Is Contact Force Sensing in Your Future?

The quest for improvement in safety and efficacy of catheter ablation procedures continues. Catheter tip-to-tissue contact impacts both safety and efficacy. Subjective surrogates for contact have been operator tactile feedback, fluoroscopic evidence, and electrogram changes. Recent studies, however, have reported on methods to directly measure contact force.The following is a review of these studies and of the various tools that are being developed.

Current Approaches to Evaluating the Quality of a Lesion

A variety of factors contribute to the size of a radiofrequency (RF) lesion. These include power delivery, electrode temperature, duration of delivery, impedance, electrode radius, the geometry of the electrode, intramyocardial blood flow and interchamber blood flow, the remote electrode size, properties of the tissue, RF delivery characteristics, irrigation, electrode material, and electrode-tissue contact.¹ We are able to control some of these factors through electrode design and others through control of the RF generator. Irrigation is also within the operator’s control, whereas blood flow and tissue properties are not. Impedance drop had been seen as a possible surrogate for good contact, as had been electrogram parameters,² but actual contact force (CF) has been a quality one could only speculate about. Contact force is a predictor of lesion quality and is also a contributor to procedural safety, as excessive CF may lead to perforation and tamponade.

Why do we care about CF? Inadequate contact leads to lesions that are non-continuous or not transmural, and no contact at all leads to no lesion. Excessive contact will lead to deeper lesions and possible damage to the esophagus, phrenic nerve injury, steam pops, and perforation. A study by Shah et al helped to point out why CF is an area of concern.³ A human study was conducted to determine how 11 experienced investigators estimated the CF exerted during a right and left atrial mapping procedure. Operator-estimated CF was compared to real-time CF. The TactiCath^® catheter from Endosense was used. Operators were blinded to actual CF; mapping took place in six sites in the right and left atria during arrhythmia, sinus rhythm, and atrial pacing. Subjective assessment of good contact actually represented a wide range of CFs, sometimes as high as over 100g during manipulation. There was also a wide inter-operator and site-specific variation seen. Recent studies have suggested that the optimal CF range is 10–40g; this suggests that experienced operators may unknowingly be exceeding a safe range of force.^4-5 It seems that additional data about CF could greatly improve procedural safety.

Current Contact Force Systems and Current Trials in the United States

There are at least four different systems in development to measure CF. The four discussed here have been approved in Europe. None are approved for use in the US, but at least three have ongoing clinical trials in the US.

The ThermoCool^® SmartTouch™ CF sensing irrigated catheter by Biosense Webster Inc. is used with the Carto^® 3 Mapping and Navigation System and the Carto^® SmartTouch™ Software Module. The software module combines a graphical display of force and contact information. The tip of the ThermoCool^® SmartTouch™ catheter is mounted on a precision spring and allows deflection. CF is measured by magnetic sensors every 50 ms. The tip of the electrode is coupled to a transmitter coil, distal to the spring, and emits the location reference signal. The CF is then averaged over 1 second. The system senses the sensor location, calculates force based on the spring characteristics, and displays this on the 3D map.⁶ The system is approved for use in the European Union. A clinical trial for patients having symptomatic paroxysmal atrial fibrillation has begun enrolling in the US. Six sites have been approved.⁷

The Sensei^® X Robotic Catheter System, the Artisan™ Control Catheter and the IntelliSense^® force sensing system are products of Hansen Medical. The IntelliSense^® Fine Force Technology™ interface provides proximal measurement of the force transmitted along the shaft of the catheter from tissue contact. The IntelliSense Vibe and IntelliSense Flex provide physicians with feedback in tactile and visual ways. IntelliSense^® Vibe is a tactile vibration feature. The vibration felt is proportional to the measurement displayed visually on the workstation. IntelliSense^® Flex adjusts the scale ratio between the workstation’s instinctive motion controller (IMC) and a steerable catheter. As forces rise, the motion scale ratio is adjusted so the IMC motion results in a smaller catheter tip motion. In 2010, a Sensei^® X trial began in the US for drug-refractory AF patients, called ARTISAN AF. This trial is still enrolling patients.⁸

EnSite Contact™ is a system from St. Jude Medical consisting of both hardware and software components.⁹ The system uses a three-terminal circuit model which isolates and measures impedance at the tip-to-tissue interface and derives a measure called the electrical coupling index (ECI). The ECI is then made available on the EnSite NavX system as a scrolling waveform, as a meter, and on the catheter as a colored beacon. The end result is thought to be a measurement that reproducibly describes electrical catheter contact. This system is approved in Europe.

