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Background/Objectives: The QDOT‐MICRO™ catheter allows very high‐power and short‐duration (vHPSD) ablation. This study aimed to investigate lesion characteristics using different ablation settings. Methods: Radiofrequency applications (90 W/4 s, temperature‐control mode with 55°C or 60°C target) were performed in excised porcine myocardium using three different approaches: single (SA), double nonrepetitive (DNRA), and double repetitive applications (DRA). Applications were performed with an interval of 1 min for DNRA, and without interval for DRA. Results: A total of 480 lesions were analyzed. Lesion depth and volume were largest for DRA followed by DNRA and SA regardless of catheter direction (depth: 3.8 vs. 3.3 vs. 2.6 mm, p <.001 for all comparisons; volume: 176.6 vs. 145.1 vs. 97.0 mm3, p <.001 for all comparisons). Surface area was significantly larger for DRA than for SA (45.1 vs. 38.3 mm2, p <.001) and larger for DNRA than for SA (44.5 vs. 38.3 mm2, p <.001), but was similar between DRA and DNRA (45.1 vs. 44.5 mm2, p =.54). Steam‐pops more frequently occurred for DRA than for SA (15.6% vs. 4.4%, p =.004) and DNRA (15.6% vs. 6.9%, p =.061), but the incidence was similar between SA and DNRA (4.4% vs. 6.9%, p = 1). Although surface area and lesion volume were larger in lesions with steam‐pops than without steam‐pops (46.5 vs. 38.1 mm2, p =.018 and 128.3 vs. 96.8 mm3, p =.068, respectively), lesions were not deeper (pop(+): 2.5 mm vs. pop(−): 2.6 mm, p =.75). Conclusions: DNRA produces larger lesions than SA without increasing the risk of steam‐pops. DRA produces the largest lesions among the three groups, but with an increased risk of steam‐pops. Even with steam‐pops, lesions do not become deeper in vHPSD ablation. [ABSTRACT FROM AUTHOR]

Introduction: Radiofrequency ablation (RFA) utilizing half‐normal saline (HNS) irrigation is a promising intervention to circumvent commonly encountered limitations during radiofrequency ablation of deep myocardial substrate. Few studies to date have analyzed the morphologic changes in the human myocardium following HNS RFA. Methods and Results: Three patients with symptomatic ventricular tachycardia (VT) who underwent RFA with HNS irrigation underwent pathological specimen examination at time of autopsy or following native heart explant at the time of cardiac transplantation. Gross evaluation of the heart was performed fresh and after fixation in 10% formalin. A routine examination was performed with fixation in 10% formalin. Sections of lesioned tissue were paraffin embedded and evaluated using standard hematoxylin and eosin (H&E) staining. Conclusion: Irrigated RF ablation with HNS irrigant produces coagulative necrosis as well as several delayed histopathological changes with a deeper field of effective ablation. Transmurality may not be obtained in the ventricular myocardium with endocardial, epicardial, or sequential unipolar HNS ablation. [ABSTRACT FROM AUTHOR]

Background: Local impedance (LI) can indirectly measure catheter contact and tissue temperature during radiofrequency catheter ablation (RFCA). However, data on the effects of catheter contact angle on LI parameters are scarce. This study aimed to evaluate the influence of catheter contact angle on LI changes and lesion size with two different LI‐sensing catheters in a porcine experimental study. Methods: Lesions were created by the INTELLANAV MiFi™ OI (MiFi) and the INTELLANAV STABLEPOINT™ (STABLEPOINT). RFCA was performed with 30 W and a duration of 30 s. The contact force (CF) (0, 5, 10, 20, and 30 g) and catheter contact angle (30°, 45°, and 90°) were changed in each set (n = 8 each). The LI rise, LI drop, and lesion size were evaluated. Results: The LI rise increased as CF increased. There was no angular dependence with the LI rise under all CFs in the MiFi. On the other hand, the LI rise at 90° was lower than at 30° under 5 and 10 g of CF in STABLEPOINT. The LI drop increased as CF increased. Regarding the difference in catheter contact angles, the LI drop at 90° was lower than that at 30° for both catheters. The maximum lesion widths and surface widths were smaller at 90° than at 30°, whereas there were no differences in lesion depths. Conclusion: The LI drop and lesion widths at 90° were significantly smaller than those at 30°, although the lesion depths were not different among the 3 angles for the MiFi and STABLEPOINT. [ABSTRACT FROM AUTHOR]

