Measurement of ECI for Contact Sensing: Experience at Southlake Regional Health Centre

Atul Verma, MD and Luc Soucie, Senior Technical Specialist*
Staff Electrophysiologist, Co-director Research & Complex Ablation, Heart Rhythm Program, Southlake Regional Health Centre, Newmarket, Ontario, Canada; *St. Jude Medical Canada, Mississauga, Ontario, Canada

Atul Verma, MD and Luc Soucie, Senior Technical Specialist*
Staff Electrophysiologist, Co-director Research & Complex Ablation, Heart Rhythm Program, Southlake Regional Health Centre, Newmarket, Ontario, Canada; *St. Jude Medical Canada, Mississauga, Ontario, Canada

In this article, the authors discuss the measurement of electrical coupling index for contact sensing, which may help to improve lesion delivery.

Introduction

Catheter ablation has emerged as an effective therapy for atrial fibrillation (AF). In patients with paroxysmal AF, the primary technique for ablation involves wide antral isolation of the pulmonary veins (PVs).

This is based on extensive data showing that the PVs are the most important source of triggers for AF and that procedural success is directly related to the completeness of PV isolation.1

However, achievement of complete and durable isolation of the PVs is challenging, primary limited both by operator experience and also the limits of currently available ablation technology. Repeat procedures are often required in 30–50% of patients to achieve PV isolation.2 Even then, reported success rates remain only 65–85% even after two procedures when patients are followed rigorously over one year. Late recurrences beyond one year also occur and may be as high as 5% per year.2 Almost all recurrences are mediated by some PV reconnection when repeat procedures are performed — either early or late.1 Reconnection often occurs because of an inability to deliver adequate, transmural lesions that will create persisting lines of block between the PVs and the rest of the left atrium (LA). While there are many factors that may contribute to inadequate lesion delivery, a lack of proper tissue-catheter contact is one of the most important contributors. Assessing real-time tissue contact is very difficult for the operator and is often based on indirect measures such as electrogram voltage amplitude, appearance of catheter movement on fluoroscopy, and manual “feel” on the catheter. None of these have been shown to be reliable assessments of contact force and are at best limited surrogates.

Newer technologies to quantify the degree of contact between tissue and catheter surfaces can help to optimize delivery of radiofrequency (RF) energy, allowing for creation of more complete and durable lesions, while minimizing the risk of complications.

 

Background of Electrical Coupling Index

Electrical-based contact sensing is a novel technology that may help to ensure adequate lesion delivery. The technology involves the use of the tip-to-tissue surface electrical coupling index (ECI), which is based on a three-terminal circuit model to measure the complex impedance at the catheter tip-tissue interface (EnSite Contact, St. Jude Medical, St. Paul, MN) (Figure 1).

As the catheter touches the tissue surface, the ECI value increases. As the catheter moves away from the tissue surface, the ECI value decreases (Figure 2). Changes in ECI have been shown to correlate directly with the degree of tissue contact that the catheter tip has with the tissue.

Since impedance will vary from patient to patient because of variations in body habitus and composition, the ECI does not have a fixed value for “contact” or “no contact” that can be applied to every patient. Instead, the ECI values for contact must be calibrated at the start of the case for every patient. Initially, the catheter is placed in a region, typically the middle of the left atrium, where the operator knows for sure that there is absolutely no contact. This is where the baseline calibration is set. The software then adds another 80 points above this value to create a “maximum” limit for contact. The amount of contact can then be displayed in several ways, including an ECI Coupling Meter, a scrolling ECI Coupling Wave showing upper and lower limits, and changes in the color of the catheter tip (Coupling Beacon), where green indicates good contact, and bars around the tip, the size of which increase as ECI is maximized (Figure 3).

Anatomical Optimization

One important contribution of contact sensing is in the building of more accurate anatomies for guiding ablation procedures. While modern mapping systems allow for rapid creation of an anatomical shell (e.g., Velocity system, St. Jude Medical, USA), certain areas may be somewhat underrepresented during the collection of the anatomy using standard multipolar or ablation catheters. In the left atrium (LA), for example, the septum and anterior wall, particularly around the transseptal access site, can be difficult to map. Contact sensing can allow one to go back to various regions of the anatomy and test whether touching the map surface equates to good contact sensing. In some areas, contact sensing may indicate regions that have not been optimally mapped, and it may be necessary to add to (or subtract from) the created anatomical shell (see online article for Figure 4).

More accurate anatomical representations can help operators guide their lesion sets based solely on the map and reduce potential fluoroscopic exposure.