The TactiCath^® catheter by Endosense contains a force sensor that allows continuous and real-time measurement and display of force applied by the tip.¹⁰ A fiber optic sensor combined with an infra-red laser waveform that passes to the catheter tip experiences microdistortion when force is exerted on the catheter tip, thus modifying the wavelength. In addition, lateral and axial forces are measured by the sensor, providing force vector. The catheter is compatible with 3D navigation and recording systems. The system consists of a base station, a splitter and a graphical user interface. The system is approved in Europe. A clinical trial is currently enrolling in the US.¹¹

In Vivo and In Vitro Studies Testing Contact Force

The following is a review of some of the recent studies in animals and animal models that have contributed to our understanding of the role of CF. In addition, some of the studies have been conducted to test the new CF systems. Issues of impedance, perforation, and force required for adequate lesions are explored.

In 2008, Yokoyama et al conducted an open-irrigated catheter study to measure CF using the TactiCath^®.⁴ The study included both a bench test and a canine muscle preparation. Larger and deeper lesions were produced by lower RF power (30W) at greater contact force (30–40g) than with higher power (50W) but lower CF (2 to 10g). Greater CF increased the incidence of steam pops and thrombus. Impedance before RF application did not correlate with electrode contact force. In 2010, Thiagalingam et al, also using the TactiCath^®, applied ablation lesions to 11 freshly excised hearts from pigs that were superfused.¹² The one-minute ablations were delivered with three power control strategies: impedance control; fixed power at 20W and 30W power; and CF at 2g, 20g, and 60g. Tip-tissue CF proved to be as important as RF power in the final lesion size. For this study, CF correlated with a high initial impedance drop.

Another study attempted to provide an exact correlation between constancy of contact and lesion size. A bovine tissue model simulating a beating heart was used along with the TactiCath^® catheter.¹⁰ The investigators were evaluating a force-time integral (FTI) as a predictor of lesion size. FTI expresses the accumulated energy delivered per ablation. The protocol included the following test combinations: 20 and 40 W of power applied with constant contact at 20g; variable contact with 20g at peak and 10g at nadir; and intermittent contact with 20g at peak and 0 for the nadir of loss of contact. Lesion size was found to correlate with the measured contact FTI. Constant contact produced the largest lesions. Another study, conducted in Oklahoma with the TactiCath^®, looked at variables which may be predictive of steam pops when high CF and moderate RF power were employed.¹³ They found that steam pops were not well predicted by initial impedance, impedance decrease during RF, electrode-tissue angle, systolic CF, diastolic CF, or force-time integral.

Information about the force required to perforate a cardiac chamber during an ablation would be helpful in averting such a situation during a procedure. A variety of animal studies have been conducted which have explored this issue. A study by Shah et al in 2011 explored forces required to perforate all chambers of a porcine heart.¹⁴ The TactiCath^® was used to measure CF. The investigators found that the threshold for perforation was lower for right- versus left-sided chambers, and was lower in previously ablated tissue. Use of a sheath allowed perforation to occur more rapidly in the left ventricle because of the prevention of catheter buckling. A swine study from Massachusetts General Hospital sought to determine the contact force required to perforate the swine atria.⁶ Contact force was measured with the ThermoCool^® SmartTouch™ investigational open irrigated-tip RF catheter. The investigators performed 111 cardiac perforations in 7 pigs. The average force required to perforate was 175.8±60.4g. This number was reduced to 151.8±49.9g after tissue had undergone 30 seconds of RF delivery. The right and left atrial perforation values did not differ significantly. The lowest force-causing perforation was 77g.

A swine study conducted in Los Angeles with the ThermoCool^® SmartTouch™ and Carto 3 System from Biosense Webster Inc. sought to compare the intracardiac electrogram (EGM) voltage and the impedance between high and low CF in the right atrium.¹⁵ The investigators found that high CF was associated with morphology changes and PQ interval duration in unipolar electrograms, but this was not true for bipolar electrograms or impedance.

Holmes et al¹⁶ studied the use of the electrical coupling index (ECI) (local resistance and reactance between catheter tip and tissue surface) from the EnSite Contact™ system as a variable for lesion depth estimation. An irrigated catheter was used to deliver RF lesions to hearts and thighs of swine. Power was set at 30W for 20 or 30 seconds for intracardiac lesions, and 30–50W for 10–60 seconds for the thigh. Intracardiac lesions with ≥12% reduction in ECI were more likely to be transmural. A lesion depth algorithm was used with the ECI subcomponents. The use of ECI factored into the lesion depth algorithm was thought to help predict efficacy of the lesion.

