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.     

Temperature monitoring during radiofrequency catheter ablation of accessory pathways.

BACKGROUND. Animal studies have suggested that the temperature of the electrode-tissue interface during radiofrequency catheter ablation accurately predicts lesion size. The purpose of the current study was to evaluate the utility of continuous temperature monitoring during radiofrequency catheter ablation in patients with Wolff-Parkinson-White syndrome. METHODS AND RESULTS. Twenty patients with manifest preexcitation were included in the study. The ablation catheter was positioned on the ventricular side of the mitral annulus for left-sided accessory pathways and on the atrial side of the tricuspid annulus for right-sided and septal accessory pathways. A thermistor imbedded in the distal electrode of the ablation catheter allowed continuous temperature monitoring during each energy application. To define the relation between power and temperature, radiofrequency current was applied several times at each site using outputs of 20, 30, 40, and 50 W. The accessory pathways were successfully ablated in each of the 20 patients. Because of marked variability in the efficiency of heating between sites, power output did not predict temperature. However, at any given site, there was a positive dose-response relation between power and temperature. Radiofrequency energy applications on the atrial side of the tricuspid annulus produced lower temperatures than did applications on the ventricular side of the mitral annulus (49 +/- 7 versus 60 +/- 16 degrees C, p = 0.0001). Transient block in the accessory pathways occurred at a mean of 50 +/- 8 degrees C, whereas permanent block was seen at a mean of 62 +/- 15 degrees C (p = 0.0001). Less than half of the applications at outputs < or = 40 W produced temperatures adequate to interrupt accessory pathway conduction. An abrupt rise in impedance caused by coagulum formation occurred only at temperatures between 95 and 100 degrees C. CONCLUSIONS. Temperature monitoring may facilitate radiofrequency catheter ablation of accessory pathways. By adjusting power output to ensure that adequate but not excessive temperatures have been achieved, a rise in impedance can be avoided and the total number of energy applications and procedure duration may be reduced.     

Ablation with an internally irrigated radiofrequency catheter: Learning how to avoid steam pops

OBJECTIVES: The aim of this study was to assess the feasibility of using electrode temperature, impedance, and power to predict and thereby potentially prevent steam pops during cooled radiofrequency (RF) ablation. BACKGROUND: When myocardial temperature reaches 100 degrees C during RF catheter ablation, steam explosions are seen. Saline-cooled RF ablation reduces temperatures at the electrode-tissue interface, but excessive intramyocardial heating still may occur. METHODS: In anesthetized swine, 26 cooled RF applications were made in the right and left atria while observing with intracardiac echocardiography (ICE). Power delivery was increased gradually until a steam explosion was seen or a maximum output of 50 W was reached. RESULTS: ICE identified steam explosions in 21 RF applications. Steam explosions were associated with a large impedance increase, >25 Omega in only three cases, whereas small increases <10 Omega (mean 5.3 +/- 2.6 Omega) occurred in 18 cases. Mean electrode temperature at the time of steam explosion was 43.6 degrees C +/- 5.3; 18 of 21 explosions occurred when temperature reached >/=40 degrees C. Mean power and impedance drop were similar for applications with and without steam explosions. Five steam explosions were associated with a sudden drop in electrode temperature. CONCLUSIONS: Steam explosions are common when cooled electrode temperature exceeds 40 degrees C and are not predictable from power or impedance drop. Small impedance rises and sudden drops in measured electrode temperature indicate possible steam formation. Maintaining cooled electrode temperature <40 degrees C during RF likely will reduce the risk of steam explosions.     

Temperature and Impedance Monitoring During Slow Pathway Ablation in Patients with AV Nodal Reentrant Tachycardia

Slow Pathway Ablation. Introduction : Successful radiofrequency ablation of an accessory pathway has been demonstrated to be associated with an electrode-tissue interface temperature of approximately 60°C or an impedance change of −5 to −10 Ω. However, the temperature and impedance changes associated with ablation of AV nodal reentrant tachycardia (AVNRT) using the slow pathway approach have not been reported. Therefore, the purpose of this study was to define the temperature and impedance changes achieved during ablation of AVNRT.
Methods and Results : The study included 35 consecutive patients with AVNRT undergoing radiofrequency ablation of the slow pathway with a fixed power output of 32 W, and using a catheter with a thermistor bead embedded in the distal 4-mm electrode. The procedure was successful in each patient. The steady-state electrode-tissue interface temperature during successful applications of energy was 48.5 ± 3.3°C (range 42° to 56°C), and the steady-state temperature during ineffective applications was 46.8°± 5.5°C (P = 0.03). The mean impedance change during all applications of energy was −1.4 ± 2.8 ω, and did not differ significantly during effective and ineffective applications. Coagulum formation resulted during five applications (2.7%) in two patients (5.7%). There were no recurrences during 114 ± 21 days of follow-up.
Conclusions : Successful ablation of AVNRT using fixed power output is achieved at an electrode-tissue interface temperature of approximately 48°C and is associated with a drop in impedance of 1 to 2 ω. These findings suggest that slow pathway ablation requires less heating at the electrode-tissue interface than does accessory pathway or AV junction ablation.     

