Impact of electrode tip shape on catheter performance in cardiac radiofrequency ablation Argyrios Petras, PhD,*1 Zoraida Moreno Weidmann, MD,†1 Marina Echeverría Ferrero, MS,‡ Massimiliano Leoni, PhD,* Jose M. Guerra, MD, PhD,†2 Luca Gerardo-Giorda, PhD*x2 From the *RICAM, Austrian Academy of Sciences, Linz, Austria, †Department of Cardiology, Hospital de la Santa Creu i Sant Pau, IIB SANT PAU, Universitat Autonoma de Barcelona, CIBERCV, Barcelona, Spain, ‡BCAM–Basque Center for Applied Mathematics, Bilbao, Spain, and xInstitute for Mathematical Methods in Medicine and Data-Based Modelling, Johannes Kepler University, Linz, Austria. BACKGROUND The role of catheter tip shape on the safety and ef- tip become comparable to those generated by the cylindrical tip. ficacy of radiofrequency (RF) ablation has been overlooked, The 2 tips feature different safety profiles: CF and power are the although differences have been observed in clinical and research main determinants of pops for the spherical tip; power is the fields. main factor for the cylindrical tip; and CF has a marginal effect. The cylindrical tip is more prone to pop generation at higher powers. OBJECTIVE The purpose of this study was to analyze the role of Saline irrigation and blood flow effect do not depend on tip shape. electrode tip shape in RF ablation using a computational model. CONCLUSION Tip shape determines the performance of ablation METHODS We simulated 108 RF ablations through a realistic catheters and has a major impact on their safety profile. The cylin- 3-dimensional computational model considering 2 clinically used, drical tip shows more predictable behavior in a wide range of CF open-irrigated catheters (spherical and cylindrical tip), varying con- values. tact force (CF), blood flow, and irrigation. Lesions are defined by the 50 C isotherm contour and evaluated by means of width, depth, KEYWORDS Cardiac ablation; Computational modeling and simula- depth at maximum width, and volume. Ablations are deemed as tion; Electrode tip; Lesion science; Open irrigation; Radiofrequency; safe, critical (tissue temperature .90 C), and pop (tissue temper- Steam pop ature .100 C). RESULTS Tissue–electrode contact is less for the spherical tip at (Heart Rhythm O2 2022;3:699–705) © 2022 Heart Rhythm Society. low CF but the relationship is inverted at high CF. At low CF, the cy- Published by Elsevier Inc. This is an open access article under the CC lindrical tip generates deeper and wider lesions and a 4-fold larger BY-NC-ND license (http://creativecommons.org/licenses/by-nc- volume. With increasing CF, the lesions generated by the spherical nd/4.0/). Introduction ences in terms of pop formation when comparing various Radiofrequency (RF) catheter ablation is extensively used for currently available, open-irrigated RF ablation catheters, sug- the treatment of tachyarrhythmias. Several improvements in gesting that catheter tip shape also may play a significant the technology have been implemented over the years to role.2–4 However, classic research approaches have not maximize lesion size while minimizing complications. been able to elucidate which characteristics of tip shape Despite an optimal equilibrium with the current available may have an impact on the safety and efficacy of lesion technology, complications still occur and are not always pre- formation and how they can be used to optimize ablation. dictable. The study and development of RF ablation has been based It is well known that contact force (CF), power, and abla- mainly on in vivo and in vitro experiments, which have tion time are the main determinants of lesion characteristics important limitations that are difficult to circumvent, mostly and safety.1 However, some groups have observed differ- related to the large number and variability of factors interre- lated in lesion formation. The recent development of computational models for RF 1 Dr Argyrios Petras and Dr Zoraida Moreno Weidmann contributed equally ablation is a powerful tool to evaluate interacting factors in to this manuscript and are joint first authors. 