Cooled-Tip Ablation Results in Increased Radiofrequency Power Delivery and Lesion Size in the Canine Heart: Importance of Catheter-Tip Temperature Monitoring for Prevention of Popping and Impedance Rise

Since myocardial lesion size during radio-frequency (RF) ablation is limited at high power by impedance rise when electrode tip temperature exceed 100 °C, controlling tip temperature by continuous intraelectrode saline infusion could permit generation of larger lesion. (1) Two dogs randomly received either standard or cooled tip RF ablation at 4 to 6 separate LV sites. Power output of 30 W was delivered via modified 7 Fr deflectable catheter with 4 mm tip for up to 120 sec or until impedance rise occurred. (2) Six dogs randomly received cooled tip RF ablation at power output of 20, 30, 40 W for 120 sec. (3) Three dogs randomly received cooled tip RF ablation using room temperature saline (21–25 °C) or chilled saline (1–4 °C) infusion. Results: Overall, peak tip temperature was lower for cooled vs standard RF deliveries (97±17 °C vs. 42±8 °C). Lesion depth and volume were significantly larger for cooled burns. Lesion depth and volume and the incidence of abrupt impedance rise/popping did not differ between room temperature saline and chilled saline infusion. The catheter-tip temperature at the onset of popping and abrupt impedance rise was 54±5 °C(48–60 °C) and 59±10 °C(50–75 °C). Conclusion: Cooled tip RF current delivery at high power is associated with increased myocardial lesion size which may facilitate successful ablation of ventricular tachycardia associated with acquired structural heart disease. Catheter-tip temperature should be maintained below 45 °C to prevent popping and abrupt impedance rise during RF energy delivery.     

Comparison of the Temperature Profile and Pathological Effect at Unipolar, Bipolar and Phased Radiofrequency Current Configurations

With a multi-electrode catheter, phased radiofrequency (RF) delivers current between each electrode and a backplate as well as between adjacent electrodes. This study compared the tissue heating and lesion dimensions created by phased and standard RF. Ablation was performed on the in vivo thigh muscles in 5 pigs. Six lesions were created on each thigh muscle using phase angle 0° RF, 127° RF, 180° RF with and without a backplate, and standard RF in bipolar and sequential unipolar configurations. Two plunge needles, each with 6 thermocouples 1mm apart, were inserted into the tissue with one needle beside an electrode and the other midway between electrodes for tissue temperature measurement. The 0° RF created lower tissue temperatures and smaller lesions between electrodes than those beside electrode. With 127° and 180° RF, tissue temperature and lesion dimensions between electrodes were similar to beside electrode, while the 127° RF created higher tissue temperature and deeper lesions than 180° RF (both with and without a backplate) at both sites. Standard RF bipolar ablation created similar tissue temperatures and lesion depths at both sites, but required greater power than the 127° RF. Standard RF sequential unipolar ablation created only a slight temperature increase and no lesions between electrodes 3 and 4. As judged by tissue temperature, lesion depth and uniformity, and RF power requirement, 127° RF may be a better energy configuration for linear ablation than the other RF modalities tested.     

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.     

Determinants of Lesion Size During Radiofrequency Catheter Ablation: The Role of Electrode-Tissue Contact Pressure and Duration of Energy Delivery