Background: Local impedance (LI) has emerged as a new technology that informs on electrical catheter‐tissue coupling during radiofrequency (RF) ablation. Recently, IntellaNav StablePoint, a novel LI‐enabled catheter that equips contact force (CF) sensing, has been introduced. Although StablePoint and its predecessor IntellaNav MiFi OI share the common technology that reports LI, distinct mechanics for LI sensing between the two products raise a concern that the LI‐RF lesion formation relationship may differ. Methods: In an ex vivo swine cardiac tissue model, we investigated the initial level and range of a reduction in LI during a 60‐s RF ablation and the resultant lesion characteristics at nine combinations of three energy power (30, 40, and 50 W) and CF (10, 30, and 50 g) steps. Correlations and interactions between CF, LI, wattage, and formed lesions were analyzed. Incidence of achieving LI drop plateau and that of a steam pop were also determined. Results: Positive correlations existed between CF and initial LI, CF and absolute/relative LI drop, CF and lesion volume, and LI drop and lesion volume. At the same LI drop, wattage‐dependent gain in lesion volume was observed. Steam pops occurred in all CF steps and the prevalence was highest at 50 W. LI drop predicted a steam pop with a cutoff value at 89Ω. Conclusion: In StablePoint, wattage crucially affects LI drop and lesion volume. Because 30 W ablation may by underpowered for intramural lesion formation and 50 W often resulted in a steam pop, 40 W appears to achieve the balance between the safety and efficacy. [ABSTRACT FROM AUTHOR]

Background: The local impedance (LI) reflects the electrical catheter‐tissue coupling and correlates with the local tissue temperature. However, there have been few clinical studies showing the recommended method for LI monitoring catheters. This study aimed to investigate the optimal ablation setting for this catheter in an in vitro experimental model. Methods: LI monitoring catheters were used in an excised swine heart experimental model. The tissue contact force (CF) was directly monitored from an external weight scale. Radiofrequency ablation was performed with a combination of various energy power settings (30, 40, and 50 W), and various CFs (10, 30, and 50 g) for 60 s. The correlation between the LI‐related indexes, power, and CF with the lesion formation was statistically analyzed. Results: A positive correlation between the LI or lesion formation and CF was observed under all powers. Although the LI drop always correlated with the maximum lesion depth, lesion diameter, and lesion volume, the coefficient of the correlation value was lower under a high CF (lesion depth, diameter, and volume; 10 g, r = 0.8064, r = 0.8389, r = 0.8477; 30 g, r = 0.7590, r = 0.8063, r = 0.8060; 50 g r = 0.5555, r = 0.5701, and r = 0.5678, respectively). Steam pops occurred only under a 50 W ablation and the LI drop cutoff value for steam pops was 46 Ω. Conclusion: The same LI drop did not always lead to the same lesion size when the CF differed. Monitoring the LI and not exceeding 46 Ω would be useful for a safe ablation. [ABSTRACT FROM AUTHOR]

Background: Steam pops are a rare complication associated with radiofrequency (RF) ablation and are hard to predict. The aim of this study was to assess the influence of coverage between the RF ablation electrode and cardiac tissue on steam pop incidence and lesion size. Methods and results: An ex vivo model using porcine cardiac preparations and contact force sensing catheters was designed to perform RF ablations at different coverage levels between the RF electrode and cardiac tissue. During coverage level I, only the distal part of the ablation electrode was in contact with tissue. During coverage level II half of the ablation electrode, and during coverage level III the entire ablation electrode was embedded in tissue. RF applications (n = 60) at different coverage levels I–III were systematically performed using the same standardized ablation protocol. Ablations during coverage level III resulted in a significantly higher rate of steam pops (100%) when compared to ablations during coverage level II (10%) and coverage level I (0%), log rank p <.001. Coverage level I ablations resulted in significantly smaller lesion depths, diameters, and impedance drops when compared to higher coverage level ablations, p <.001. In the controlled ex vivo model, there was no difference in applied contact force or energy between different coverage levels. Conclusions: The level of coverage between RF electrode, cardiac tissue, and the surrounding fluid significantly influenced the incidence of steam pops in an ex vivo setup. Larger coverage between RF electrode and tissue resulted in significantly larger lesion dimensions. [ABSTRACT FROM AUTHOR]

