Catheter ablation is one of the primary treatments for

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1 Assessment of Catheter Tip Contact Force Resulting in Cardiac Perforation in Swine Atria Using Force Sensing Technology Francesco Perna, MD; E. Kevin Heist, MD, PhD; Stephan B. Danik, MD; Conor D. Barrett, MD; Jeremy N. Ruskin, MD; Moussa Mansour, MD Background Force sensing is a recently developed technology that allows the determination of the contact force (CF) at the tip of the catheter during electrophysiology procedures. Previous studies suggested that the optimal CF for adequate catheter contact ranges between 10 and 40 g. The aim of this study was to determine the CF needed to cause perforation in the swine atria. Methods and Results Pericardial access was obtained at the beginning of the study in a swine model to drain pericardial effusions. Electroanatomic maps of the right atrium (RA) and left atrium (LA) were constructed. Ablation was performed using an irrigated-tip radiofrequency catheter equipped with force-sensing technology (30 W, 30 ml/min, for 30 seconds). Perforations of the LA and RA wall were intentionally performed in different locations with and without radiofrequency ablation. CF values preceding each perforation were recorded. A total of 111 cardiac perforations were achieved in 7 pigs. The overall average CF resulting in perforation was g (range, 77 to 376 g). This was significantly lower after 30 seconds of radiofrequency delivery: g versus g (P ). The average value of CF resulting in perforation was not statistically different between the RA and the LA ( g versus g) (P 0.29). Conclusions Perforation of the atrial wall in a swine model can occur over a wide range of CF values. Perforation can occur with a CF as low as 77 g. Ablation reduces the perforating force by 23%. (Circ Arrhythm Electrophysiol. 2011;4: ) Key Words: contact force force sensing cardiac perforation catheter ablation cardiac tamponade Catheter ablation is one of the primary treatments for symptomatic drug-refractory atrial fibrillation. It is also considered a first-line therapy for other atrial arrhythmias. 1 8 One of the major complications of this procedure is cardiac perforation and tamponade, which can occur in all cardiac chambers Contact force (CF) is an important predictor of radiofrequency ablation efficacy. More importantly, it is also a very important factor for the safety profile of electrophysiology procedures because excessive CF can result in severe cardiac complications, including perforation and tamponade. Previous studies have shown that electrogram parameters and impedance are poor predictors of CF, 13 thus highlighting the strong necessity of a reliable tool to measure the actual CF at the catheter tip. Other preclinical and clinical studies showed that real-time information about electrode-tissue CF during mapping and ablation procedures can help reduce the risk of periprocedural complications Force sensing is a recently developed technology that allows determination of the CF at the tip of the catheter during mapping and ablation procedures. Studies using this technology suggest that the optimal CF to achieve adequate electrode tip tissue contact and to reduce the cardiac complication rate ranges between 10 and 40 g. 14,17 At this time, however, a CF limit value resulting in cardiac perforation in the beating heart has not yet been identified. Clinical Perspective on p 224 In this preclinical in vivo study, we aim to determine the CF needed to cause perforation in the beating swine atria during catheter manipulation and ablation. Methods Animal Model Setup and Monitoring of Life Parameters and Complications The study was performed in the Experimental Electrophysiology Laboratory at Massachusetts General Hospital (Boston, MA). The procedures were performed on 7 male Yorkshire pigs, obtained from a United States Department of Agriculture licensed facility (Tufts University, Boston, MA). This study protocol was reviewed and approved by the Subcommittee of Research Animal Care, which serves as the Institutional Animal Care and Use Committee for the Received August 20, 2010; accepted December 16, From the Heart Center, Massachusetts General Hospital, Boston, MA. Correspondence to Moussa Mansour, MD, Cardiac Arrhythmia Service, Massachusetts General Hospital, GRB 109, 55 Fruit St, Boston, MA mmansour@partners.org 2011 American Heart Association, Inc. Circ Arrhythm Electrophysiol is available at DOI: /CIRCEP