Determinants of Radiofrequency Lesion Size. Introduction: Radiofrequency (RF) catheter ablation is a promising modality in the management of cardiac arrhythmias, but the optimum protocol for energy delivery has not yet been determined. The purpose of this study was to examine the effects of varying electrode-tissue contact pressure and varying duration of RF energy delivery on the size of the resultant lesion in an in vitro model of isolated perfused and superfused canine myocardium. Methods and Results: RF power (500 kHz) was delivered through the tip of a specially-designed thermistor-tipped 1.6-mm diameter electrode mounted on an adjustable fulcrum so that contact force could be varied. RF power was adjusted to maintain a constant electrode-tissue interface temperature of 80°C during lesion production. Seventy-nine lesions were created with a 90-second RF energy delivery, and a contact force that was varied between 0 and 400 newtons (N) (0-41 g). Lesions produced with a small contact force (10 N) were significantly larger than those with a contact force of zero (width 5.5 mm vs 3.8 mm, P <0.0002), but not significantly different from those produced with the maximum contact force of 400 N (width 6.5 mm, P = NS). However, the greater contact force significantly decreased the power required to maintain a constant electrode-tissue interface temperature. Ninety-six lesions were then created using a constant contact force, but duration of energy delivery was varied from 10 to 600 seconds. Lesion size grew monoexponentially with time. The t_1/2 of lesion growth was 7.6 and 9.6 seconds for depth and width, respectively. Conclusion: Thus, close electrode-tissue contact is essential for adequate lesion formation during RF ablation of myocardium, but increasing contact force does not significantly increase lesion size if power is adjusted to maintain a constant electrode-tissue interface temperature. In order to approach steady-state and maximize lesion size, duration of RF energy delivery should be at least 40 seconds.     

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.     

Time to electrode rewarming after cryoablation predicts lesion size

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.     

Area Under the Real‐Time Contact Force Curve (Force–Time Integral) Predicts Radiofrequency Lesion Size in an In Vitro Contractile Model

FTI Predicts RF Lesion Size in Contractile Model. Introduction: Electrode tissue contact, radiofrequency (RF) power and duration are major determinants of RF lesion size. Since contact forces (CF) vary in the beating heart, we evaluated contact force–time integral (FTI) as a predictor of lesion size at constant RF power in a contractile bench model simulating the beating heart. Methods and Results: An open‐tip irrigated catheter was attached to a movable mount incorporating a dynamic force sensor allowing closed loop control to achieve desired force variations between the catheter tip and bovine skeletal muscle placed on a ground plate. RF energy (20 and 40 W for 60 seconds, 17 cc/min irrigation) was delivered during (1) constant contact (C) at 20 g, (2) variable contact (V) with a 20 g peak and 10 g nadir, and (3) intermittent contact (I) with a 20 g peak and 0 g nadir with loss of contact. V and I protocols were performed at 50 and 100 catheter movements/min and 2 systole:diastole time ratios (50:50 and 30:70). The area under the CF curve was calculated as the FTI. Measured FTI was highest in C, intermediate during V and lowest during I and correlated linearly with lesion volume (P < 0.0001 for 20 and 40 W). Lesion volume was highest in group C, intermediate in V and lowest in group I (P < 0.05 for C vs V, V vs I, and C vs I). Conclusions: Lesion size correlates linearly with measured contact FTI. Constant contact produces the largest and intermittent contact the smallest lesions despite constant RF power and identical peak contact forces. (J Cardiovasc Electrophysiol, Vol. 21, pp. 1038‐1043, September 2010)     

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.     

Is There a Potential Benefit to Increased Irrigation Channels During Radiofrequency Ablation? Results From a Two‐Center Prospective Randomized Study

Potential Benefit to Increased Irrigation Channels During Radiofrequency Ablation. Introduction: Open irrigation during radiofrequency (RF) application allows a higher power delivery in the setting of temperature‐controlled ablation, without causing blood clots. This study sought to evaluate the clinical value of the additional 6 supplementary channels at the proximal catheter tip compared to a standard irrigated RF catheter with 6 conventional channels present at the distal tip only. Methods and Results: Ninety‐five consecutive patients were prospectively randomized to cavotricuspid isthmus ablation using an 3.5 mm tip ablation catheter with 6 distal irrigation channels (6C; 48 patients) or an 4 mm tip ablation catheter with 12 irrigation channels (12C; 47 patients) disposed at the distal (6 channels) and proximal (6 additional channels) catheter tip. There was no significant difference between the 12C and the 6C irrigated‐tip catheter concerning the total procedural duration, the RF duration, the fluoroscopic duration, and the amount of irrigation. Conversely, there were significantly more patients who experienced at least one steam pop while using the 12C as compared to the 6C irrigated‐tip catheter (0% vs 13%, respectively, P = 0.018). Conclusion: The addition of proximal irrigation holes at the catheter tip do not facilitate lesion formation during RF ablation, but significantly increases the risk of steam pop. This is probably the consequence of an increase distortion of the temperature feedback . (J Cardiovasc Electrophysiol, Vol. 22, pp. 516‐520 May 2011)     

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.     