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.     

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.     

Indication of the radiofrequency induced lesion size by pre-ablation measurements.

BACKGROUND: During radiofrequency ablation of arrhythmias tissue heating and hence lesion size depend on electrode-tissue contact and cooling of the electrode tip caused by cavitary blood flow. These factors are unique and unknown for each catheter placement in the beating heart. A tool for assessing these factors prior to ablation may indicate the lesion size which will be obtained for any given catheter position. METHODS AND RESULTS: Radiofrequency ablation was performed in vitro on strips of left ventricular porcine myocardium during two different levels of convective cooling (0 or 0.1 m/s), two different contact pressures (10 or 30 g) and parallel or perpendicular electrode-tissue orientation using 7F 4 mm tip catheters. Prior to ablation the impedance rise (DeltaIMP) caused by the obtained contact and the temperature rise with a 0.6 W 5 s test pulse (DeltaT) were measured. Subsequently, during unchanged conditions, radiofrequency ablation was performed as either temperature-controlled, power-controlled or irrigated tip ablation and lesion size was determined. DeltaIMP increased significantly (P < 0.05) by improved contact, whereas it was not affected by convective cooling. DeltaT was significantly increased by increasing contact pressure (P < 0.05) and significantly decreased by increased cooling (P < 0.001). DeltaT was not systematically affected by electrode orientation. The product of DeltaT and DeltaIMP showed a significant correlation between the obtained lesion size and power output for temperature-controlled and between lesion size and tip temperature for power-controlled ablation (P < 0.001). CONCLUSIONS: Pre-ablation measurement of DeltaIMP and DeltaT can indicate the lesion size resulting after ablation in temperature-controlled, power-controlled and irrigated ablation in vitro, since DeltaT reflects cavitary cooling and to a smaller extent electrode-tissue contact, and DeltaIMP reflects only electrode-tissue contact.     

Target Temperatures of 48°C versus 60°C During Slow Pathway Ablation:

INTRODUCTION: The relationship between temperature at the electrode-tissue interface and the loss of AV and ventriculoatrial (VA) conduction is not established, and the optimal target temperature for the slow pathway approach to radiofrequency ablation of AV nodal reentrant tachycardia (AVNRT) is unknown. Therefore, the purpose of this study was to compare target temperatures of 48 degrees C and 60 degrees C during the slow pathway approach to ablation of AVNRT. METHODS AND RESULTS: The study included 138 patients undergoing ablation for AVNRT. Patients undergoing slow pathway ablation using closed-loop temperature monitoring were randomly assigned to a target temperature of either 48 degrees C or 60 degrees C. The primary success rates were 76% in the patients assigned to 48 degrees C and 100% in the patients assigned to 60 degrees C (P < 0.01). The ablation procedure duration (33 +/- 31 min vs 26 +/- 28 min; P = 0.2), fluoroscopic time (25 +/- 15 min vs 24 +/- 16 min; P = 0.5), and mean number of applications (9.3 +/- 6.5 vs 7.8 +/- 8.1; P = 0.3) were similar in patients assigned to 48 degrees and 60 degrees C, respectively. The mean temperature (46.1 degrees +/- 24.8 degrees C vs 48.7 +/- 3.2 degrees C; P < 0.01), the temperature associated with junctional ectopy (48.1 degrees +/- 2.0 degrees C vs 53.5 degrees +/- 3.5 degrees C, P < 0.0001), and the frequency of VA block during junctional ectopy (24.6% vs 37.2%; P < 0.0001) were less in the patients assigned to 48 degrees C compared to 60 degrees C. The frequency of transient or permanent AV block was similar in each group (2.8% vs 3.6%; P = 0.2). In the 60 degrees C group, only 12% of applications achieved an electrode temperature of 60 degrees C. During follow-up of 9.9 +/- 4.2 months, there was one recurrence of AVNRT in the 48 degrees C group and none in the 60 degrees C group. CONCLUSIONS: Compared to 48 degrees C, a target temperature of 60 degrees C during radiofrequency slow pathway ablation is associated with a higher primary success rate and a higher incidence of VA block during junctional ectopy induced by the radiofrequency energy. AV block is not more common with the higher target temperature, but only if VA conduction is aggressively monitored during applications of radiofrequency energy.     

