Transcatheter Cryoablation of Ventricular Myocardium in Dogs

Introduction: Surgical cryoablation, a highly effective technique used during antiarrhythmic surgery, produces voluminous, histologically uniform and discreet myocardial lesions. In contrast, radiofrequency (RF) catheter ablation, which as a result of its less invasive nature has largely supplanted antiarrhythmic surgery, produces smaller, histologically heterogeneous myocardial lesions. Since small lesion size and heterogeneity may reduce antiarrhythmic efficacy, we sought to reproduce the large, histologically homogeneous lesions created by surgical cryoablation, using a catheter cryoablation system (Cryogen, Inc., San Diego, CA) in the canine ventricle. Methods and Results: In seven dogs, nineteen ventricular lesions (two right and seventeen left) were created with a 10F cryoablation catheter with either a 2 or 6[emsp4 ]mm tip. In one dog AV node ablation was also performed. For each 'freeze', catheter tip nadir temperature, lesion width, depth, and transmurality were recorded, and lesion volume calculated. Average tip nadir temperature was –79.6±4.9°C. Cooler nadir tip temperature was associated with deeper (p=.007) and more voluminous lesions (p=.042), and a greater likelihood of lesion transmurality (p=.034). Average lesion volume was 500±356[emsp4 ]mm³. No other variables predicted lesion volume or transmurality. Histologically, the catheter cryoablation lesions were sharply demarcated and homogeneous. The single freeze performed at the AV junction produced complete AV block. One complication, catheter rupture following its repetitive use, resulted in a coronary air embolus and death. Conclusion: Catheter cryoablation of canine ventricular myocardium produced voluminous, discrete, transmural lesions, which might be effective for ablation of ventricular tachycardia. Lesion volume and transmurality were dependent on catheter tip nadir temperature.     

Counter Intuitive Relations Between in vivo RF Lesion Size, Power, and Tip Temperature

Background: Radiofrequency (RF) lesion size in vitro is positively correlated with applied power and catheter tip temperature. However, the relation between RF lesion size, power, and tip temperature in vivo remains unclear. We hypothesized that due to flow, anatomy and tip contact effects in vivo, increased tip temperature would be inversely related to applied power and RF lesion size. Methods: RF lesions were created on the endocardium of 16 pigs using 5, 6, and 7 Fr catheters. The ablation generator was set to achieve a temperature of 70°C. RF lesions were created in different regions of the heart so as to encompass a wide range of blood flow and catheter movement conditions. RF lesions were measured acutely (DIMEN, mm) and correlated with average power applied (POWER, W), and average tip temperature (TEMP, °C). The POWER and TEMP relation was also examined. Results: For TEMPs below 55°C, the power output from the generator was typically maximized at 50 W. At TEMPs above 55°C, POWER decreased exponentially with increasing TEMP {POWER = 50 – exp(-((41-TEMP)/7)), r = 0.98, p < 0.05}. Further, DIMEN tended to be inversely related to TEMP (Slope: –0.07 ± 0.04, r = –0.15, p = 0.07); but, was positively related to POWER (Slope: 0.04 ± 0.02, r = 0.23, p < 0.05). These relations varied by tip size and estimated local blood flow characteristics. Conclusion: In vivo, variable tissue contact and flow yield DIMEN-POWER-TEMP relations opposite to those found in vitro. These counterintuitive results suggest that maximum in vivo RF lesion size is achieved when power is maximized at tip temperatures between 50 and 60°C.     

Radiofrequency Delivery Through a Cooled Catheter Tip Allows the Creation of Larger Endomyocardial Lesions in the Ovine Heart