Why a Large Tip Electrode Makes a Deeper Radiofrequency Lesion:

Increase in RF Lesion Depth with Larger Electrode. 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) same 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.     

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.     

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.     

The effect of temperature-guided radiofrequency ablation of ventricular myocardium

The safety and feasibility of temperature-guided radiofrequency(RF) ablation of ventricular myocardium were assessed in anopen-chest animal model. RF ablation was performed in the ventriclesof 19 pigs using ablation catheters with a tip electrode of2 or 4 mm length. The energy was delivered in a bipolar (2mmtip electrode only) or unipolar configuration. Set temperatureranged from 60 to 90°C. Pulse duration was 30 s. Histologicalexamination showed coagulation necrosis with a haemorrhagiczone. However, lesions with a deep cleavage were found afterfive unipolar (2mm tip electrode) energy applications. No suchlesions were found after unipolar energy applications with a4 mm tip electrode. During or shortly after ablation, prematureventricular beats and non-sustained VT were frequently observedand in some instances ventricular fibrillation. In the bipolarmode we found a positive correlation between lesion area/volumeand peak temperature with a correlation coefficient of 0.48and 0.56, respectively (P < 0.05). However no correlationwas found between lesion size and applied energy. In the unipolarconfiguration there was no correlation between lesion size andthese parameters. Median depth and area were: bipolar: 4.0 mm/23.5mm², unipolar (2mm tip electrode): 3.5 mm/12.2 mm², unipolar(4 mm tip electrode); 4.0 mm/15.7mm² We conclude that in a beatingheart it is djfficult to predict lesion size from temperatureor energy.     

NMR spectrometry

In the present investigation, suitability of nuclear magnetic resonance (NMR) spectrometry for total stool fat quantification in patients with normal or impaired exocrine pancreatic function (chronic pancreatitis) has been analyzed in comparison with a conventional chloroform-methanol extraction technique. Basic temperature-dependence studies of NMR spectrometry (90°/180° radiofrequency pulse sequence) on 21 chloroform-methanol extracted pure total stool lipid standards (weight range: 0.05–1.6 g) revealed significantly (P<0.05) improving correlations between NMR signal amplitudes and corresponding weights at increasing temperatures (r=0.952/40° C,r=0.965/60° C,r=0.988/80° C), thus indicating 80° C as optimal temperature for NMR spectrometric total stool fat quantification. In subsequent comparative measureemnts of lyophilized stool samples, NMR spectrometry (at 80° C) and conventional chloroform-methanol extraction provided significantly (P<0.001) correlated results with respect to total fecal fat contents/day of quantitatively collected and homogenized stools in 93 patients with known exocrine pancreatic function (secretin-pancreozymin test), irrespective of whether correlations were determined for all 93 patients (r=0.983) or separately for patients with normal (N=45;r=0.867), moderately reduced (N=31;r=0.946), or highly reduced (N=17;r=0.992) exocrine pancreatic function and correspondingly increased total fecal fat excretions.     

