tip and the small force generated by the magnetic field allow for safe navigation of the catheter within the heart and minimal risk of perforation. The Stereotaxis system has demonstrated feasibility and safety in mapping and ablation of arrhythmias including atrial flutter, regular supraventricular tachycardias, AF and VT. In addition, it has been used with good effect in patients with complex, often surgically corrected congenital heart disease to treat unusual atrial and ventricular arrhythmias. Owing to the need for dedicated magnetic tipped catheters in the Stereotaxis systems, early studies noted difficulty in achieving transmurality during cavotricuspid isthmus (CTI) line ablation and pulmonary vein isolation (PVI) ablation owing to lack of availability of an irrigated catheter for use with Stereotaxis. In addition, extensive charring was noted on the non-irrigated catheter tip in one-third of patients.(2) These issues have been resolved with the introduction of newer irrigated catheter tips.

Kim et al assessed the outcomes of catheter ablation cases in a variety of arrhythmias with 127 robotic cases and 594 manual cases.(4) Fluoroscopy times were significantly lower using Stereotaxis for AF ablation (–29 minutes, p<0.001), AVNRT ablation (–14 minutes, p<0.001) and AVRT ablation (–18 minutes, p=0.045). However, overall procedure times were significantly increased for AF ablation (+36 minutes, p=0.003) and atrial flutter cases (+116 minutes, p=0.016). AF procedure time diminished with increasing number of cases performed. In this series, two cases (two of 91, 2.2%) of tamponade occurred during manual AF ablation, whereas no tamponade occurred with 75 AF ablations using Stereotaxis.(4) Outcome studies with Stereotaxis utilising the new irrigated catheter tip included a study by Chun and colleagues in Hamburg.(5) Left atrial mapping and PVI was undertaken using Niobe II Stereotaxis and a novel, first generation 3.5mm tip open-irrigated catheter (Thermocool Navistar RMT, Biosense Webster) (Group 1 = 28 patients). Despite the introduction of open irrigation, tip charring was still noted in 17/28 (61%) patients with embolic complications in 3/28 (11%) patients while using the first-generation irrigated catheter. 
This resulted in a catheter redesign and no charring was subsequently noted in 28 patients who underwent ablation with the second-generation irrigated tip catheter (Group 2 = 28 patients). Procedural duration was 370 minutes (230–550 minutes) in Group 1 and 243 minutes (125–450 minutes) in Group 2. Overall sinus rhythm was maintained in 35/50 (70%) of patients over a follow-up period of 545 days, comparable to manual ablation procedures. Arya and colleagues performed ablation of ventricular tachycardia using magnetic navigation in 30 patients.(6) They achieved non-inducibility in 80% of patients using 41.2±23.3 minutes of radiofrequency ablation, and with total procedure and fluoroscopy times of 158±47 minutes and 9.8±5.3 minutes respectively. No acute complications occurred during these cases. The long-term procedural success rate was 70%, comparable to results from manual VT ablation.

Magnetecs system
Similar in concept to the Stereotaxis system, the Magnetec system consists of eight coil-core electromagnets arranged semi-spherically and attached to a uniquely designed spherical steel structure, surrounding the subject’s torso on a standard fluoroscopy table (Figure 5). The eight-coil electromagnetic system generates a shaped dynamic magnetic field within the region of the subject’s heart, with a maximal measured uniform field strength of 0.14T. By controlling the field magnitude, direction and gradient, the generated magnetic fields can exert sufficient torque and force on a specially constructed magnetised catheter tip to cause it to be pushed/pulled and torqued within the cardiac chambers.(7) A real-time CGCI computer controller calculates the instantaneous current values for the eight coils to rapidly shape the magnetic field and thus hold or move the catheter. By independently controlling each coil, six degrees of freedom of movement are imparted to the catheter tip. When a desired target is selected on the 3D representation of a cardiac chamber, the controller computer instantaneously computes the eight-coil currents necessary to generate the required dynamic field, which facilitates the movement of the catheter tip from actual to desired site. Unlike the Stereotaxis system, this novel system of shaping magnetic fields allows the magnets to remain static. The subject’s position on the fluoroscopy table is monitored with the use of optical fiducial patches, which provide dynamic compensation for gross body movements during the procedure. 