RF Lesions From Cooled Electrode. Introduction: The delivery of radiofrequency (RF) energy through conventional catheter electrodes is often associated with coagulation necrosis at the tissue-electrode interface, with resultant impedance rise and limited lesion size. This study was performed to examine the effects of catheter tip cooling during RF delivery, to test the hypothesis that such cooling would decrease the likelihood of impedance rise and allow the creation of larger endomyocardial lesions. Methods and Results: The experiments were performed in eight open chest, anesthetized sheep. RF lesions were created within both ventricular chambers of each animal through a catheter tip that could he cooled with a saline perfusate. Assignment of cooled versus non-cooled RF delivery to either ventricle was alternated from one animal to the next. In each set of experiments, lesion volumes relative to the mode of RF delivery were compared. The mean power delivered via the cooled electrode (22.04 ± 4.51 W) was significantly higher than that delivered via the noncooled electrode (6.10 ± 2.47 W; P < 0.001). The mean duration of RF delivery was 42.7 ± 11.2 sec for noncooled lesions versus 49.2 ± 6.8 sec for cooled lesions (P < 0.01). Mean lesion volume was 436.07 ± 177.00 mm³ for noncooled RF delivery versus 1247.78 ± 520.51 mm³ for cooled RF delivery (P < 0.001). This significantly larger lesion size with cooled RF delivery was associated with no instance of impedance rise in 27 attempts versus 11 impedance rises in 28 attempts with noncooled RF (P < 0.001). Conclusions: Delivery of RF energy through a cooled catheter tip allows the creation of larger endomyocardial lesions by limiting the occurrence of impedance rise despite the delivery of greater energy. These observations suggest that, under certain conditions, resistive tissue heating at a distance from the site of current delivery may play an important role in RF ablation therapy.     

Catheter-Based Cryoablation Produces Permanent Bidirectional Cavotricuspid Isthmus Conduction Block in Dogs

Introduction: Catheter-based cryoablation has been shown to produce punctate and effective cardiac lesions to treat focal targets. However, sequential applications are required to produce a continuous linear lesion needed to cure macroreentrant arrhythmias with large critical isthmuses. The purpose of this study was to compare and contrast linear cardiac lesions produced with sequential applications using a novel cryoablation system to those produced using radiofrequency (RF) energy. Methods and Results: Seven mongrel dogs were prepared for femoral venous insertion of the ablation catheter (either a 10-French, 6-mm tipped, bipolar cryoablation catheter (N = 5, cryo group) or a 7-French, 4-mm tipped, bipolar RF ablation catheter (N = 2, RF group)). Ablation of the cavotricuspid isthmus was performed by delivering applications at sequential locations across the isthmus. The cryo group received 6 to 10 (mean 8 ± 1.4), 5-minute applications at 3 to 5 separate sites with a mean nadir temperature of –74.5 ± 1.6°C. Each dog in the RF group received 9, 90-second applications with a mean temperature and power of 62 ± 0°C and 32 ± 3.6 W. No acute or chronic complications were associated with either form of ablation. Immediately and six weeks after the index procedure, electroanatomical mapping showed the presence of bidirectional isthmus conduction block in all dogs. Gross and histopathologic examination revealed the presence of linear lesions, which were continuous and transmural. Conclusion: Similar to RF ablation, catheter-based cryoablation can produce linear, transmural lesions in the cavotricuspid isthmus resulting in permanent bidirectional isthmus conduction block.     

Basic Aspects of Radiofrequency Catheter Ablation

RF Ablation. Radiofrequency (RF) catheter ablation has become the treatment of choice for many symptomatic cardiac arrhythmias. It is presumed that the primary cause of tissue injury by RF ablation is thermally mediated, resulting in a relatively discrete homogeneous lesion. The mechanism by which RF current heats tissue is resistive heating of a narrow rim (< 1 mm) of tissue that is in direct contact with the ablation electrode. Deeper tissue heating occurs as a result of passive heat conduction from this small region of volume heating. Lesion size is proportional to the temperature at the electrode-tissue interface and the size of the ablation electrode. Temperatures above 50°C are required for irreversible myocardial injury, but temperatures above 100°C result in coagulum formation on the ablation electrode, a rapid rise in electrical impedance, and loss of effective tissue heating. Lesion formation is also dependent on optimal electrode-tissue contact and duration of RF delivery. Newer developments in RF ablation include temperature monitoring, longer ablation electrodes coupled to high-powered RF generators, and novel ablation electrode designs.     