Temperature Measurement as a determinant of Tissue Heating During Radiofrequency Catheter ablation: An Examination of Electrode Thermistor Positioning for Measurement Accuracy

Thermometry and Radiofrequency Catheter Ablation. Introduction: Temperature monitoring has been proposed as a control for lesion occurrence and dimension during radiofrequency transcatheter ablation. Effective temperature measurement depends on thermistor positioning relative to the heated cardiac tissue and the convective cooling effects of the circulation. But the accuracy of a single tip thermistor as a measure of peak electrode-tissue interface temperature is unknown. Methods and Results: A standard 8-French, 4-mm electrode catheter with 5 thermistors (1 tip thermistor, 4 radial thermistors) was used to deliver radiofrequency energy in vitro to 3 porcine right ventricles and in vivo to 7 mongrel dogs. In vitro, the catheter orientation was varied. In vivo the catheter was positioned under fluoroscopy at a variety of atrial, tricuspid annular, and ventricular sites, with no attempt to adjust catheter orientation. In both cases varied discrete power levels were used so that a wide temperature range was attained. Lesions created in vivo with a standard, single thermistor tipped electrode were compared to those of a catheter with a thermistor extending 1 mm from the tip. Power was varied and tip thermistor temperatures recorded. All lesions were examined pathologically. Comparisons of radial thermistor temperature to tip thermistor temperature for 3 catheter orientations in vitro resulted in tip thermistor underestimation of peak electrode-tissue interface temperature by a median of 0.5°C in 35% of the perpendicular orientations, 1.9°C in 82% of the 45 orientations, and 5°C in 83% of the parallel orientations. During in vivo trials, the tip thermistor underestimated the peak electrode-tissue interface temperature during 2 of 51 lesions by 1.2°C and 7.6°C. There was a sudden rise in electrical impedance in 17 of 51 radiofrequency energy deliveries. Only one case was observed where the peak electrode-tissue interface temperature was below 95°C. The normal to extended tip thermistor configurations analysis showed similar relationships between lesion size and temperature. Conclusions: Accuracy of a single tip thermistor was found to be dependent upon catheterlissue orientation. With routine catheter positioning in vivo, the tip thermistor was a good indicator of peak electrode-tissue interface temperature. Thus with power regulation to avoid temperatures greater than 90°C, a single flush-mounted tip thermistor is probably adequate for temperature monitoring of lesion formation and avoidance of impedance rises.     

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.     

Epicardial radiofrequency ablation of ventricular myocardium: Factors affecting lesion formation and damage to adjacent structures

We evaluated the factors affecting epicardial radiofrequency (RF) lesion formation in normal ventricular myocardium. In 16 dogs, a minithoracotomy was made and a sheath was placed in the pericardial space. Standard ablation lesions (4-mm tip catheter; 70 ( composite function) C/60 seconds) were created in each ventricle under fluoroscopy guidance (n = 7) or hand-held with direct visualization of the catheter to assure optimal electrode-tissue contact (n = 6). In the latter, thermally-shielded (TS) electrodes (50% tip surface along its 4 mm length) were used in 3/6 dogs. Catheter tip (4 mm) irrigation (13 mL/minutes; 40 ( composite function) C/60 seconds) was employed with conventional techniques in 3 additional dogs. RESULTS: With optimal electrode-tissue contact (11 lesions), power (3.4 +/- 2.3 W vs. 16 +/- 13 W; p < 0.001) and pacing thresholds (0.2 +/- 0.0 mA vs. 3.6 +/- 5.7 mA; p = 0.004) were lower than standard RF (25 lesions). However, lesion dimensions were similar and transmural lesions did not occur (depth 2.8 +/- 1.1 mm vs. 3.0 +/- 1.5 mm). Catheter irrigation allowed high power outputs (43 +/- 6.1 W; p < 0.001) generating transmural lesions, 5/9 (55%), depth 6.4 +/- 2.1 mm. At constant power (2 W), catheter-tip temperature (52 +/- 5.2( composite function) C vs. 57 +/- 6.6( composite function) C; p = NS) and lesion (10 in each group) dimensions were similar for conventional and TS electrodes, but damage to parietal pericardium and lungs occurred with conventional electrodes only (70% vs. 0% p = 0.02). CONCLUSION: Standard epicardial RF ablation does not produce deep lesions and exhibits a significant energy loss probably due to poor electrode-tissue contact. Catheter irrigation allows delivery of high power outputs to the epicardium consistently creating deeper lesions than standard ablation. TS electrodes may reduce damage to neighboring structures during epicardial RF ablation.     