2 Perna et al Perforating Force Assessed With Force Sensing 219 Figure 1. Novel open irrigated-tip ablation catheter equipped with the force sensing technology is depicted. A, Scheme of the inner structure of the catheter showing the transmitter coil, the precision spring, and the location sensors (as indicated by labels). B, Photograph of the catheter, external view. C, Photograph of the catheter showing the transmitter coil (asterisk) and the precision spring (arrow). Massachusetts General Hospital, according to the American Association for Laboratory Animal Care standards for proper research animal care. After an overnight fast, the animals were induced with tiletamine HCl/zolazepam HCl (Telazol, 4.4 mg/kg), intubated, and maintained under general anesthesia with isoflurane (1% to 3%); they were kept in a supine position on the operating table, and vital parameters, including invasive blood pressure measurement, were constantly monitored. Pericardial access was obtained at the beginning of the study to assess for subsequent perforation and to drain pericardial effusions. A 10F linear phased array ultrasound catheter for intracardiac echography (AcuNav, Biosense Webster Inc, Diamond Bar, CA) was advanced into the right atrium (RA) to guide the transseptal puncture and to help detect early pericardial effusions. After the transseptal puncture, an intravenous bolus dose of unfractionated heparin of 100 IU/kg was administered. Electroanatomic Mapping and Radiofrequency Ablation Surface electrocardiograms and bipolar endocardial electrograms from the coronary sinus and the mapping/ablation catheter were continuously monitored and stored on a computer-based digital amplifier/recorder system (Prucka CardioLab, GE HealthCare). Intracardiac electrograms were filtered from 30 to 500 Hz. Electroanatomic mapping of the RA and the left atrium (LA) were performed using a magnetic-based electroanatomic mapping system (Carto System, Biosense Webster Inc, Diamond Bar, CA). Openirrigated radiofrequency ablation was also performed within the atrial chambers (30 W, 30 ml/min). A long, deflectable introducer sheath (Agilis, St Jude Medical) was used to improve catheter stability and obtain better transmission of the CF to the endocardial surface. Force-Sensing Technology An investigational, open irrigated-tip radiofrequency ablation catheter, equipped with a force sensor, was used for mapping and ablation (ThermoCool SmartTouch, Biosense Webster Inc, Diamond Bar, CA) (Figure 1). The force-sensing capability of this catheter is based on the electromagnetic location technology used in the Carto System. The catheter tip electrode is mounted on a precision spring that permits a small amount of electrode deflection. A transmitter coil that is coupled to the tip electrode, distal to the spring, emits a location reference signal. Location sensor coils placed at the proximal end of the spring detect micromovement of the transmitter coil, representing movement of the tip electrode on the spring. The system senses the location information of the sensor and calculates the associated force based on the known spring characteristics. Atrial Chamber Perforations Perforations of the LA and RA wall were intentionally performed in different locations by progressively increasing the force applied to the ablation catheter. When ablation was performed, radiofrequency energy was applied for 30 seconds at 30 W and 30 ml/min irrigation before increasing the pressure on the catheter tip. The occurrence of perforation was assessed by means of (1) fluoroscopy, (2) electroanatomic location of the catheter tip, (3) loss of good atrial electrogram on the ablation catheter tracing of the recorder system (Figure 2), (4) sudden drop of the CF value (Figure 3), (5) initial rise in impedance followed by a sudden fall, and (6) evidence of atrial perforation at necropsy (Figure 4). Maximum CF values closely preceding each perforation were recorded. Perforations were performed in the RA in the appendage, the free wall, and the roof. In the LA, perforations were performed in the appendage and the roof. The locations of perforations were evenly spread throughout the atria and were assessed by electroanatomic mapping to prevent overlap. The diagnosis of early pericardial effusion was detected by intracardiac echography. When blood started to accumulate in the pericardial space, it was withdrawn through the preexisting pericardial access and it was then reinfused via a central vein. Statistical Analysis Results are expressed as mean SD. Contact force values were tested for normality with the use of the Shapiro-Wilk test in the statistical software, STATA (program swilk ). When these were found to be nonnormal, all CF values were log-transformed before any testing of statistical hypotheses (swilk confirmed that these log-transformed CF values were compatible with the assumption of a normal distribution of errors). Hypotheses regarding differences in perforation CF values were tested with a 3-factor ANOVA model (Analysis of Variance, program anova in STATA) with 2 fixed effects, RA versus LA location of perforation and presence or absence of prior radiofrequency ablation, and 1 random effect, pig, to account for the multiple perforations per pig in the face of correlations of CF values within the pig. An interaction term was included in the model to test for a location*ablation interaction. When this interaction was found to be significant, 2 separate linear models were run for the 2 chambers, LA and RA. Results Seven male pigs weighting kg (36 to 54 kg) were used. A total of 111 perforations were performed. Fifty-four perforations were performed in the RA, 24 during ablation. In the LA, 57 perforations were performed, 28 during ablation. Contact Force Value Resulting in Perforation The overall average CF value resulting in perforation was g. The lowest CF value resulting in perforation was 77 g and the highest was 376 g.