Effect of Catheter Tip Length and Position on Lesion Volume in Temperature Controlled RF Ablation in Canine Tricuspid Valve Annulus

Background: Radiofrequency (RF) energy has been delivered to the tricuspid valve annulus (TVA) in humans with both 4 mm and 8 mm long catheter tip electrodes to treat atrial flutter. However, lesion volume with temperature controlled RF delivery systems has not been previously characterized.Methods: In 10 anesthetized canines, a single pulse of temperature controlled RF energy at a 70°C set point, 30 second duration was delivered with either a 7 Fr/4 mm tip or a 7 Fr/8 mm tip electrode in a position both anterolateral and posteroseptal to the tricuspid valve annulus (TVA). Surface echocardiogram was obtained prior and after ablation. The animals were sacrificed after ablation and the lesions underwent gross and histological examination.Results: Lesion size, tip temperature and power were related to tip electrode surface area (SA). Eight mm tips (SA = 59 mm2) tended to create significantly larger lesions than 4 mm tips (SA = 29 mm2). Median lesion volume was 22 vs. 1.5 mm3, respectively. Eight mm tips were also associated with higher power requirements and lower temperatures than 4 mm tips. Posteroseptal TVA lesions tended to be larger than anterolateral lesions. No significant complications were noted.Conclusions: Using temperature controlled RF ablation, large lesions may be safely created on the canine TVA using 7 Fr catheters with 8 mm long tips.     

Biophysical aspects of high frequency catheter ablation. Studies of the significance of sudden changes in impedance]

To determine the effects and the underlying mechanisms of sudden rise of impedance during radiofrequency (RF) catheter ablation, 60 RF applications were delivered to isolated preparations of ventricular myocardium at three different power levels (mean: 3.7, 11.3, 19.3 watts). Pulse duration was 30 s, current voltage and catheter tip temperature were continuously monitored. Impedance rise occurred during 34 of 60 applications; the incidence of impedance rise increased at higher power levels. Impedance rise was significantly more often observed when the preparations were superfused with heparinized blood compared to saline solution (p less than 0.05). Catheter-tip temperature during radiofrequency application without impedance rise was significantly lower compared to applications with impedance rise (mean = 108 degrees C vs. 121 degrees C, p less than 0.01). The increase of catheter-tip temperature and maximal-tip temperature following impedance rise was significantly higher in blood when compared to saline solution (mean = +48 degrees C vs. +13 degrees C (p less than 0.001), Tmax: 121 degrees C vs. 245 degrees C). Following impedance rise, insulation defects of the electrode catheter and vaporized crater formation of the myocardium was often observed. Conclusions: During radiofrequency catheter ablation impedance rise occurs following overheating of the catheter electrode (greater than 110 degrees C). After impedance rise, catheter-tip temperature markedly increases. Insulation defects of the catheter and vaporized craters in the myocardium frequently occur after impedance rise. The results have important implications for the clinical use of RF-currents for catheter ablation; energy application should be immediately stopped after the occurrence of impedance rise.     

Effect of Saline Irrigation Flow Rate on Temperature Profile during Cooled Radiofrequency Ablation

Introduction: Cooled radiofrequency ablation has been developed clinically for the treatment of ventricular tachycardia. Although clinical studies employ a constant saline flow rate for cooling, we hypothesized that varying the flow rates might optimize the temperature profile at depth.Methods: In excised ovine left ventricle, we compared the temperature profile from a catheter tip electrode thermocouple to those placed at depths of 0.0mm, 1.0mm, and 2.0mm. We compared the following settings: 20Watts without flow, 20Watts with 0.3cc/sec flow, 20Watts with 0.5cc/sec flow, and 70°C surface temperature without flow (temperature control).Results: The temperatures decreased from 77.5±10.5°C, 91.7±6.3°C, 65.5±11.8°C, and 52.5±11.8°C at 20W without saline irrigation at the tip, 0.0mm, 1.0mm, and 2.0mm, respectively, to 33.0±1.4°C*, 63.4±7.0°C*, 57.1±5.8°C*, 49.9±5.8°C+ at 20W with 0.5ml/sec flow (*P<0.01, +P=0.09). The lesion volumes were 79.6mm³ for 20W without flow, 64.1mm³ for 20W with 0.3ml/sec flow, 47.5mm³ for 20W with 0.5ml/sec flow, and 28.6mm³ for temperature control.Conclusions: We conclude that 1) the temperature profile greatly depends upon the rate of saline flow for cooling; 2) at high flow rates, the 0.0mm and 1.0mm temperatures are similar; 3) even at high irrigation rates, lesion size is greater than for temperature control; 4) the tip temperature significantly underestimates the surface temperature and improved methods of measuring temperature are needed.     