Introduction: The influence of power (Watt [W]) and total energy (Joule [J]) on lesion size and the optimal overlap ratio remain unclear in laser balloon (LB) ablation for atrial fibrillation. We aimed to evaluate lesion size and visible gaps after LB ablation with various energy settings and different overlap ratios in vitro model. Methods and Results: Chicken muscles were cauterized using the first‐generation LB with single applications of full and a half duration of six energy settings (5.5 W/30 seconds [165 J] to 12 W/20 seconds [240 J]) and varying power (5.5‐12 W) at the constant total energy (160 J). Three overlapped ablations with different ratios (25% and 50%) for each energy setting were also performed to evaluate the visible gap degree categorized from 1 (perfect) to 3 (poor). Twenty lesions were evaluated for each energy setting. In single applications of full duration, lesion depth, lesion volume, and maximum lesion diameter increased according to the total energy (all, P < .001) and were greater than in those of half duration in each energy setting (all, P < .05). However, applications with larger power created larger lesion volume and maximum lesion diameter at constant total energy (P < .05). The visible gap degree was better in all energy settings with 50% overlapped ablation than in those with 25% (all, P < .001). Conclusion: Lesion size depends not only on power but also on total energy in the LB ablation. Sufficiently overlapped ablations allow continuous lesion formation. [ABSTRACT FROM AUTHOR]

Environmental Impedance and Ablation Lesions Introduction The electrical conductivity of the environment surrounding a catheter electrode may affect radiofrequency (RF) current. A higher impedance environment enveloping a catheter tip can be insulating and thus direct RF preferentially into targeted cardiac tissue. We sought to evaluate whether different environmental impedances will affect RF lesions. Methods An ex vivo model using bovine myocardium, load cell, and 4 mm ablation catheter was tested using a circulating bath of normal (0.9%; NS) and half normal (0.45%; HNS) saline. Ablations were performed at 50 W. In addition, using a porcine thigh prep model, warmed heparinized blood was circulated at 350 mL/min. Prior to ablation, circulating bath impedance was altered with NS and HNS. RF was applied at 30 W with a force-sensing, open irrigated catheter. Results Ex vivo RF with a 4 mm catheter using an HNS bath led to larger lesions, 286.7 ± 64 mm³ versus 198.9 ± 61 mm³ (P < 0.001), compared to an NS bath. In the porcine thigh prep, for RF in the high impedance (HNS) bath, initial and final impedances were higher, and the measured impedance reductions during ablation were larger, compared to RF in the low impedance (NS) bath. In vivo ablation within a higher impedance bath created larger lesion sizes, compared to a lower impedance bath, 222.4 ± 63.4 mm3 versus 135.4 ± 61.7 mm3 (P < 0.001). Conclusions A higher impedance surrounding an RF catheter can facilitate more effective RF and thereby increase lesion size. This may have implications for RF safety and efficacy, especially in the epicardium where pericardial fluid can be altered with infusion of ionic solutions. [ABSTRACT FROM AUTHOR]

Background: The use of carbon nanotubes (CNTs) in oncology has been proposed for the purpose of sensitizing tumors to radiofrequency (RF) ablation. We hypothesize that myocardial tissue infiltrated with CNTs will improve thermal conductivity of RF heating and lead to altered ablation lesion characteristics. Methods: An ex vivo model consisting of viable bovine myocardium, a circulating saline bath at 37 °C, a submersible load cell, and a deflectable sheath was assembled. A 4-mm nonirrigated ablation catheter was positioned with 10 gm of force over bovine myocardium infiltrated with CNTs, 0.9% saline, or sham injections. A series of ablation lesions were delivered at 20 and 50 W, and lesion volumes were acquired by analyzing tissue sections with a digital micrometer. Tissue temperature analyses at 3 and 5 mm depths were also performed. Results: Myocardial tissue treated with CNTs resulted in significantly larger lesions at both low and high power settings. The electrical impedance was increased in CNT treated tissue with a greater impedance change observed in the CNT infiltrated myocardium. The thermal conductivity of heat generated by application of RF in the tissue was altered by the presence of CNTs, resulting in higher temperatures at 3 and 5 mm depths for both 20 and 50 W. Conclusions: Myocardial tissue treated with CNTs resulted in significantly larger lesions at both low and high power settings. The electrical and thermal conductivity of heat generated by application of RF in myocardial tissue was altered by the presence of CNTs. Further research is needed to assess the in vivo applicability for this concept of facilitated ablation with CNTs. [ABSTRACT FROM AUTHOR]