3 220 Circ Arrhythm Electrophysiol April 2011 Figure 2. Evidence of cardiac perforation by means of electroanatomic mapping, fluoroscopy, and loss of a good atrial electrogram. A, Electroanatomic map of the RA. Brown dots (arrows) represent perforation points. Magenta dots represent points inside the pericardium. Light pink tags represent the tricuspid annulus. B, Left to right: Progression of the ablation catheter during perforation of the LA appendage. C, Loss of good atrial electrogram on the ablation catheter (arrows) during perforation of the atrial wall. Abl indicates ablation catheter tracing; CS, coronary sinus catheter tracing.

4 Perna et al Perforating Force Assessed With Force Sensing 221 Figure 3. Real-time CF measurement during a perforation using force-sensing technology. A sudden drop of the CF tracing is reported (arrow). Tracing appears green when the CF is within the targeted range, red when an excessive force is applied, and yellow when the CF is suboptimal. Red number (circle) represents CF at the time of perforation. Scheme on the right of the tracing (asterisk) shows the incidence angle of the catheter tip on the endocardial surface. RA Versus LA Log perforation CF values were found to be not different between the 2 chambers in the overall ANOVA model. The value of CF recorded before perforation in the RA was g compared with g in the LA (Figure 5). Mean perforation CF values are reported rather then the logarithms of these values, which were used only for hypothesis testing. Effect of Ablation Ablation at 30 W and 30 ml/min irrigation was applied for 30 seconds before perforation in 24 sites in the RA and 28 in the LA. The CF causing perforation was lower after 30 seconds of radiofrequency delivery: g versus g. Using the same overall ANOVA model, the interaction term location*ablation for log CF values was found to be significant (P ). Subsequently, separate ANOVA models were run for each chamber. In the RA, mean log CF values were significantly different between perforations without radiofrequency and perforations with radiofrequency ( g versus g, P 0.01). Similarly, in the LA, these differences were also significant ( g versus g, P ). Discussion This animal study is the first to assess the contact force resulting in perforation in the atria in the beating heart. It Figure 4. Evidence of RA posterior wall perforation after excision of the swine heart. resulted in several findings: (1) perforation of the atrial wall can occur over a wide range of CF values; (2) contact force as low as 77 g may cause atrial perforation; (3) radiofrequency ablation reduces the perforating force by 23%; and (4) there is no difference in the CF needed to perforate the RA compared with the LA. Percutaneous radiofrequency catheter ablation is one of the primary treatment of cardiac arrhythmias. This treatment modality, however, has a complication rate of 2% to 5% according to some studies. 3,5 7,18 Cardiac perforation leading to tamponade, caused by catheter manipulation within the heart chambers, is one of the most serious complications associated with this procedure. 10 Since CF has emerged as an important determinant of radiofrequency lesion efficacy and safety, precise calibration of the force applied to the catheter during mapping and ablation procedures could be a crucial advance in reducing this sort of complication. Previous experiences highlighted the critical need for an accurate and reliable tool to measure CF during ablation procedures. Surrogate measures of CF, such as electrogram parameters (injury current, amplitude and dv/dt) and electrode tip impedance, have been shown to be inaccurate. 13 Force sensing is a recently developed tool that allows a direct real-time measurement of CF during catheter manipulation and radiofrequency ablation. Different technologies are currently being investigated for this purpose. The technology used in this study is based on the electromagnetic location properties of the Carto System and is capable of providing a precise estimate of the actual CF (sensitivity 1 g). Prior studies performed on animals and humans concluded that real-time contact force sensing is feasible and that it can improve both the efficacy and safety of catheter ablation procedures. 16,17 In their study on a canine thigh muscle preparation, Yokoyama et al 14 found that tissue temperature, radiofrequency lesion size, and incidence of steam pop and thrombus increase significantly while raising electrode CF at a constant radiofrequency power output. In another recently published study in an ex vivo model, catheter tip CF was found to have an important impact on both the ablation lesion size and the incidence of steam pops during irrigated-tip radiofrequency ablation. 19 The studies conducted to date suggested to us setting the target range of CF value between 10 g and 40 g. Keeping CF within this range, radiofrequency lesion size should be