2Dr Jose M. Guerra and Dr Luca lesion formation in an independent manner.5,6 This approach Gerardo-Giorda share last authorship. Address reprint requests and corre- spondence: Dr Jose M. Guerra, Department of Cardiology, Hospital de la allows study of specific catheter designs7 and assessment of Santa Creu i Sant Pau, Sant Antoni M. Claret 167, 08025 Barcelona, Spain. new protocols, such as high-power/short-duration applica- E-mail address: jguerra@secardiologia.es. tions.8 2666-5018/© 2022 Heart Rhythm Society. Published by Elsevier Inc. This is an open access article https://doi.org/10.1016/j.hroo.2022.07.014 under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 700 Heart Rhythm O2, Vol 3, No 6PA, December 2022 Tissue characteristics KEY FINDINGS We studied virtual porcine cardiac tissue of 20-mm thick- - Electrode tip shape plays a crucial role in terms of the ness.11 The biophysical and mechanical parameters are sum- safety and efficacy of radiofrequency ablation. marized in the Supplementary Material. - The cylindrical tip has a more predictable behavior in a Lesion assessment wide range of contact force values but seems to be Considering irreversible tissue damage at 50 C,12 lesions more prone to steam pop generation. were identified by the 50 C isotherm contour. The measured - Each catheter design needs different specific settings dimensions were depth (D), lesion volume (V), width (W), to achieve similar ablation efficiency while avoiding and depth of maximum width (DW), considering the zero complications. at the undeformed endocardial surface (Figure 1). The purpose of this study was to analyze the role of cath- Ablation settings eter tip shape in RF ablation using our validated advanced For both electrode tips, we simulated 30 seconds of ablation computational model,9 which accounts for the elastic defor- for several protocols in terms of power (20–50 W) and CF mation of the electrode–cardiac tissue interface, thus allow- (5g–20g). All simulated ablations are performed perpendic- ing a full evaluation of different scenarios. Recent ular to the endocardium, resulting in progressive deformation experimental studies have highlighted the correlation be- of the tissue. The ablation protocols use a power-control tween catheter contact area and lesion size.10 Because the mode without temperature limit. catheter contact area depends on tissue deformation, which, Two blood flow protocols commonly used in in vitro ex- in turns, depends on both CF and tip shape, our model repre- periments were considered2: high blood flow (0.5 m/s), rep- sents a natural candidate to study, in an independent manner, resenting heart areas of high flow such as the cavotricuspid the different tip shapes and their role in RF ablation. The re- isthmus, ventricle, or outflow tracts; and low blood flow sults will provide a better understanding of the pathophysi- (0.1 m/s), representing flow in areas such as below a valve ology of RF ablation and pop occurrence and possibly leaflet, the pulmonary veins, or the atrium. Two saline irriga- identification of hazardous ablation protocols. tion rates were simulated that typically are used in RF: low rate (17 mL/min) and high rate (30 mL/min). Complication assessment Methods We considered a steam pop to occur when the tissue temper- Computational model ature reached 100 C.12 Whenever a pop occurred, the simu- Our computational model of RF is based on an in vitro exper- lation was terminated, and the time to pop was recorded. Due imental setup reported by Guerra et al.2 In brief, the model to slight fluctuations of the temperature at which steam pop simulates a porcine heart considering fixed conditions, which occurs,13 we defined a critical area, which corresponds to allows for standard comparison of different ablation settings. simulated lesions in which the tissue temperature reaches be- The mathematical model, detailed in the Supplementary Ma- tween 90 C and 100 C. Although reaching a boiling temper- terial, is built within a 3-dimensional computational frame- ature does not automatically result in a pop, given the work,10 which considers tissue elasticity, blood flow, and seriousness of this complication, we equated the risk of pop its interaction with the irrigated saline and the thermoelectric with the occurrence of it. Each virtual lesion was classified interaction between the electrode and the target ablation area. as pop, critical, or safe, based on the maximum tissue temper- Simulations were performed using our self-developed ature (Tmax) reached. For each electrode tip shape, a color- software in FEniCS-HPC (www.fenics-hpc.org) on BCAM coded risk map of the protocol was drawn based on CF and  in-house HPC cluster (18 nodes/624 cores with 4-TB RAM). power. The ablation protocols that led to Tmax ,90 C after No patient data or animals were used in this study. The 30 