Robotic Ablation of Arrhythmias

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Citation
European Cardiology - Volume 4 Issue 2;2008:4(2):75-77
DOI
http://dx.doi.org/10.15420/ecr.2008.4.2.75

The 4mm deflectable radiofrequency (RF) ablation catheter enabled small necrotic lesions to be created anywhere in the heart, revolutionised the treatment of tachycardias mediated by localised abnormalities such as atrioventricular nodal re-entrant tachycardia (AVNRT), accessory pathways and focal atrial tachycardias1 and fuelled the rapid growth of cardiac electrophysiology. The first limitations of the catheter design emerged only with attempts to ablate typical right atrial flutter, for which multiple contiguous and transmural lesions were required in the cavotricuspid isthmus. Procedure times to achieve isthmus block with 4mm catheters were considerable, and drove the development of both the 8mm-tip and irrigated-tip RF ablation catheters, which produced larger lesions and could rapidly establish bi-directional block in the isthmus.2,3 Atrial fibrillation (AF) ablation took the challenge further with even more extensive areas of conduction block being necessary to prevent ectopy from the pulmonary veins.4 Both the 8mm- and irrigated-tip catheters were able to achieve successful AF ablation, but even these improved catheters could not overcome the long procedures and high clinical recurrence rates. On the background of these shortcomings and the vast numbers of patients with AF, the innovation in new technologies for AF ablation was exponential.

New Technologies for Atrial Fibrillation Ablation

The cornerstone of AF ablation is circumferential ablation around the pulmonary veins with proven electrical isolation.5 In order to achieve this degree of extensive, contiguous and transmural lesion around the pulmonary veins, a variety of circular ablation catheters have been tested. Some of these use modified forms of RF energy (Mesh Ablator, Bard Inc., US, and the pulmonary vein ablation catheter [PVAC], Ablation Frontiers Inc., US), while others use new modes of tissue injury such as thermal injury (Arctic Front, CryoCath Inc., Canada) and high-intensity focused ultrasound (HIFU, ProRhythm Inc., US). These catheters have all been successful at producing pulmonary vein isolation with circumferential ablation, albeit with the intermittent requirement for additional conventional ablation, and often with reduced procedure times.6–9 However, all of these are constrained by their ability to deliver only the designated lesion set, and increasingly it is being suggested that ablation beyond the pulmonary veins may be required with lesions at sites of ganglionic plexi or regions of complex fractionated electrogram. Furthermore, if there is conversion to atrial tachycardia, linear lesions such as roof or mitral isthmus lines may also be needed. Therefore, despite these developments there remains scope for further improvement in catheter technology for AF ablation.

Remote Navigation Systems

Remote navigation systems have been developed on the premise that the primary limitation of the conventional RF ablation catheter is the reliance on operator hand-skill. The precision movement afforded by such technology could improve outcomes by enabling placement of adjacent lesions with closer proximity, thereby producing contigious and therefore more effective lesions. There would also be no limitations placed on the strategy of ablation that is adopted. Therefore, in principle a remote navigation approach to AF ablation could overcome the limitations of both the manual method and the single circumferential ablation technologies.

Magnetic Remote Navigation – Stereotaxis Inc.

There are currently two remote navigation catheter control systems available for clinical use. The first to be developed was the Niobe™ system (Stereotaxis, US). This uses two permanent magnets positioned on either side of the patient to apply a low-intensity magnetic field (0.08 Tesla) across the patient. The mapping catheter contains three inner magnets that align parallel to the applied external magnetic field. Catheter navigation is achieved by altering the magnetic field using the orientation of the large external magnets. A motor drive attached to the catheter enables remote navigation by advancing or retracting the catheter. The technical feasibility and safety of the system for ablation of arrhythmias has been widely published.10–14 The stability of the catheter during ablation may produce better lesions, and specific clinical applications have been identified, such as the ability to manoeuvre within the pericardial space.14 Clinical outcomes have not been shown to improve in spite of the theoretical advantages over conventional manual ablation. This is largely because current use of the system has been limited to non-AF cases, where manual outcomes are already excellent. Technical problems with the magnetically controlled irrigated-tip catheter and demonstration of char formation on the non-irrigated catheter in the left atrium has dissuaded the majority of operators from performing AF ablation using the Stereotaxis system.15 As a result, the full potential of this system has not been evaluated. Despite this, the system is popular with its advocates, who are able to work sitting outside the area of radiation exposure without lead-shielding garments. Although this is often dismissed as a minor, unscientific reason for using remote navigation systems, it is undeniably an attraction for high-volume operators keen to avoid health problems from both radiation exposure and heavy lead coats.

This form of mechanisation of catheter movement has the potential, unlike other forms of ablation, to enable automated computer-controlled catheter movement. The magnetic catheters have the advantage of being soft, and the force that can be generated is unlikely to cause perforation. This has allowed the development of auto-mapping, where the virtual geometry of the left atrium can be created without operator guidance. It is likely that if the technical problems with the irrigated catheter can be overcome and combined with further advances in automation, there is a real possibility of improving AF outcome and maybe procedure times with the Stereotaxis system.