A live canine model was used to compare irrigation catheter tip contact established by intracardiac echo with the CF measured by an in-line sensor in grams.¹⁷ Perforation threshold testing was also performed in the left atrium. The in-line CF sensor was validated by ICE visualization. Force characteristics varied between chambers. There were no perforations at forces greater than 80g.

Okumura et al conducted a canine study delivering ablation lesions to the right and left atria.¹⁸ Hansen Medical’s Sensei Robotic Catheter System was used, as well as an irrigated tip catheter. ICE was used to validate CF. The system incorporates an in-line mechanical sensor. Contact force of 10–20 and ≥20 grams produced full-thickness lesions; lesser lesions occurred with <10 grams. The investigators suggested that mapping required lower force to avoid image distortion. Di Biase et al also conducted a canine study using the Hansen Medical robotic sheath and IntelliSense^®.⁵ The study examined the relationship between lesion formation, pressure, and complications. Lesions were placed in the left atrium at six settings. A power setting of ≤35W and lower/medium contact pressure showed ‘relative’ sparing. Pressures between 20g to 30g and a power setting of 40W achieved transmurality and preserved safety. Power settings of 45W and pressure >40g more often associated with char and crater formation (66.7%).

In summary, varying results have been seen with attempts to correlate impedance and CF. Much data has been gathered about what a safe CF range should be. Questions about safe and effective CF ranges in the beating heart remain. Algorithms to quantitate variable CF are being developed.

Clinical Trials in Humans

All the studies that follow were conducted in Europe and represent studies with each system that was discussed previously.

A study using TactiCath^® examined the relationship between CF and gap formation at three-month follow-up in 45 patients.¹⁹ The antrum of each pulmonary vein was divided into eight segments. The CF and FTI information were collected for each segment. The conclusion was that if low FTI occurred in a segment, late gap occurrence was predicted, suggesting that FTI could become a measure of procedural success. Another TactiCath^® study looked at the dynamics of CF through the cardiac cycle.²² Blinded investigators applied CF levels they judged appropriate to six predefined sites in the left atrium. Dynamic range (DR) was the difference between systolic and diastolic peak forces. There were 21 patients at six centers, and 10 operators. The investigators found that dynamic behavior of CF is different in different sites of the left atrium. They proposed that controlled CF may contribute to more stable catheter placement, making the procedure safer and more effective.

A European study using the ThermoCool^® SmartTouch™ by Biosense Webster Inc. evaluated locations of high CF during mapping of the left atrium and pulmonary veins.²⁰ Limited RF power was used during ablation in the high CF sites. AF ablation was performed in 17 patients. The investigators found that high CF often occurs at the LA roof directly beneath the ascending aorta. RF power for ablation was based on CF, with: CF<10g: 35–45 W; 11–30g: 25–34 W; 31–50g: 15–24 W; and >51g: 0–14W. No steam pops or impedance rises occurred in any patients. The authors stated that RF power based on CF may prevent steam pops and impedance rises.

A study of 20 patients with persistent AF utilized the robotically navigated Sensei^® X by Hansen Medical.²¹ Each RF application lasting at least 40 seconds required a stable catheter tip position and contact pressure in the range of 20–40g. This was achieved with the IntelliSense^® System and perpendicular wand contact. Lesion success was assessed with differential bidirectional pacing. Lesion lines included an antrum-oriented pulmonary vein isolation, a roof line between left and right pulmonary veins, and an anterior mitral line connecting the roof line with the superior mitral annulus. After 6.2 months, two patients required re-isolation of the pulmonary veins. Forty percent of the others had recurrence of PAF, which was now successfully suppressed with medication. The authors believed this contact pressure-controlled procedure simplified lesion line creation and was highly effective for the persistent AF population.

A study by Piorkowski et al enrolled 12 patients for AF ablation.⁹ Physicians were blinded to electrogram amplitudes, pacing thresholds, and impedances at catheter tip-to-tissue interface. The EnSite Contact™ system from St. Jude Medical was used. Local resistance and reactance between catheter tip and tissue surface were measured and combined in an ECI. Increased catheter contact led to an increase in ECI. The authors concluded that the measurement of local impedances between catheter tip and tissue was feasible for describing electrical catheter contact.

Where Are We?