In Vitro Comparison of Open-Irrigated Catheters Introduction Open-irrigated radiofrequency ablation catheters with slight differences in tip architecture are widely used, although limited comparative data are available. The purpose of this study was to compare the lesion size and potential complications produced by commercially available open-irrigated catheters in an in vitro porcine heart model. Methods and Results Six catheters were tested (Biosense Webster Thermocool, Boston Scientific Open irrigated, St. Jude CoolPath, St. Jude CoolPath Duo, Biosense Webster Thermocool SF, St. Jude Cool Flex) at 20 and 35 W power-control, under 2 different blood flows (0.1 and 0.5 m/s) and at 2 target durations (30 and 60 seconds). A total of 601 lesions were made in 26 in vitro preparations. The tip temperature profile showed significant differences between the catheters (P < 0.001) with the Thermocool SF registering the lowest. Only the surface diameter and the depth at maximum diameter of the lesion were influenced by the design of the ablation electrode. The lesion volume did not show significant differences between catheters for any power, application duration or blood flow condition. Char and pops occurred more often at 35 W with only slight differences between the catheters. Conclusions Tip design of the 6 different irrigated catheters does not affect the lesion total volume, although a slight difference in lesion geometry in terms of surface diameter and depth at maximum diameter is present. The catheters show a slight different in vitro safety profile [ABSTRACT FROM AUTHOR]

Introduction: The relative lesion sizes created by large electrode cryoablation catheter and irrigated radiofrequency (RF) ablation are not known. The purpose of this study was to directly compare lesion sizes created by cryoablation and irrigated RF under controlled conditions. Methods and Results: Ablation lesions were created in freshly harvested porcine left ventricular myocardium in a blood-filled tissue bath using an 8-mm-tip cryoablation catheter and a 3.5-mm-tip open-irrigated RF ablation catheter. Lesions were created under all permutations of the following conditions: electrode orientation vertical (perpendicular) or horizontal (parallel) to the tissue, electrode contact pressure at 6 or 20 g, and blood flow at 0.2 or 0.4 m/s over the electrode-tissue interface. The largest lesion volumes created with cryoablation were 961 ± 103 mm3, compared with the largest lesions volumes created with RF of 680 ± 48 mm3 (P < 0.001). The 3-way interactions among electrode orientation, contact pressure, and superfusate blood velocity accounted for the variation in lesion volumes for both catheters (both r2= 0.97, both P < 0.0001). The greater contact pressure increased lesion size for both cryoablation and RF. For cryoablation, lesion sizes were increased by the horizontal orientation and by the lower blood flow velocity. For open-irrigated RF, lesion sizes were significantly reduced by the horizontal orientation, however. Conclusions : Depending on conditions of electrode orientation, contact pressure, and blood velocity, either 8-mm-tip cryoablation or open-irrigated RF may produce the larger lesion volumes. Open-irrigated RF lesion sizes are reduced in the horizontal catheter orientation. [ABSTRACT FROM AUTHOR]

Introduction: There are no methods in clinical use to assess tissue cooling during catheter cryoablation. Cryoablation electrode temperature may be a poor predictor of lesion size. The purpose of this study was to determine whether the time necessary for the cryoablation electrode to cool to target temperature or to rewarm after cryoablation can predict lesion size. Methods and Results: Cryoablation was performed on live porcine left ventricle in a saline bath (37°C) using 8-mm-tip catheter. Cryoablation was given for 300 seconds under all permutations of the following conditions: electrode orientation vertical or horizontal, contact pressure 6 or 20 g, superfusate flow over electrode–tissue interface at 0.2 or 0.4 m/s (N = 10 each condition set, total 80 experiments). The time intervals necessary to cool the electrode to the target temperature of −75°C and to rewarm to + 30°C after termination of cryoablation were recorded. Lesion volume was predicted best by the time necessary to rewarm the elctrode to +30°C (r2= 0.65, P < 0.0001), followed by electrode temperature (r2= 0.28, P < 0.0001) and time to cool the electrode to −75°C (r2= 0.24, P < 0.0001). Time to +30°C and time to −75°C were associated with superfusate flow rate, contact pressure, and electrode orientation (r2= 0.80 and 0.61, respectively, both P < 0.0001). Superfusate flow rate, contact pressure, and orientation were also highly predictive of lesion volume (r2= 0.93, P < 0.0001). Conclusions: Time to cryoablation electrode rewarming is a better predictor of cryoablation lesion size than is electrode temperature. Time to cryoablation electrode rewarming reflects important determinants of cryoablation lesion formation—convective warming, contact pressure, and electrode orientation—that are not ascertainable during clinical ablation procedures. [ABSTRACT FROM AUTHOR]