Homogeneity and Diameter of Linear Lesions Induced With Multipolar Ablation Catheters: In Vitro and In Vivo Comparison of Pulsed Versus Continuous Radiofrequency Energy Delivery

Background: For invasive treatment of atrial fibrillation, linear lesions induced with multipolar ablation catheters (MAC) are needed to prevent recurrence. The aim of the study was to compare the efficacy of pulsed versus continuous radiofrequency (RF)-energy delivery using MAC. Methods: In vitro tests were performed using endomyocardial preparations of fresh pig hearts in a 10-liter-bath of physiologic saline solution (37°C) at constant flow conditions (1.5[emsp4 ]l/min). The MAC were placed with a constant pressure of 20 ponds onto the endocardium. The energy (generator: Osypka HAT 200 S) was delivered either pulsed (4 electrodes simultaneously, 5[emsp4 ]ms duty-cycle) or continuously (each electrode separately). In vivo experiments were performed in 6 anesthetized pigs using fluoroscopic positioning of MAC at 40 different intracardial positions and with similar conditions as in vitro experiments. Lesion volume (LV) was calculated after measuring lesion diameter with a microcaliper. The homogeneity of the lesions (LH) was classified from 1–4; with 1 as highest homogeneity. Results: Pulsed energy delivery produced more homogeneous linear lesions in significantly less time. There was no difference in electrode temperature values (50.2±0.8 and 51.3±1.4°C) in vitro and in vivo. In the in vivo experiments, lesion depth and calculated lesion volume were less in both modes of energy delivery but pulsed energy delivery was superior regarding lesion depth and homogeneity. Conclusion: With pulsed energy delivery it is possible to create linear lesions of significantly greater homogeneity. Moreover, larger lesions are induced in less time by pulsed energy delivery in vitro and in vivo.     

Observations on electrode-tissue interface temperature and effect on electrical impedance during radiofrequency ablation of ventricular myocardium

The purpose of this study was to correlate changes in electrical impedance with the electrode-tissue interface temperature and to characterize the associated events occurring at the catheter tip electrode. In a canine model, lesions were created in vitro (n = 49) and in vivo (n = 31) and radiofrequency power settings were varied. Electrode-tissue interface temperature, delivered current, and voltage were recorded, and impedance was calculated. A sudden rise in electrical impedance was seen in only two of 17 ablations in vitro and in one of 16 ablations in vivo with a peak electrode-tissue interface temperature of less than 100 degrees C compared with 29 of 32 ablations in vitro (p = 0.0001) and 12 of 15 ablations in vivo with a temperature of more than 100 degrees C (p = 0.0001). This phenomenon was associated with the observation of boiling and popping at the tip in in vitro preparations and tissue avulsion and thrombus formation on the catheter tip in in vivo studies. The lesion size was directly proportional to the peak temperature for all ablations but not to the peak power, current, or voltage during radiofrequency catheter ablation in the heart. Maintaining electrode-tissue interface temperature at less than 100 degrees C during radiofrequency catheter ablation in the heart may avoid the complications associated with the sudden rise in electrical impedance.     

Acute and Chronic Effects of Cryoablation of the Pulmonary Veins in the Dog as a Potential Treatment for Focal Atrial Fibrillation

Background: Atrial fibrillation (AF) may be triggered by premature atrial depolarizations originating in the pulmonary veins (PV). Radiofrequency catheter ablation of PV foci may prevent recurrence of AF, but may cause PV stenosis. Therefore, a safer method for ablation of PV foci is needed. This study evaluated the acute and chronic effects of PV ablation using a cryocatheter ablation (CCA) system, which may be less likely to cause PV stenosis. Methods: CCA was performed by freezing for 5 minutes or more in one or more PVs in 10 anesthetized dogs. Pacing threshold and vessel diameter were measured before and after PV cryoablation. All dogs were restudied at 4.0 ± 1.64 months (range 2–7) in a manner identical to baseline. Results: CCA was performed in 27 PVs (range 1–4/dog), with a mean freeze time of 8.62 ± 5.42 minutes per vein (range 5.23–22.06). Mean temperature for all freezes was –65 ± 5.3°C. Mean PV diameter was 6.49 ± 1.73 vs 6.24 ± 1.83 mm (p = NS) and mean pacing threshold 1.32 ± 0.75 vs 9.36 ± 5.93 mA (p < .01), before vs. acutely after ablation. At followup, at the ablation sites PV diameter (7.02 ± 1.88 mm) was unchanged from baseline, whereas pacing threshold remained elevated (2.54 ± 1.44 mA, p < .05 vs baseline). There were no acute or long-term complications. Conclusions: (1) CCA of PVs produced a significant rise in acute and chronic pacing threshold indicating loss of atrial conductivity. (2) CCA of PVs did not cause PV stenosis or other complications. (3) The data suggest that CCA of PVs may be a safe and effective method for treating focal AF.     