5 222 Circ Arrhythm Electrophysiol April 2011 Figure 5. Chart showing the average of CF values recorded in the RA and LA with and without radiofrequency ablation. RF indicates radiofrequency. optimal and cardiac complications may be minimized. 14,19 21 Without knowledge of the real-time CF, transient forces 100 g were commonly recorded even among experienced operators. 22 Although forces even much greater than 100 g may not usually lead to complications, and although the average perforating force was above this value in our experience ( g), we found that perforation can occur over a wide range of CF, and it can even happen with CF as low as 77 g. On the other hand, no perforations occurred in our experience in these animals when CF was within the target range of 10 to 40 g. It thus appears clear that the safety margin of CF, especially during radiofrequency delivery, is relatively small. This finding has utmost clinical implications because it underscores the crucial importance of having an accurate feedback on real-time CF. Other investigators also studied forces resulting in perforation and found that these values were always above 100 g. 23 This study, however, was performed ex vivo on explanted animal hearts. The myocardial wall movement toward the catheter tip in the beating heart may have played a role in easing the perforating process in our study. Moreover, the in situ beating heart of a living animal indeed represents a more physiological condition, namely the closest to the real environment in which ablations are actually performed, given the particular nature of this study. Irrigated-tip radiofrequency ablation reduced the CF needed to perforate the atrial wall by 23%. This finding was in agreement with ex vivo studies conducted by other groups. 19,23 Irrigated-tip catheters allow more power to be delivered, therefore facilitating the accomplishment of larger and transmural ablation lesions, but irrigation also alters the utility of catheter tip temperature monitoring. 27 As a result, further information, like real-time CF measurement, would be important to improve the safety of this type of ablation, particularly when using a long sheath. The ability to monitor CF during radiofrequency application would allow the operators to reduce the risk of perforation while preserving an effective ablation lesion. The lowest perforating CF of 77 g was recorded during ablation in our study; nevertheless, some perforations were performed in our study with CF 100 g even in the absence of radiofrequency delivery. Therefore, real-time CF measurement has the potential to improve the safety of catheter manipulation related complications during both cardiac mapping and ablation procedures. Study Limitations The data provided herein must be viewed with consideration of some limitations. Forces causing perforation may differ in humans as compared with pigs. Moreover, the animal model we used may not account for the effects of age and underlying atrial pathology that can be present in patients affected by atrial tachyarrhythmias. For these reasons, caution must be taken in translating our results into the clinical practice. The risk of cardiac tamponade may not be completely eliminated in clinical practice by avoidance of macroscopic perforation because microperforation and tamponade in the setting of full heparinization might occur with lower CF. Therefore, complete elimination of this type of complication might not be achievable even using the force-sensing technology because of the high variability of cardiac anatomy and coagulation status of individual patients. The small number of animals used in this work could have reduced the statistical power of our analyses. Nevertheless, we performed several perforations in different areas of both atrial chambers in order to improve the reproducibility of our findings. Because we used long sheaths to improve catheter support, we cannot provide data regarding the CF needed to perforate in the absence of long sheaths; however, we could not achieve perforation without the support of a long sheath because of catheter shaft buckling, despite multiple attempts to perforate in this manner at multiple LA and RA sites. These observa-

