Contact force sensing for atrial fibrillation ablation

Background

The use of catheter ablation for the treatment of symptomatic atrial fibrillation (AF) has increased significantly over the past decade.  Cardiac electrophysiologists are continually trying to refine and improve procedural outcomes for patients having catheter ablation for AF. Pulmonary veins (PVs) play a dominant role in the pathophysiology of AF and consequently, pulmonary vein isolation (PVI- electrical isolation of the pulmonary veins from the left atrium) is the cornerstone of catheter ablation therapy for all types of AF. Techniques and tools have been refined over the years to ensure the procedure is safe and effective, but technological advances continue unabated in the search to improve both safety and patient outcomes (1).
Nevertheless, the Achilles heel of catheter ablation for AF is the rate of recurrence of atrial tachyarrhythmias, the predominant cause of which has been shown to be electrical reconnection of the PVs (2,3,4). Acute reconnection is frequent and ranges from 33%-93% in published studies (5,6). Elimination of these reconnections may improve long-term success rates (7,8). More durable radiofrequency lesions, that are not only transmural but permanent may overcome this limitation. 
A multitude of factors interplay during catheter ablation to result in the final lesion formation and these include: radiofrequency (RF) power and duration, electrode size, irrigation, electrode temperature, local blood flow and finally the catheter tip-tissue contact which is what contact force sensing technology relates to (9).  

1 - Importance of real time contact force information

Maintaining optimal catheter tip-tissue contact is vital for delivering robust RF lesions but is a constant challenge during catheter ablation. Improved contact reduces dissipation of energy into the circulating blood pool resulting in better energy coupling to tissue. The importance of contact force in creating RF lesions has been well documented in pre-clinical studies and is particularly pertinent in light of the fact that contemporary irrigated tip catheters (irrigation with saline) preclude accurate tissue temperature monitoring during ablation (9-10). It has also been postulated that additional real-time contact force information would help to create more predictable and more reliable lesions and potentially improve both the safety and efficacy of RF ablation (9).
The ability to measure the contact force between catheter tip and tissue in real-time has remained elusive and up until the recent development of contact force sensing technologies, this parameter was assessed indirectly (subjectively) by the operator. This was done using methods such as imaging of the catheter tip fluoroscopically, tactile feedback from catheter manipulation, and changes in intracardiac electrograms and impedance. The accuracy of these surrogate measures is poor and has not been validated. In fact, it is suggested that these methods correlate poorly with tissue–electrode contact (11,12).

2 - Available contact force sensing technologies

Technologies to allow the direct measurement of contact force between the catheter tip and the target myocardium in real-time are now available. One such system uses a unique sensor located at the distal tip of an irrigated RF catheter (ThermoCool® SmartTouch™, Biosense Webster, USA). This catheter has a 3.5mm externally irrigated tip electrode that is connected by a tiny precision spring to the shaft. The degree of deformation of this spring is measured and transmitted every 50ms via a magnetic transmitter at the tip and three location sensors at the shaft, and has a resolution of <1g in bench tests. It has been designed to allow full integration with the widely used Carto 3 electroanatomical mapping system (Carto 3 System, Biosense Webster, USA), and is able to provide both graphical and numerical displays of contact force and three-dimensional force vectors in real time, on one screen, without the need for an additional workstation.
Other contact force measuring tools that are currently commercially available include the TactiCath® (Endosense, SA) catheter, which uses proprietary fibreoptic sensor technology mounted on a 3.5mm open irrigated-tip ablation catheter. The micro-deformation of optical fibres within the catheter tip provides real-time contact force  measurements on a dedicated (additional) workstation, with a sensitivity of less than 1 gram (similar to the SmartTouch catheter). The Tacticath catheter was the first contact force  sensing technology that has been evaluated in humans with clinical in vivo data demonstrating its feasibility and safety in human subjects (13). The third system, IntelliSense® (Hansen Medical Inc., Mount View CA, US), is a robotic catheter navigation module which incorporates a system-based force sensing technology. This provides visual and vibration feedback to the operator with published data of its successful use in human paroxysmal AF (14). One limitation of the IntelliSense technology is that it can only be used in conjunction with use of the robot system for catheter ablation. The fourth system calculates the tip-tissue contact via impedance measurements (EnSite Velocity™ Contact™; St. Jude Medical, St. Paul, MN, USA).

3 - Pulmonary vein reconnection and current evidence base

Patients who get a recurrence of AF or atrial tachycardia following a PVI procedure have rates of PV reconnection above 80%. This implies that PV reconnection plays an important mechanistic role in these recurrences (15,16). 
The Toccata study, published in 2011, was the first in human clinical investigation of a contact force catheter (TactiCath®) demonstrating that it was as safe as conventional irrigated catheters during RF ablation of AF and right-sided supraventricular tachycardias. It also highlighted that high contact force values can occur even during catheter manipulation as well as during actual ablation, which could help avoid complications such as perforation (13). 
With regards to AF ablation, a recent clinical study from 2012 demonstrated the potential benefits of contact force information (ThermoCool® SmartTouch™) during PVI. The principal finding from this study was that using contact force data resulted in an exceptionally lower rate of acute (within 60 mins) PV reconnection in patients, than without using this data (15% vs. 70%; p=0.001). Furthermore, it showed that areas of acute PV reconnection were typically associated with lower contact force values during ablation. It also reaffirmed the fact that tactile feedback was a poor surrogate for contact force, as blinding the operator to contact force data intra-procedurally resulted in significantly lower values of mean contact force (20). 
Additionally, the use of contact force technology has been shown to reduce ablation time significantly when compared to a conventional catheter (39mins vs. 55 mins; p=0.007) whilst reconfirming a comparable safety profile of the contact force catheter to that of a conventional irrigated-tip catheter (21).

Conclusion

The evidence base for contact force sensing technology is in its early stages but is already compelling. What is currently not known is the impact of contact force use during AF ablation on long-term clinical outcomes. If the significantly reduction in acute electrical PV re-connection turns out to be a long-lasting effect, borne out in studies with appropriate longer term follow-up, then it will have a significant impact for AF patients. Contact force sensing technology, therefore, has the potential to become the recommended standard of care for all patients undergoing AF ablation.