A rewarming maneuver demonstrates the contribution of blood flow to electrode cooling during internally irrigated RF ablation

Introduction: Titration of radiofrequency (RF) ablation energy to avoid steam pops while maximizing lesion size is difficult because of disparities between electrode and tissue temperature. We hypothesized that the variable contribution of local blood flow to electrode cooling contributes to electrode-tissue temperature disparity, even with an irrigated RF catheter.
Methods and Results: Forty RF lesions were made in the atria of live swine with an internally irrigated cooled RF catheter. Prior to RF delivery, the catheter tip was cooled and then allowed to rewarm to body temperature by stopping irrigant flow. The rewarming time (RT) was noted. The catheter was again cooled and RF energy delivered, with power titrated to achieve electrode temperature of 40°C, which was previously defined as the approximate upper limit of safety. Lesion formation and steam pops were observed with intracardiac echocardiography. The RT ranged from 10 to 47 seconds. Nine steam pops were seen. The mean RT for lesions with pops was faster than those without (20 vs. 33 seconds, P = 0.003). The 20 lesions with faster RT required more power to achieve target temperature than the 20 lesions with slower RT (P = 0.01). Eight of nine steam pops occurred in the group with faster RT.
Conclusions: RT quantifies convective heating by blood prior to RF. Faster RT predicts greater cooling by blood during irrigated RF, a greater power requirement to reach target electrode temperature, and a higher risk of steam pops during fixed-temperature ablation. RT therefore can demonstrate the variation in blood cooling with each lesion and can be used to tailor energy delivery to maximize efficacy and safety.     

Avoiding microbubbles formation during radiofrequency left atrial ablation versus continuous microbubbles formation and standard radiofrequency ablation protocols: comparison of energy profiles and chronic lesion characteristics

BACKGROUND: Radiofrequency (RF) energy parameters and chronic lesion characteristics associated with the microbubbles formation have not been yet fully elucidated. OBJECTIVES: The objective of this study was to compare the energy profiles and chronic lesion characteristics associated with RF ablation of the pulmonary vein antrum using three different ablation protocols: (1) avoiding microbubbles; (2) continuous microbubble formation; (3) temperature-guided ablation. METHODS: A 4-mm tip ablation catheter was used for creating RF ablation lesions in 15 adult mongrel dogs. All ablation lesions were created at the posterior aspect of the PV antrum in each animal. Avoiding microbubbles (group 1, n = 5 dogs, 23 lesions), continuous microbubble formation (group 2, n = 5 dogs, 22 lesions), and temperature-guided (group 3, n = 5 dogs, 19 lesions, target temperature 60 degrees C/power limit 50 W) ablation lesions were analyzed. RESULTS: Group 1 showed significantly lower power (19 +/- 8.6 W), lower temperature (50 +/- 4.8 degrees C), higher efficiency-of-heating index (2.9 +/- 0.8 degrees C/W), and lower impedance (109 +/- 24.4 Omega) than groups 2 (38 +/- 8.4 W; 63 +/- 10 degrees C; 1.8 +/- 0.8 degrees C/W; 148 +/- 34.4 Omega) and 3 (44 +/- 12 W; 57 +/- 2.4 degrees C; 1.4 +/- 0.5 degrees C/W; 139 +/- 23.1 Omega) (P < 0.001 vs groups 2 and 3). During ablation, no significant events were detected in group 1, but 11 cases of audible pop, 11 cases of catheter tip charring, and 1 case of fatal myocardial perforation were observed in groups 2 and 3. Transmural lesions were more frequently created in group 1. CONCLUSION: RF energy delivery applying "avoiding microbubbles" protocol seems to be associated with higher degree of safety and efficacy when compared to temperature-guided and continuous microbubble-formation ablation protocols.     