Introduction: The initial success or failure of radiofrequency ablation (RFA) does not always reflect the long-term outcome that can lead to complications such as late atrioventricular block or recurrence of accessory pathways. We hypothesize that these occurrences may be due to a change in lesion size over time. Methods and Results: Intramural RFAs were performed on five greyhounds at thoracotomy using an epicardial approach into the left ventricular (LV) wall. Twenty-one gauge needle electrode ablations were created in the anterior aspect of the left ventricle. Radiofrequency energy was delivered at 600 Hz for 60 seconds and at an electrode temperature of 90°C. Eight ablations were created in each greyhound and the chest was closed. After 3 weeks, a further eight ablations were created under the same conditions in the lateral aspect of the LV, ensuring they were well away from the chronic lesions, and the dogs were sacrificed an hour later. All lesions were removed, stained with Gomori Trichrome and measured. There was no significant difference in lesion size detected in the 1-hour-old lesions compared with 3-week-old lesions. Acute lesions were well demarcated by an area of fibrous scar and a central necrotic region. Chronic lesions showed chronic inflammatory cells and strands of collagen. Conclusions: This study shows no change in lesion dimension over time and hence a change in size may not contribute to a change in RFA outcome over time. [ABSTRACT FROM AUTHOR]

Standard and Irrigated RF Ablation Lesions. Introduction: Radiofrequency ablation is successful for treating some arrhythmias but not for CAD-VT, possibly due to insufficient lesion size. Irrigated electrodes were developed to apply higher power for longer duration to create larger lesions. Our objective was to characterize and compare irrigated and standard ablation in terms of lesion size, crater, and coagulum formation. Additionally, a method Is proposed for creating large irrigated lesions without craters. Methods and Results: Three ablation protocols were conducted in canine ventricles. Protocol I: standard ablation was performed in power mode at 10, 20,30, and 50 W, and electrode-temperature mode at 70° and 90°C (120 sec). Protocol II: irrigated ablation was conducted with 30 and 50 W (30 and 120 sec). Protocol III: to create large lesions without craters, irrigated ablation was performed at 20 W (5 and 10 min). With a standard electrode, the largest lesions were created using 20 W (358 ± 194 mm3) and using 90°C (301 ± 130 mm3). Ablation duration decreased with power for the power mode standard ablations. The largest irrigated lesions were formed using 50 W (986 ± 357 mm3). large lesions without craters were created with irrigation using 20 W for 10 minutes (602 ± 175 mm3). Coagulum was seen for most standard ablations but infrequently for irrigated ablations. Craters were observed with 30 and 50 W Irrigated ablation but were not observed with 20 W irrigated ablation. Conclusion: irrigated ablation created larger lesions than standard; large lesions may be created without craters using moderate power and long duration. [ABSTRACT FROM AUTHOR]

Introduction: Increasing electrode size allows an increase in radiofrequency lesion depth. The purpose of this study was to examine the roles of added electrode cooling and electrode-tissue interface area in producing deeper lesions. Methods and Results: In 10 dogs, the thigh muscle was exposed and superfused with heparinized blood. An 8-French catheter with 4 or 8-mm tip electrode was positioned against the muscle with a blood flow of 350 mL/min directed around the electrode. Radiofrequency current was delivered using four methods: (1) electrode perpendicular to the muscle, using variable voltage to maintain the electrode-tissue interface temperature at 60°C; (2) sane except the surrounding blood was stationary; (3) perpendicular electrode position, maintaining tissue temperature (3.5-mm depth) at 90°C; and (4) electrode parallel to the muscle, maintaining tissue temperature at 90°C. Electrode-tissue interface temperature, tissue temperature (3.5- and 7.0-mm depths), and lesion size were compared between the 4- and 8-mm electrodes in each method. In Methods 1 and 2, the tissue temperatures and lesion depth were greater with the 8-mm electrode. These differences were smaller without blood flow, suggesting the improved convective cooling of the larger electrode resulted in greater power delivered to the tissue at the same electrode-tissue interface temperature. In Method 3 (same tissue current density), the electrode-tissue interface temperature was significantly lower with the 8-mm electrode. With parallel orientation and same tissue temperature at 3.5-mm depth (Method 4), the tissue temperature at 7.0-mm depth and lesion depth were greater with the 8-mm electrode, suggesting increased conductive heating due to larger volume of resistive heating because of the larger electrode-tissue interface area. Conclusion: With a larger electrode, both increased cooling and increased electrode-tissue interface area increase volume of resistive heating and lesion depth. [ABSTRACT FROM AUTHOR]