seconds were classified as safe and represented in green; computer model was validated in a previous study by Petras for Tmax between 90 C and 100 C, the ablation protocol et al.9 was deemed critical, represented as yellow; and for Tmax 5 100 C, the protocol was deemed pop, represented in red. We registered the time to critical temperature as a surro- gate of the safe ablation interval time, defined as the time Electrode tip shape from the beginning of an ablation to Tmax .90 C. We considered 2 different electrode tip shapes, based on commercially available and clinically used, open-irrigated Study design catheters: one with a hemispherical tip and one with a cylin- The study was performed in 3 parts. (1) In the first set of ex- drical tip. Both electrodes have diameter of 2.33 mm (7F) and periments, we studied the safety of both catheter shapes at length of 3.5 mm, and feature 6 irrigation pores (0.5-mm different ablation protocols in terms of power (20–50 W) diameter) (Figure 1). and CF (5g–20g). For this set, we maintained constant blood Petras et al Impact of Electrode Shape on Cardiac RF Ablation 701 Figure 1 A: The computational model and the 2 electrode tip shapes studied. B, C: Simulated lesions with each catheter. The area of irreversible tissue lesion is delimited by the blue line, which represents the 50 C isotherm. Lesion measurements are depth (D), width (W), and depth of maximum width (DW) flow (0.5 m/s) and low saline irrigation rate (17 mL/min), whereas very low CF offers a wide safety margin at any po- which are the values used in the in vitro experiments against wer tested (up to 50 W). An increase in 1 of the 2 parameters which the computational model was validated. (2) The sec- over the other shows a progressive reduction in safety. ond set of experiments aimed to compare the impact of CF For the cylindrical tip, power proves to be the main factor in terms of lesion size for both catheter tip shapes. For that contributing to tissue overheating. Whereas power ,30 W purpose, we chose a routine clinical protocol of 30 W for seems to be generally safe at any CF, at 35 W the risk of 30 seconds and 4 different CF levels (from 5g to 20g). Blood pop rises suddenly even at low CF. In contrast, variations flow and irrigation rate were kept at 0.5 m/s and 17 mL/min, of CF seem to have a marginal effect on the risk of pop. respectively. (3) Finally, we analyzed the impact of blood The time to critical temperature (ie, safe ablation interval flow and saline irrigation level on safety and efficacy. For time) is shown in Figure 3. This interval is larger at lower po- each catheter, we focused on the highest power not producing wer, lower CF, and with the spherical tip rather than with cy- a pop identified from the previous experiments at a standard lindrical one. Stepwise increases in CF lead to an exponential CF of 10g. decline of this time for the spherical tip, whereas the decline of the curve is smoother for the cylindrical tip. Results Impact of CF for each tip shape A total of 108 RF catheter ablation lesions were simulated, We compared 4 different CFs in a standard clinical scenario combining tip shape, CF, RF power, saline irrigation rate, (30 W, 30 seconds), and the results are summarized in and blood flow. Table 1 and Figure 4. We observed that at low CF, the cylin- drical tip generates deeper and wider lesions (and a 4-fold Impact of catheter tip shape on safety larger volume) than the spherical tip. With increasing CF, The 2 catheter tip shapes show very different risk profiles the lesions generated by the spherical tip become progres- (Figure 2). For ablations with the spherical tip, a combination sively closer to those generated by the cylindrical tip. More- of CF and power is needed to lead to pop. A low-power de- over, increasing CF from 5g to 15g results in a 6-fold larger livery (up to 25 W) would lead to safe lesions at any CF, lesion for the spherical tip but a mere 50% increment for the 702 Heart Rhythm O2, Vol 3, No 6PA, December 2022 Figure 2 Contact force–power steam pop risk maps for each electrode tip shape. Blood flow and irrigation rate were kept at 0.5 m/s and 17 mL/min, respec- tively. Ablation time was 30 seconds. Safe protocols are represented in green, critical in yellow, and pop in red. cylindrical tip. A similar pattern can be observed for Tmax. safety profiles of the 2 tip shapes are very different. (4) Inde- For the spherical tip Tmax is 61 C at 5g and produces a pop pendent of