Protection of the coronary arteries during epicardial radiofrequency ablation with intracoronary chilled saline irrigation: assessment in an in vitro model

INTRODUCTION: The coronary arteries can be damaged during epicardial radiofrequency ablation (RFA) procedures. We hypothesized that intracoronary irrigation with chilled saline may be a useful technique for minimizing heat-induced damage to the coronary artery endothelium during this procedure. METHODS AND RESULTS: Twenty-nine ablation procedures were performed on 17 freshly excised ovine hearts. Radiofrequency current was delivered through an internally cooled, 4-mm-tip ablation catheter placed directly over the coronary artery (24 applications) and over noncoronary epicardium (5 applications). An Amplatz coronary catheter was used to internally irrigate the coronary artery with either 37 degrees C or 5 degrees C 0.9% saline (12 ablations each group). Fluroptic temperature probes were placed within the artery lumen under the ablation site and 15 mm distal from the ablation site. The peak intracoronary temperature directly under the ablation catheter was significantly lower (P = 0.001) in the chilled than in the nonchilled saline irrigation group (23.6 degrees C, interquartile range [IQR] 15.7-39.8 vs 54.6 degrees C, IQR 48.9-58.6). Blue tetrazolium stained lesion sections showed that the median distance between the ablation lesion and the artery wall was significantly higher (P = 0.004) for the chilled versus the nonchilled saline irrigation group (0.42 mm, IQR 0.25-0.70 vs 0.00 mm, IQR 0.00-0.28). CONCLUSIONS: Intracoronary irrigation with chilled saline may protect the coronary artery endothelium from heat-induced damage during epicardial RFA.     

Prediction of Lesion Size Through Monitoring the 0°C Isothermic Period Following Transcatheter Cryoablation

A prototype steerable 8.5F bipolar catheter fitted with a feedback thermocouple was tested in 7 anaesthetized pigs (30 kg) guided by the electrocardiogram in order to modify the AV nodal and His-Purkinje system conductive properties. Thermal energy was delivered by a pressurized N2O tank (>650 psi) via a cardiac cryo unit (Spembly, Hampshire, UK) into the catheter wherein gas expands resulting in a tip temperature as low as–70± 2°C within 10 seconds. Cryoablation under fluoroscopic and electrocardiographic guidance was applied at distinct sites in both ventricles for 60 or 120 seconds. After a follow-up period of 6 weeks, the ablation lesions found were well demarcated with small margins of hypertrophy of myocardial cells. With respect to lesion volume variability (8–207 mm3) and geometry, a relationship between the 0°C isothermic period and cryolesion volume was found. Results of an in vitro model corroborated this relationship. Therefore, an isothermic period probably can predict the lesion size and its geometry in terms of lesion depth. This potential therapeutic mode of transcatheter cryoablation deserves further investigation.     

In Vitro Performance Characteristics of Reused Ablation Catheters

Background: Prior studies have found that there is a widespread practice of catheter reprocessing in cardiac electrophysiology laboratories. Effects of reprocessing of ablation catheters on temperature sensing and mechanical deflection are not fully known. Methods: Twenty-four new and used ablation catheters were studied. Deviation of temperature sensing by catheters from the temperature of a heated saline bath was measured. The angle of deflection of digitally scanned catheters at 75% and 100% handle deflection was also measured. New and used catheters were compared with respect to their temperature sensing accuracy and deflection characteristics. Results: Overall, there was 0.7 ± 0.1°C (mean ± standard error) deviation of the sensed temperature from the bath temperature, with no significant difference between new and used catheters. Similarly, there was no significant difference in the angle of deflection between new (66.7° ± 6.2° and 24.3° ± 6.8° at 75% and 100% deflections, respectively) and used (59.6° ± 5.6° and 28.7° ± 9.9° at 75% and 100% deflections, respectively) catheters. The difference in the angle of deflection between matched new and used catheters was 18.9° ± 4.2° and 10.9° ± 2.4° at 75% and 100% deflections, respectively, with a relatively broad range (5.0°–35.6° and 0.4°–19.0° at 75% and 100% deflections, respectively). Conclusions: This study found no significant overall difference in temperature sensing accuracy and deflection angle of new and used ablation catheters. Nevertheless, individual differences in deflection characteristics between new and used catheters are occasionally seen and warrant screening of reprocessed catheters prior to their reuse.     