Robotic Remote Navigation – Hansen Medical Inc.

More recently, a novel electromechanical robotic master–slave system (Hansen Medical, US) has been developed that is capable of remotely steering a guide catheter to enable precise positioning and manipulation of any type of electrophysiological catheter placed within the sheath and use it for mapping and ablation. The system comprises three linked components: the physician’s workstation (Sensei™ robotic catheter system), remote catheter manipulator (RCM) and steerable guide catheter (Artisan™ control catheter) (see Figure 1). The steerable guide catheter confers both flexibility and stiffness to the sheath, maintaining the catheter position by the tensile strength of four pullwires so that the shape adopted by the sheath is uniquely suited to the point of interest at which the catheter is positioned. This is in contrast to the manual approach, where the operator has to dynamically apply torque and flexion to prevent the catheter displacing from the point of interest. The Sensei system also incorporates a pressure sensor system (Intellisense™ Fine Force Technology), which calculates the proximal force along the shaft of the ablation catheter using the differential resistance when continuously dithering the catheter in and out of the Artisan sheath (see Figure 2). This tissue contact pressure is unique to this system, and not only enhances the validation of tissue contact but also provides for the first time the ability to identify a pressure curve for optimal lesion production.
The Sensei system has the immediate advantage of being compatible with any catheter that fits inside the sheath and therefore, since receiving regulatory approval, has been used primarily for AF ablation. Early results suggest that it may be a more effective approach than conventional ablation for AF.16–18 Although procedure times do not appear to be reduced at this stage, it is clear that robotic ablation with the Sensei system may have other advantages, which need further investigation.

Optimal clinical outcomes require permanent transmural lesions to be created in the correct locations. However, determining transmurality can be difficult and it has been suggested that a 90% reduction in signal amplitude is a marker of transmurality. In experimental models, the size of RF lesion is proportional to power delivery, electrode temperature, electrode radius and contact pressure.2 In clinical settings, the electrode size is fixed and power can be optimised to an extent, leaving tissue contact as the most important factor in achieving the tissue temperatures required to produce transmural lesions. However, the efficiency of energy delivery can be poor – as low as 10% depending on catheter stability and catheter position relative to intracavitary blood flow. Current methods of assessing contact (fluoroscopy, tactile feedback or electrogram characteristics) appear to be inadequate for judging the efficiency of tissue heating.19 Therefore, poor lesion quality, as a result of inconsistent catheter tissue contact, may be a significant reason for the clinical recurrence rate in circumferential pulmonary vein ablation for AF. The pressure sensor in the Sensei system was primarily developed to convery contact and tip force, but it has the serendipitous benefit of providing a margin of safety. Animal studies have suggested that a contact pressure of 20g is optimal for producing transmural lesions with power limited to 25W and saline flow rate of 17ml/min. The improved tissue contact using Intellisense together with catheter stability also appears to produce transmural lesions more rapidly, and a 30-second lesion with 40% signal reduction appears to create transmural lesions.20,21 These data indicate that tissue injury using the system is more effective, but raises the possibility that excessive ablation may risk perforation. Pericardial tamponade with the system has been reported and probably reflects the higher contact pressures and powers utilised by early users. The optimal pressure, power and time settings will need to identified to ensure that the incidence of pericardial effusion is no different from that seen with conventional ablation, but should also reduce ablation times and achieve more consistent transmurality compared with the manual approach.22
In order to achieve pulmonary vein isolation, the lesions in a circumferential pulmonary vein ablation have to be contiguous. This requires precise positioning of each lesion adjacent to the prior lesion, and necessitates experienced hand-skill when using a standard manual catheter where the catheter tip is controlled by deflection and rotation. The Sensei robotic control system uses a 3D hand-operated joystick (see Figure 1B) that is registered to the fluoroscopic image and controls the Artisan sheath (instinctive motion controller [IMC]).

This registration allows the direction of hand movement to be the same as the direction of catheter movement and, in theory, enables the operator to respond to the image on the screen without performing any complex hand manoeuvres.17 Furthermore, the IMC handle response can be scaled to achieve finer movements with the same degree of movement of the joystick. In combination, these features should reduce the hand-skill required for circumferential pulmonary vein ablation.
At present, the Sensi system not only provides the benefits of reduced operator radiation exposure and a safe working posture, but also enhances conventional ablation technology for AF ablation by providing tissue contact information and instinctive precision catheter control. Similar to the Stereotaxis system, the full potential of the technology may not be realised until robotic assistance with automation becomes available. Robotic ablation provides the versatility to respond to the prevailing strategy for AF ablation, but with distinct advantages over the manual approach. Randomised studies of AF ablation using the robotic approach are already under way and will further determine the importance of this technology in the treatment of AF.

Disclosures

Consulting for Hansen Medical and Ablation Frontiers.

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