Contact sensing provides a highly accurate means to titrate radiofrequency ablation lesion depth

RF Ablation Lesion Depth Estimation Using Contact Sensing . Background: Transmural lesions are essential for efficacious ablation. There are, however, no accurate means to estimate lesion depth. Objective: Explore use of the electrical coupling index (ECI) from the EnSite Contact? System as a potential variable for lesion depth estimation. Methods: Radiofrequency (RF) ablation lesions were created in atria and the thighs of swine using an irrigated RF catheter. Power was 30 W for 20 or 30 seconds intracardiac and 30–50 W for 10–60 seconds for the thigh. Intracardiac, the percentage change in ECI during ablation was compared with transmurality and collateral damage occurrence. For the thigh model, an algorithm estimating lesion depth was derived. Factors included: power, duration, and change in the ECI subcomponents (ΔECI+) during ablation. The ΔECI+ algorithm was compared to one using power and duration (PD) alone. Results: Intracardiac, lesions with ≥12% reduction in ECI were more likely to be transmural (92.3% vs. 59.4%, P < 0.001). Twenty‐second lesions were less likely to cause collateral damage compared to 30 seconds (33% vs. 70%, P = 0.003), while transmurality was similar. With the thigh model, ΔECI+ had a better correlation than the PD algorithm (P < 0.01). Accuracy of the ΔECI+ algorithm was unimproved with inclusion of tip orientation, while PD improved (R²= 0.64). Discussion: Change in ECI provides evidence of transmural versus nontransmural swine intracardiac atrial lesions. A lesion depth estimation algorithm using ECI subcomponents is unaffected by tip orientation and is more accurate than using PD alone. Conclusion: Use of ECI as a factor in a lesion depth algorithm may provide clinically valuable information regarding the efficacy of intracardiac RF ablation lesions. (J Cardiovasc Electrophysiol, Vol. 22, pp. 684‐690, June 2011)     

The LETR-Principle: A Novel Method to Assess Electrode-Tissue Contact in Radiofrequency Ablation

Minimal Power RF Application. Introduction: Stable electrode-tissue contact is crucial for successful radiofrequency ablation of cardiac tachyarrhythmias. In this in vitro study, a custom-made radiofrequency generator was used to evaluate the correlation between tip temperature response to a minimal radiofrequency power delivery (Low Energy Temperature Response: LETR-Principle) and electrode-tissue contact as well as lesion size.
Methods and Results : A battery-powered radiofrequency generator (LETR-Box, 500 kHz, 0.1 to 0.3 W) could measure the temperature increase at the tip electrode with 0.01°C accuracy. The device was tested in vitro using isolated porcine ventricular tissue. For various electrode-tissue settings (i.e., 0 to 0.89 N contact force), the temperature increase (δT) due to 0.1-W power delivery for 10 seconds was recorded. Subsequently, for the same electrode-tissue contact, a temperature-controlled radiofrequency ablation was performed (70°C target temperature. 50-W maximum output, 30 sec). Thereafter, the lesion size was measured histologically. To prove the safety of the applied LETR-Principle, the tissue was inspected microscopically after continuous radiofrequency power delivery of 0.3 W for 1 hour with high contact pressure (1.33 N). The delivery of 0.1-W radiofrequency power resulted in an average δT of 0.18° plusmn; 0.13°C. During temperature-controlled radiofrequency ablation, the tip temperature was 59° 8.5°C, resulting in a lesion depth of 4.8 ± 0.6 mm. The correlation coefficient between δT and contact force was 0.97 and 0.81. respectively, for lesion depth. No lesion was microscopically visible after power delivery of 0.3 W for 1 hour with 1.33 N contact pressure.
Conclusion : The LETR-Principle safely indicates electrode-tissue contact and lesion depth under in vitro conditions and can be useful for catheter positioning during radiofrequency ablation procedures.