Optimized Radiofrequency Delivery

Catheter contact is directly related to both lesion depth and surface area, and completeness of transmural lesions can be predicted by the total force application.3,4 Furthermore, adequate lesion formation cannot be achieved at even very high delivered powers where inadequate tissue force is delivered.5 Based on these observations, it can be hypothesized that operators who have ECI may be better able to deliver durable RF lesions, and in this way, maximize procedural success. Preliminary evidence from very small clinical trials have already suggested an inverse relationship between delivered force and the occurrence of conduction gaps around the PVs when patients are reassessed at the time of a repeat procedure.6 Similarly, data from Sommer et al also demonstrated fewer conduction gaps in circumferential PV lesions with faster PV isolation time when contact was used compared to not. Indeed, during initial PV encircling, with EnSite Contact monitoring, isolation was achieved in 61% of PV compared to 29% without EnSite Contact monitoring (p < 0.02). Furthermore, when touch-up lesions were necessary, only 4.9 ± 4.1 vs. 9.0 ± 8.6 (p < 0.03) lesions were necessary with EnSite Contact monitoring compared to no monitoring.7

ECI may also help to improve the safety profile of catheter ablation. Although the overall risk for AF ablation is low, there continues to be important procedure-related complications that limit wider applicability in the treatment of AF. Cardiac perforation still occurs in 0.5–2.0% of patients. Thromboembolic complications also occur in 1–2% of cases. Lethal complications, such as atrial-esophageal fistulae, are very rare, but are devastating when they occur. All of these complications are directly related to applied ablation force. Animal data has shown that the incidence of tissue perforation is directly related to contact, even more so than delivered power.8 Contact also predicts both tissue pops and thrombus formation, which can lead to stroke, in spite of the use of an irrigated tip catheter.5,8 Operators who are aware of ECI while ablating may be able to titrate energy and force delivery to avoid excessive tissue contact, and potentially avoid serious adverse events from AF ablation.

Clinical Applications of Contact Sensing

While the utility of contact sensing could be beneficial in any ablation case, it is likely to particularly transform the performance of complex ablation, particularly for AF and ventricular tachycardia (VT). These are the two most commonly performed complex ablation procedures. In AF ablation, the key is finding a balance between robust RF delivery required to perform a durable PV isolation, while at the same time avoiding important risks such as perforation and stroke. More durable PV isolation may help make the outcome of AF ablation more reproducible while minimizing the number of repeat ablation procedures. Contact sensing may also assist in mapping and complete ablation of specific electrograms, such as complex fractionated electrograms (CFE). For VT ablation, substrate-based ablation has become more common, in which patients are ablated in sinus rhythm in scar or scar border-zone regions defined by electrogram voltage. By ensuring good tissue-catheter contact, contact sensing can help distinguish between true scar regions and regions with low signal amplitude secondary to poor contact. Improved accuracy of scar maps can help better guide RF lesion sets for elimination of VT. Other complex ablation procedures which would benefit from contact sensing would include left atrial flutter or tachycardia, any right atrial atypical flutter, any left ventricular case, and even ablation in the right ventricular outflow tract.

Future Directions

Measurement of ECI for contact sensing is only the beginning for this technology. ECI may also further optimize RF delivery by giving an assessment of lesion completion. As RF is delivered, ECI typically falls. Both the magnitude of the fall in ECI and the partial recovery of ECI during ablation may provide feedback as to when a full transmural lesion has been achieved. ECI is also typically lower in recently ablated regions versus normal regions, which again can help distinguish areas that need further RF delivery from those that do not. Preliminary evidence is already available demonstrating the ability of ECI changes to correlate with transmural lesion formation.9

Southlake as First North American Center

The Southlake Heart Rhythm Program has always been committed to providing the most advanced care to our patients. Our program is fully invested in clinical investigation, participating in and leading several clinical trials. We have also had a history of beta-testing new technologies, often being the first both nationally and internationally in evaluating new software and hardware for performing complex ablations. We are proud to be the first center in North America to pilot the new contact sensing technology. As part of this program, we will be mentoring other centers in North America on its optimal usage. We are also performing active investigation into the best settings for ECI and how ECI may be able to help us beyond just contact sensing. We also plan to lead a clinical trial comparing traditional ablation to that guided by contact sensing. We hope to be able to publish our findings in the near future.

References

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  2. Weerasooriya R, Khairy P, Litalien J, et al. Catheter ablation for atrial fibrillation: Are results maintained at 5 years of follow-up? J Am Coll Cardiol 2011;57:160–166.
  3. Shah DC, Lambert H, Nakagawa H, et al. Area under the real-time contact force curve (force-time integral) predicts radiofrequency lesion size in an in vitro contractile model. J Cardiovasc Electrophysiol 2010;21:1038–1043.
  4. Thiagalingam A, D’Avila A, Foley L, et al. Importance of catheter contact force during irrigated radiofrequency ablation: Evaluation in a porcine ex vivo model using a force-sensing catheter. J Cardiovasc Electrophysiol 2010;21:806-811.
  5. Yokoyama K, Nakagawa H, Shah DC, et al. Novel contact force sensor incorporated in irrigated radiofrequency ablation catheter predicts lesion size and incidence of steam pop and thrombus. Circ Arrhythm Electrophysiol 2008;1:354–362.
  6. Vijaykumar R, Locke AH, Ahmed H, et al. Novel visualization of catheter-tissue contact force during pulmonary vein isolation [abstract]. Heart Rhythm 2010;7:S100.
  7. Sommer P, Gaspar T, Sih H, et al. Electrical coupling index guided radiofrequency ablation of the pulmonary veins. Eur Heart J 2010;31(Suppl):708.
  8. Perna F, Heist EK, Barrett C, et al. Assessment of the catheter tip contact force resulting in cardiac perforation in the swine atria [abstract]. Heart Rhythm 2010;7:S66.
  9. Holmes D, Fish JM, Byrd IA, et al. Contact sensing provides a highly accurate means to titrate radiofrequency ablation lesion depth. J Cardiovasc Electrophysiol 2011;22:684-690.