A 7F quadripolar radiofrequency ablation catheter with 4mm distal ablation electrodes is fitted with a high-coercive magnetic pellet of neodymium-based nano-crystalline material and standard platinum electrode design and spacing. The distal-most 10cm of the catheter is of a ‘floppy’ construction, thus allowing for safe guidance and manoeuvrability in the heart. The CGCI system includes integration with a digital fluoroscopic unit (Ziehm Imaging, Inc, Nuremberg, Germany), multichannel ECG recording and digital storage, intracardiac echocardiographic (ICE) display and storage, and full integration with a three-dimensional electroanatomical mapping system. Two modes of catheter control are available to the operator of the CGCI system. Firstly, a manual magnetic mode (‘man-in-the-loop’) is a joystick-controlled mode that offers a nearly instantaneous responsive way to direct the catheter tip within a cardiac chamber to an operator-designated site. Secondly, CGCI logic routines can plan a path to reach the targeted location, determine the optimal contact direction and guide the catheter tip until it makes firm and continuous tissue contact. A tissue-impedance measurement algorithm aids in determining the point of catheter-tissue contact, while tissue-contact sensing filters continue advancing the tip until continuous contact is reached over the entire cardiac cycle. Artificial intelligence routines are used in the following manner: if the catheter slips from a designated tissue location, the sensing system immediately alerts the magnetic field regulator and the tip is rapidly guided back into contact at the desired tissue location. If an anatomical obstacle is detected during the catheter’s journey to a designated point, the location is automatically marked and a new path is planned until tissue contact is achieved. In animal models, catheter navigation was shown to be highly reproducible, accurate and time efficient. Two systems have now been installed in Spain and South Korea, with further installations planned in Europe and the USA. More than 40 patients in Spain have had clinically indicated ablation with this system and clinical results are pending.

Conclusions
There is a clear clinical need to improve the operational efficiency and clinical outcome of treatment for complex cardiac arrhythmias, particularly AF. It is feasible to facilitate such ablations using catheter guidance systems and studies to date have shown similar clinical success as well as complications rates compared to conventional ablation. Unfortunately, procedural times have not been shortened but X-ray doses to patients and operators have been reduced. Furthermore, being able to perform long procedures without the need to wear a lead apron is likely to reduce operator fatigue. Continuing advances in catheter guidance technology with features such as automated ablation procedures and improved catheter designs including contact-sensing catheters, coupled with increasing operator experience and familiarity with remotely controlled ablation procedures, are likely to be key drivers towards making complex arrhythmia ablation safer, quicker and more successful in the coming years.

References
  1. Koa-Wing M et al. Robotically assisted ablation produces more rapid and greater signal attenuation than manual ablation. J Cardiovasc Electrophysiol 2009;20:1398–404.
  2. DI Biase L et al. Ablation of atrial fibrillation utilizing robotic catheter navigation in comparison to manual navigation and ablation: single-center experience. J Cardiovasc Electrophysiol 2009;20:1328–35.
  3. Steven D et al. Reduced fluoroscopy during atrial fibrillation ablation: benefits of robotic guided navigation. J Cardiovasc Electrophysiol. 2010;21:6–12.
  4. Kim AM et al. Impact of remote magnetic catheter navigation on ablation fluoroscopy and procedure time. Pacing Clin Electrophysiol 2008; 31:1399–404.
  5. Chun KR et al. Remote-controlled magnetic pulmonary vein isolation using a new irrigated-tip catheter in patients with atrial fibrillation. Circ Arrhythm Electrophysiol 2010; 3: 458–64.
  6. Arya A et al. Catheter ablation of scar-related ventricular tachycardia in patients with electrical storm using remote magnetic catheter navigation. Pacing Clin Electrophysiol 2010; 33: 1312–18.
  7. Gang ES et al. Dynamically shaped magnetic fields: initial animal validation of a new remote electrophysiology catheter guidance and control system. Circ Arrhythm Electrophysiol 2011; 4: 770–7.