Safety profiles and lesion size of different radiofrequency ablation technologies: a comparison of large tip, open and closed irrigation catheters

INTRODUCTION: Different technologies have been developed for radiofrequency ablation (RFA), which include increasing electrode (tip) size and cooling the tip through irrigation either internally (closed-loop) with D5W or externally (open-loop) with saline. Although these catheters are widely used clinically, the propensity for adverse events and the lesion profiles of each of these catheter technologies have not been directly compared under a wide range of controlled conditions. METHODS AND RESULTS: Freshly excised canine thigh muscle was placed in a chamber filled with circulating, heparinized blood heated to 37 degrees C. Five different catheters were tested: 4 mm tip, 10 mm tip single thermistor, 10 mm tip multitemperature sensor, 4 mm closed-loop irrigated cooled-tip, and 4 mm open-loop irrigated cooled tip at several different contact and power settings. The catheter and tissue interface was continuously monitored with intracardiac echocardiography (echo) (Acuson). During the RFA, any bubbling generated from the tip and/or popping seen on echo was noted, and after each RFA, the catheter and lesion were examined for the presence of thrombus. For all of the catheters, complications correlated to the electrode tip temperature and power setting. All of the catheters experienced complications at any lesion size except for the open-irrigated catheter, which only had complications at the largest lesions. Overall, the cooled tip catheters experienced an at least sixfold greater odds of popping, bubbling, and impedance rises than the 4 mm, but the majority occurred at power levels greater than 20 W. The open-irrigated catheters created eccentric lesions that extended away from the tissue-catheter interface, in the direction of blood flow. In addition, it produced saline filled blisters at the lesion site in 16.7% of the burns. The 10 mm catheter had an at least twofold greater odds of thrombus, charring, and bubbling, but larger lesions than the 10 mm multitemperature sensor catheter. CONCLUSIONS: Catheter type, contact conditions, and power settings all play a role in lesion size and in the frequency of complications that occur during an RFA. Cooling the electrode tip, either internally or externally, does not prevent complications from occurring, especially at the higher power control settings. Adding more temperature sensors to the 10 mm seems to reduce the amount of complications that can occur.     

Large radiofrequency ablation lesions can be created with a retractable infusion-needle catheter

BACKGROUND: Radiofrequency (RF) catheter ablation of ventricular tachycardia is sometimes limited by inadequate lesion depth. We report the use of a novel retractable needle-tipped electrode catheter with intramyocardial (IM) saline infusion and IM RF energy delivery to create large myocardial ablation lesions. METHODS: The left ventricle was entered via the femoral artery in 6 and 11 anesthetized goats and swine (32-90 kg) with an 8-F electrode catheter with an extendable 27-gauge needle at the tip (modified for RF ablation by making the needle electrically active). The needle was advanced 5-7 mm intramyocardially and 0.9% saline was infused 1 mL/min x 60 seconds prior to, and throughout a 120-second application of RF via the active needle, with power titrated to 12 W for 9 lesions, and 30-40 W for 37 lesions, followed by a 120-second RF application using the 4-mm-tip electrode, with power titrated to achieve a 10-Omega decrease in impedance. Needle/saline lesions were compared to 18 standard 4-mm-tip control lesions (power titrated to < or =50 W, to achieve a 10-Omega impedance decrease or limited to 60 degrees Celsius) and to 17 irrigated 3.5-mm-tip lesions (power titrated to < or =50 W, temperature limited to 50 degrees Celsius, 30 mL/min infusion rate). Lesions were identified in the excised heart, fixed, serially sectioned from the endocardium, and digitally analyzed to calculate volume. RESULTS: Lesions were homogeneous and had distinct borders. Compared to 4-mm-tip and irrigated-tip lesions, high-power needle/saline lesions were significantly deeper (13 +/- 2 vs 5 +/- 1 and 8 +/- 3 mm, P < 0.001), had significantly larger volumes (1,700 +/- 750 vs 240 +/- 170 and 750 +/- 650 mm(3), P < 0.001), and had larger cross-sectional area at each millimeter depth beyond the 1 mm (P < 0.01). CONCLUSIONS: IM saline infusion and IM RF energy delivery markedly increase RF lesion size as compared to standard RF ablation and is feasible with a percutaneous catheter. This method warrants further investigation because of its potential clinical utility.     