tip shape, power and CF are the 2 major determi- (after 21.5 seconds) at 20g, whereas for the cylindrical tip nants of pop formation, whereas catheter irrigation and blood Tmax only rises from 80 C at 5g to 94 C at 20g. Hence, lesion flow have a minor impact. size and complications are determined mostly by the power for the cylindrical tip and by the combination of power and Model validation CF for the spherical tip. Our computational model was previously validated for a spherical tip against ad hoc, in vitro experimental results.9 Impact of blood flow and saline irrigation For the cylindrical tip, despite lacking a twin experimental For this evaluation, we chose, at a conventional CF of 10g, model for validation, the simulated lesion sizes were in the largest power not producing a pop for each tip shape: good agreement with the ex vivo experimental results recently 35 W for the cylindrical tip and 40 W for the spherical tip. reported by Masnok and Watanabe,10 who used a 6-hole cy- Table 2 summarizes the lesion dimensions and tissue temper- lindrical electrode. Moreover, our results align with the atures with varying blood flows and saline irrigation rates. No nonoccurrence of steam pops reported therein for 30-W/30- steam pop is observed, although all simulated protocols are second ablations in the range of CF (up to 20g) we studied deemed critical for both tip shapes. The overall variation of (Figure 2). Tmax is below 2 C. Tip shape and contact surface Discussion Heat transfer to tissue depends directly on the contact surface To the best of our knowledge, this is the first study to use an between the metallic region of the catheter tip and the cardiac advanced and realistic computational model to explore in a wall.1 Although a recent experimental study stressed the controlled and independent manner the impact of catheter importance of the contact surface between electrode and tis- tip shape on the safety and the efficacy of RF ablation. The sue using 5 different angles of the catheter and a wide range main findings are as follows. (1) The tip shape determines of CF values,10 the role of the catheter tip shape has been the contact surface with the tissue and how it changes based overlooked. Our simulations clearly show how that catheter on CF. (2) CF is more critical for the spherical tip than for the tip shape is a major determinant of the amount of surface cylindrical tip with regard to safety and efficacy. (3) The tip area that comes into contact with the tissue. As shown Figure 3 Time to critical temperature for the 2 electrode tip shapes at different power/contact force combinations. Petras et al Impact of Electrode Shape on Cardiac RF Ablation 703 Table 1 Lesion characteristics for the 2 tip shapes at different CFs this occurs much faster for the spherical tip than for the cylin- CF Tip D (mm) W (mm) DW (mm) V (mm3) Tmax ( C) drical tip, whose contact surface shows only slight variations at a wide range of CFs. Hence, at low CF the percentage in 5g Spherical 3.29 4.84 0.77 32.7 60.8 contact is much lower for the spherical tip than the cylindrical Cylindrical 4.74 7.4 1.31 124.8 80.2 tip (7% vs 13%); at moderate CF the percentage difference 10g Spherical 4.69 6.84 1.49 106.4 77.6 Cylindrical 5.22 7.87 1.62 161.2 86.5 rapidly reduces; and at CF 20g the relationship is inverted 15g Spherical 5.60 8.27 1.28 182.3 92.6 (18% vs 17%). As a result, lesions generated at low CF are Cylindrical 5.60 8.43 1.71 193.4 91.2 larger with the cylindrical tip than with the spherical tip. 20g Spherical 5.55* 8.35* 0.98* 182.3* 100.0* Increasing the CF would require a reduction in power to Cylindrical 5.92 8.85 1.42 220.2 94.0 remain in the safe zone, especially for the spherical tip. Ablation protocol is 30 W, 30 seconds. Blood flow is kept at 0.5 m/s and irrigation rate at 17 mL/min. CF 5 contact force; D 5 depth; DW 5 depth of maximum width; Impact of CF Tmax 5 maximum tissue temperature; V 5 volume; W 5 width. Our analysis clearly shows the critical role of CF in lesion *Protocol popped at 21.4 seconds. Values measured at time of pop. formation and complications for spherical tips, whereas large variations of CF have only a minor impact on cylindrical tips. In line with our results, ex vivo and in vivo experimental in Figure 4 (center panel), for the spherical tip, the tissue sur- studies based on RF lesions using a spherical catheter tip face area that is in contact with the catheter tip has a major have shown that the increase