6 Perna et al Perforating Force Assessed With Force Sensing 223 tions suggest that atrial perforation is less likely to occur in the absence of a long sheath. The risk of perforation probably depends on the mechanical properties of the ablation catheters, however, and perforation without a long sheath may be more likely with a stiffer catheter and may also be dependent on the specific geometry of the catheter tip. Conclusions Perforation of the atrial wall in a swine model can occur over a wide range of CF values. Perforation can result from a CF as low as 77 g, but in our experience it did not occur with CF below 40 g. Irrigated-tip radiofrequency ablation reduced the force required for perforation by 23% in this study. There was no statistically significant difference between the CF needed to perforate the RA and the LA. Contact force sensing may represent a useful new tool for maintaining CF in the target range and thereby may minimize the risk of cardiac perforation during electrophysiology procedures. Acknowledgments We thank Shawna M. Laferriere, BS, for expert technical support and John Newell for expert opinion for statistical analysis. Sources of Funding The study was partially funded by the Deane Institute for Integrative Research in Atrial Fibrillation and Stroke and a research grant from Biosense Webster. The authors used an electroanatomic system owned by Biosense Webster with force sensing ability to perform the experiments. Disclosures Dr Heist received research grants from Biotronik and St Jude Medical, honoraria from Biotronik, Boston Scientific, Sorin, and St Jude Medical, and serves as consultant to Biotronik, Boston Scientific, and St Jude Medical. Dr Ruskin received fellowship support from Biosense Webster, Boston Scientific, Medtronic, and St Jude Medical; he serves on the Scientific Advisory Board of CardioInsight, the Clinical Oversight Committee for CardioFocus, and the Scientific Steering Committee for CryoCath and as a consultant for Biosense Webster and Medtronic. Dr Mansour received research grant support from Biosense Webster and St Jude Medical and serves as consultant for both companies. References 1. Blomstrom-Lundqvist C, Scheinman MM, Aliot EM, Alpert JS, Calkins H, Camm AJ, Campbell WB, Haines DE, Kuck KH, Lerman BB, Miller DD, Shaeffer CW Jr, Stevenson WG, Tomaselli GF, Antman EM, Smith EM Jr, Alpert JS, Faxon DP, Fuster V, Gibbons RJ, Gregoratos G, Hiratzka LF, Hunt SA, Jacobs AK, Russell RO Jr, Priori SG, Blanc JJ, Budaj A, Burgos EF, Cowie M, Deckers JW, Garcia MA, Klein WW, Lekakis J, Lindahl B, Mazzotta G, J. Morais C, Oto A, Smiseth O, and Trappe HJ. 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7 224 Circ Arrhythm Electrophysiol April Nakagawa H, Ikeda A, Shah DC, Lambert H, Leo G, Vanenko Y, Merino J, Seres K, Sharma T, Pitha JV, Jackman WM. Role of contact force in esophageal injury during left atrial radiofrequency ablation (abstract). Heart Rhythm. 2008;5:S Shah DC, Schmidt B, Arentz T, Kuck K-H, Neuzil P, Latcu G, Hindricks G, Kautzner J, Aeby N, Lambert H. Catheter contact force during human right and left atrial mapping in humans (abstract). Heart Rhythm. 2009; 6:S Shah DC, Lambert H, Saoudi N, Vanenkov Y, Walpoth B, Aeby N, Gentil-Baron P. Catheter tip force required to mechanically perforate the cardiac free wall (abstract). Heart Rhythm. 2008;5:S Nakagawa H, Yamanashi WS, Pitha JV, Arruda M, Wang X, Ohtomo K, Beckman KJ, McClelland JH, Lazzara R, Jackman WM. Comparison of in vivo tissue temperature profile and lesion geometry for radiofrequency ablation with a saline-irrigated electrode versus temperature control in a canine thigh muscle preparation. Circulation. 1995;91: Otomo K, Yamanashi WS, Tondo C, Antz M, Bussey J, Pitha JV, Arruda M, Nakagawa H, Wittkampf FH, Lazzara R, Jackman WM. Why a large tip electrode makes a deeper radiofrequency lesion: effects of increase in electrode cooling and electrode-tissue interface area. J Cardiovasc Electrophysiol. 1998;9: Yokoyama K, Nakagawa H, Wittkampf FH, Pitha JV, Lazzara R, Jackman WM. Comparison of electrode cooling between internal and open irrigation in radiofrequency ablation lesion depth and incidence of thrombus and steam pop. Circulation. 2006;113: Petersen HH, Chen X, Pietersen A, Svendsen JH, Haunso S. Tissue temperatures and lesion size during irrigated tip catheter radiofrequency ablation: an in vitro comparison of temperature-controlled irrigated tip ablation, power-controlled irrigated tip ablation, and standard temperature-controlled ablation. Pacing Clin Electrophysiol. 2000;23:8 17. CLINICAL PERSPECTIVE Contact force is an important predictor of the efficacy of catheter-based radiofrequency ablation. It also represents a very important safety factor for electrophysiology procedures because excessive contact force can result in cardiac perforation. The optimal range for contact force needed to create adequate lesions has not been defined yet. The findings of this study demonstrated that cardiac perforation can occur with contact force values as low as 77 g during ablation. This may help define the range of optimal contact force, which is needed to improve the safety and efficacy of the ablation procedure.