Biophysics of Radiofrequency Ablation Using an Irrigated Electrode

Background: Previous reports have proposed that prevention of electrode-endocardial interfacial boiling is the key mechanism by which radiofrequency application using an irrigated electrode yields a larger ablation lesion than a non-irrigated electrode. It has been suggested that maximal myocardial temperature is shifted deep into myocardium during irrigated ablation. Purpose: To examine the biophysics of irrigated ablation by correlating electrode and myocardial temperatures with ablation circuit impedance and lesion morphology, and to perform a comparison with non-irrigated ablation modes. To assess the influence of irrigant rate, composition, temperature and blood flow velocity. Methods: I. Ablation with and without electrode irrigation was performed in vitro utilizing a whole blood-superfused system. Electrode, electrode–endocardial interface, and intramyocardial temperatures were assessed, as were ablation circuit impedance, total delivered energy, and lesion and electrode morphology. Irrigants assessed were room temperature normal saline, iced normal saline, and dextrose. Irrigant flow rates assessed were 20 and 100 cc/min. Blood flow velocities assessed were 0 and 0.26 m/s.II. Finite element simulations of myocardial temperature during irrigated ablation were performed to further elucidate irrigation biophysics and provide a more detailed myocardial temperature profile. Two models were constructed, each utilizing a different core assumption regarding the electrode-tissue boundary: 1. electrode temperature measured in vitro; 2. interfacial temperature measured in vitro. Intramyocardial temperatures predicted by each model were correlated with corresponding temperatures measured in vitro. Results: I. Ablation during electrode irrigation with normal saline was associated with greater ablation energy deposition and larger lesion dimensions than non-irrigated ablation. The mechanism underlying the larger lesion was delay or inhibition of impedance rise; this was associated with attenuation or prevention of electrode coagulum. Irrigation did not prevent interfacial boiling, which occurred during uninterrupted radiofrequency energy deposition and lesion growth. Irrigation using saline at 100 cc/min was associated with no impedance rise regardless of blood flow velocity, whereas during irrigation at 20 cc/min impedance rise was blood flow rate-dependent. Iced saline produced results equivalent to room temperature saline. Irrigation with dextrose was associated with curtailed energy application and relatively small lesions.II. The finite element simulation that used electrode–endocardial interfacial temperature as the core assumption predicted a myocardial temperature profile which correlated significantly better with in vitro than did the simulation which used electrode temperature as the core assumption. Regardless of irrigant and blood flow rates, maximal myocardial temperature was always within 1 mm of the endocardial surface. Conclusions: Radiofrequency energy application via a saline irrigated electrode resulted in a larger lesion due to attenuation or eradication of electrode coagulum, thus preventing an impedance rise. Irrigation did not prevent interfacial boiling, but boiling did not prevent lesion growth. The site of maximal myocardial temperature during irrigated ablation was relatively superficial, always within 1 mm of the endocardial surface. Irrigation with iced saline was no more effective than with room temperature saline; both were far more effective than dextrose. Higher irrigation rates immunized the electrode from the influence of blood flow. The biophysical effects of blood flow and irrigation were similar.     

Saline-irrigated radiofrequency ablation electrode with external cooling

INTRODUCTION: Open flush, irrigated ablation electrodes may improve the safety of radiofrequency catheter ablation by preventing protein aggregation and coagulum formation. This is particularly important in left-sided procedures like catheter ablation of atrial fibrillation. Electrode cooling and the inherent loss of temperature feedback, however, grossly reduce the ability to monitor tissue heating. Intimate contact may not be recognized and the delivery of nominal RF power levels may then lead to excessive tissue heating, steam explosions, and even tamponade. METHODS AND RESULTS: Standard, open flush, irrigated catheters (Sprinklr, Medtronic Inc, Minneapolis, MN) were modified by thermally insulating the irrigation channels inside the ablation electrode. Using the thigh muscle preparation, multiple lesions were created with standard and modified catheters using 60 s, 20-50 Watt applications and a constant saline flush rate of 20 cc/min. A total of 57 lesions were created on five thigh muscles of three dogs. Lesion dimensions were not significantly different between both types of catheter, but the maximum electrode temperature rise during ablation was significantly higher with the modified catheter. Insulation of the irrigation channels improved the correlation coefficient between maximum electrode temperature rise and lesion volume from 0.38 (ns) to 0.62 (P < 0.001). CONCLUSION: Thermal insulation of the irrigation channels facilitates temperature feedback during radiofrequency ablation and controllability of lesion formation.