in lesion size depends more dependence on CF. In the absence of CF, only a small area on CF than on power: lesions obtained with 30 W/20g of a spherical tip will be in contact with the tissue. In contrast, were greater in depth and width than those obtained with for the cylindrical tip, the whole base is in contact with the 40 W/10g.14 A recent study compared the lesions obtained tissue, leading to greater power delivery. As CF increases, by matching a predefined value on the lesion size indicator so does the contact surface for both catheters. However, provided by the navigation system of 2 commercially Figure 4 Contact surface percentage with the tissue according to contact force (CF) for each catheter tip shape (spherical in blue, cylindrical in orange) and comparison of the corresponding lesions generated with a 30-W/30-second ablation protocol. Blood flow and irrigation rate were kept at 0.5 m/s and 17 mL/min, respectively. In all plots, blood flows from left to right. 704 Heart Rhythm O2, Vol 3, No 6PA, December 2022 Table 2 Lesion characteristics for the 2 tip shapes applying different blood flows and saline irrigation rates Electrode tip protocol Spherical (40 W, 10g) Cylindrical (35 W, 10g) Blood flow (m/s) HBF (0.5) LBF (0.1) HBF (0.5) LBF (0.1) Saline irrigation rate (mL/min) 30 17 30 17 30 17 30 17 D (mm) 5.48 5.49 5.53 5.50 5.64 5.64 5.68 5.66 W(mm) 8.21 8.29 8.48 8.32 8.66 8.72 8.77 8.79 DW (mm) 2.33 2.34 2.28 2.35 2.24 2.23 2.18 2.36 V(mm3) 176.6 185.3 200.3 186.7 207.7 213.8 222.8 217.9 Tmax ( C) 96.2 96.2 98.0 97.1 98.2 98.0 99.9 99.0 HBF 5 high blood flow; LBF 5 low blood flow; other abbreviations as in Table 1. available catheters (Ablation Index for CARTO3, Biosense the spherical catheter, the only observed pop was at a greater Webster, Inc., Diamond Bar, CA; Lesion Size Index [LSI] CF (20g). for Ensite Precision, Abbott Laboratories, Abbott, IL), Our observations suggest the need for a tailored power similar to those considered in our study.4 Lozano Granero setting based on the anatomic region and the catheter tip et al4 also observed an inversion in lesion size according to design. Use of a spherical tip with low power may be suitable CF and attributed to a suboptimal prediction ability of the for the atrial wall where abrupt changes in CF are foreseeable, LSI when using a spherical catheter in a low-CF setting. thus avoiding the risk of pop or perforation. In contrast, use of Our results suggest that, in line with previous publications,15 cylindrical tips may be appropriate for reaching deeper ven- lesion size indicators should account for reliable estimators of tricular arrhythmogenic substrates, allowing greater heat contact surface between the tip and the tissue, beyond the CF. transfer and larger lesions. Impact of blood flow and catheter irrigation Safety We observed no significant influence on tissue temperature Both electrode tips show an increasing risk of steam pop from variations in blood flow or irrigation rate for the elec- occurrence with increased CF and power. However, for the trode tips used in our study. Available clinical and experi- cylindrical tip the risk of pop depends more on power than mental data also support that the type of irrigation (ie, on CF, whereas for the spherical tip both CF and power are discrete number of pores or surround flow designs) has no equally important. Thus, greater powers can be applied significant impact on lesion size.2,3,10,18–20 with the spherical tip if the CF is kept low, and vice versa. Our study also suggests that the incidence of steam pops is In contrast, for the cylindrical tip, setting the power up to not directly related to the amount of irrigation. Whether the 30 W will be mostly safe, independent of CF. Because power type of irrigation (discrete vs surround flow) has an impact is a selected parameter in contrast to CF, which may vary on pop occurrence remains unclear. Although clinical studies abruptly during the ablation, the behavior of the cylindrical have not reported significant differences,21 in vitro results tip seems overall more predictable than that of the spherical have shown discrepancies.2,3,18,19 tip. Knecht et al16 described differences in 2 catheters having Clinical implications different tip shapes and irrigation in terms of safety. In their Electrode tip shape plays a crucial role in terms of safety and study, the spherical tip shape showed a higher risk of pop, efficacy. The cylindrical tip shape has a more