Piorkowski has argued that force alone may be inadequate in different types of tissue (smooth vs. trabeculated) and with different tip-to surface alignments (perpendicular vs. parallel orientation), and also that CF may be difficult to apply to current catheters. He argues for use of an electrical coupling index.⁹ Further, Burkhardt asks, ‘Is the contact required for an acceptable lesion the same in a fibrillating atrium as it is in a trabeculated ventricle?’²³ In addition, he asks how CF applies to other ablation energies, and wonders if the same values will apply equally to magnetic and robotic systems.

The questions will no doubt continue. However, one cannot argue that CF is definitely a big new player on the ablation scene. The results of the US clinical trials are sure to bring valuable insights into the role contact force sensing will play.

References

1. Haines D. Biophysics of radiofrequency lesion formation. In: Huang SKS and Wood M. Catheter Ablation of Cardiac Arrhythmias. Philadelphia: Elsevier, 2006.
2. Nakagawa H, Ikeda A, Govari A, et al. Electrogram amplitude and impedance are poor predictors of electrode-tissue contact force for radiofrequency ablation (abstract). Heart Rhythm 2009:6:S12.
3. Shah DC, Schmidt B, Arentz T, et al. Catheter contact force during human right and left atrial mapping in humans. Heart Rhythm 2009;6:S274.
4. 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 pops and thrombus. Circ Arrhythm Electrophysiol 2008;1:354–362.
5. DiBiase L, Natale A, Barrett C et al. Relationship between catheter forces, lesion characteristics, “popping”, and char formation: Experience with robotic navigation system. J Cardiovasc Electrophysiol 2009;20:436–440.
6. Perna F, Heist EK, Danik SB, et al. Assessment of catheter tip contact force resulting in cardiac perforation in swine atria using force sensing technology. Circ Arrhythm Electrophysiol 2011;4:218–224.
7. www.ClinicalTrials.gov. Identifier NCT01385202. ThermoCool®SmartTouch™ Catheter for the treatment of symptomatic paroxysmal atrial fibrillation. Accessed July 9, 2011.
8. www.ClinicalTrials.gov. Identifier NCT01122173. Use of the Hansen Medical System in Patients with Atrial Fibrillation (ARTISAN AF). Accessed July 10, 2011.
9. Piorkowski C, Sih H, Sommer P, et al. First in human validation of impedance-based catheter tip-to-tissue contact assessment in the left atrium. J Cardiovasc Electrophysiol 2009;20:1366–1373.
10. 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.
11. www.ClinicalTrials.gov. Identifier NCT01278953. TOCCASTAR-TactiCath Contact Force Ablation Catheter Study for Atrial Fibrillation. Accessed July 11, 2011.
12. 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.
13. Ikeda A, Nakagawa H, Lambert H, et al. Predictors of steam pop during radiofrequency ablation at high contact force and moderate RF power in canine beating heart (abstract). Heart Rhythm 2011;8:S205.
14. Shah D, Hendrik L, Langenkamp A, et al. Catheter tip force required for mechanical perforation of porcine cardiac chambers. Europace 2011;13:277–283.
15. Liu T, Amorn A, Wang X, et al. Unipolar electrograms and contact force: What have we learned? (abstract) Heart Rhythm 2011;8:S400.
16. 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.
17. Swale MJ, Gard J, Johnson SB, Packer DL. Intracardiac echocardiography (ICE) validation of contact force sensing to guide irrigated tip ablation: What does measured contact force imply? (abstract). Heart Rhythm 2011;8:S388.
18. Okumura Y, Johnson SB, Bunch TJ, et al. A systematic analysis of in vivo contact forces on virtual catheter tip/tissue surface contact during cardiac mapping and intervention. J Cardiovasc Electrophysiol 2008;19:632–640.
19. Reddy VY, Neuzil P, Kautzer J, et al. Low catheter-tissue contact force results in late PV reconnection (abstract). Heart Rhythm 2011;8:S26.
20. Nakagawa H, Natale A, Kautzner J, et al. Locations of high contact force in atrial fibrillation ablation: Limiting RF power based on contact force may prevent steam pop with effective lesions (abstract). Heart Rhythm 2011;8:S456.
21. Schreieck J, Weig HJ, Slawomir W, et al. High efficacy of dissection of anterior left atrium by contact-controlled robotic navigated RF ablation in persistent atrial fibrillation (abstract). Heart Rhythm 2011;8:S476.
22. Herrera C, Jais P, Neuzil P, et al. Dynamics of contact force using RF ablation catheter varies in different sites of LA. Cardiostim 2010, Nice, France. 229/5.
23. Burkhardt JD, Natale A. Contact force sensing: A speedometer for a lost driver. J Cardiovasc Electrophysiol 2010;21:1044–1045.