predictable and they related the difference to a more efficient irrigation behavior at a wide range of CF values, although it seems to system that, by underestimating tissue temperature feedback, be more prone to steam pop generation at higher powers. would enable higher power delivery. Iles et al17 observed an However, its low sensitivity to CF favors the generation of increase in pop occurrence at power .40 W delivered by a more homogeneous lesions, thus helping to standardize the spherical tip on a reanimated porcine cardiac model. ablation procedures. Although the authors did not consider CF, they did consider .50% of the electrode tip surface to be in contact with the endocardium, which is more than twice the maximal percent- Study limitations age we studied. In other studies comparing many catheter tip Our calculations are based on ablations performed on a designs, a trend toward more frequent pops with spherical porcine slab of tissue representing an in vitro setting because tips also was found.2,18 Although in a recent study by Lozano that was the setting used to validate the computational et al4 more pops were observed with lesions performed by a model10; therefore, as in the case of in vitro experiments on cylindrical tip, the main factor for the occurrence was the animal models, direct extrapolation of the results to the hu- applied power (from 30 to 60 W at 10g). In contrast, for man clinical setting should be taken with care. However, Petras et al Impact of Electrode Shape on Cardiac RF Ablation 705 the aim of the present work was not to directly provide guide- References lines for clinical settings to be used. 1. Haines DE. 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Our results should force and radiofrequency lesion size and incidence of steam pop in the beating canine heart: electrogram amplitude, impedance, and electrode temperature are help in designing new catheters and safe ablation protocols poor predictors of electrode-tissue contact force and lesion size. Circ Arrhythm in the future. Electrophysiol 2014;7:1174–1180. 15. Shah D, Namdar M. Real-time contact force measurement: a key parameter for Funding Sources: Part of this work was performed when Drs Gerardo- controlling lesion creation with radiofrequency energy. Circ Arrhythm Electro- Giorda, Petras, and Leoni were with BCAM, Bilbao, Spain. Mrs physiol 2015;8:713–721. Echeverría Ferrero was supported by the Basque Government (BERC 16. Knecht S, Sacher F, Forclaz A, et al. Is there a potential benefit to increased irri- 2018-2021) and the Spanish State Research Agency (BCAM Severo gation channels during radiofrequency ablation? Results from a two-center pro- Ochoa Excellence Accreditation SEV-2017-0718). Drs Gerardo-Giorda, spective randomized study. J Cardiovasc Electrophysiol 2011;22:516–520. Petras, and Leoni are partially supported by the State of Upper Austria. 17. Iles TL, Quallich SG, Iaizzo PA. Identification of radiofrequency ablation cath- eter parameters that may induce intracardiac steam pops: direct visualization of Disclosures: Dr Guerra has served as a consultant for Biosense Webster, elicitation in reanimated swine hearts. J Cardiovasc Transl Res 2019; Boston Scientific, and Abbott; received speaker fees from Boston 12:250–256. Scientific and Abbott; and received research grants from Abbott, 18. Moreno J, Quintanilla JG, Molina-Morua R, et al. Morphological and thermody- Medtronic, and Biosense Webster. All other authors have no conflicts to namic comparison of the lesions created by 4 open-irrigated catheters in 2 exper- disclose. imental models: thermodynamic evaluation of 4 open-irrigated catheters. J Cardiovasc Electrophysiol 2014;25:1391–1399. Authorship: All authors attest they meet the current ICMJE criteria for 19. Mehta N, Morgaenko K, Sauer W, Stevenson W, Haines D. Impact of variable authorship. orientation and flow rates on radiofrequency ablation lesions created by externally irrigated catheters: an ex-vivo study. J Atr Fibrillation 2020;13:2353. Ethics Statement: No patient data or animals were used in this study. The 20. Larsen T, Du-Fay-de-Lavallaz JM, Winterfield J, et al. Comparison of ablation computer model was validated in a previous study by the authors. index versus time-guided radiofrequency energy dosing using normal and half- normal saline irrigation in a porcine left ventricular model. J Cardiovasc Electro- physiol 2022;33:698–712. Appendix 21. 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