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.     

Temperature Monitoring During Radiofrequency Ablation

Temperature Monitoring During RF Ablation. Thermal injury is the primary mechanism of lesion formation during radiofrequency catheter ablation procedures. Irreversible tissue injury requires heating to approximately 50°C. Temperatures above 100°C result in coagulum formation. Because of this importance of temperature during radiofrequency catheter ablation procedures, temperature monitoring has been proposed as a tool to facilitate catheter ablation procedures. The results of recent clinical studies demonstrate that electrode temperatures do not differ at successful and failed ablation sites, electrode temperature does not predict or eliminate the possibility of arrhythmia recurrence, and closed-loop temperature control decreases but does not eliminate the development of coagulum nor guarantees that target temperatures will be achieved. These observations are due in large part to the important distinctions between electrode temperature, the temperature at the electrode-tissue interface, and the temperature at the ablation target. Nonetheless, temperature monitoring and temperature control are valuable tools during radiofrequency ablation procedures as they provide important information regarding the adequacy of tissue heating, minimize the development of coagulum, and maximize lesion size.     

Gold‐Tip Electrodes—A New “Deep Lesion” Technology for Catheter Ablation? In Vitro Comparison of a Gold Alloy Versus Platinum–Iridium Tip Electrode Ablation Catheter

Gold-tip electrodes. Radiofrequency (RF) catheter ablation is widely used to induce focal myocardial necrosis using the effect of resistive heating through high-frequency current delivery. It is current standard to limit the target tissue-electrode interface temperature to a maximum of 60-70 degrees C to avoid char formation. Gold (Au) exhibits a thermal conductivity of nearly four times greater than platinum (Pt-Ir) (3.17 W/cm Kelvin vs 0.716 W/cm Kelvin), it was therefore hypothesized that RF ablation using a gold electrode would create broader and deeper lesions as a result of a better heat conduction from the tissue-electrode interface and additional cooling of the gold electrode by "heat loss" to the intracardiac blood. Both mechanisms would allow applying more RF power to the tissue before the electrode-tissue interface temperature limit is reached. To test this hypothesis, we performed in vitro isolated liver and pig heart investigations comparing lesion depths of a new Au-alloy-tip electrode to standard Pt-Ir electrode material. Mean lesion depth in liver tissue for Pt-Ir was 4.33+/-0.45 mm (n=60) whereas Au electrode was able to achieve significantly deeper lesions (5.86+/-0.37 mm [n=60; P<0.001]). The mean power delivered using Pt-Ir was 6.95+/-2.41 W whereas Au tip electrode delivered 9.64+/-3.78 W indicating a statistically significant difference (P<0.05). In vitro pig heart tissue Au ablation (n=20) increased significantly the lesion depth (Au: 4.85+/-1.01 mm, Pt-Ir: 2.96+/-0.81 mm, n=20; P<0.001). Au tip electrode again applied significantly more power (P<0.001). Gold-tip electrode catheters were able to induce deeper lesions using RF ablation in vitro as compared to Pt-Ir tip electrode material. In liver and in pig heart tissue, the increase in lesion depth was associated with a significant increase in the average power applied with the gold electrode at the same level of electrode-tissue temperature as compared to platinum material.     

Temperature-Controlled Irrigated Tip Radiofrequency Catheter Ablation:

Temperature-Controlled Irrigated Tip Ablation. Introduction : In patients with ventricular tachycardias due to structural heart disease, catheter ablation cures < 60% partly due to the limited lesion size after conventional radiofrequency ablation. Irrigated tip radiofrequency ablation using power control and high infusion rates enlarges lesion size, hut has increased risk of cratering. The present study explores irrigated tip catheter ablation in temperature- controlled mode, target temperature 60°C, using an irrigation rate of 1 mL/min, comparing this to conventional catheter technique, target temperature 80°C.
Methods and Results : In vivo anesthetized pigs were ablated in the left ventricle. In vitro strips of porcine left ventricular myocardium were ablated in a tissue bath. Lesion volume was significantly larger after irrigated tip ablation both in vivo (544 ± 218 vs 325 ± 194 mm³, P < 0.01) and in vitro (286 ± 113 vs 179 ± 23 mm³, P < 0.01). The incidence of cratering was not significantly different between the two groups. In vivo, no coagulum formation on part of the catheter tip was seen after irrigated tip ablation as opposed to 52% of the applications with conventional ablation (P < 0.05).
Conclusion : We conclude that temperature-controlled radiofrequency ablation with irrigated tip catheters using low target temperature and low infusion rate enlarges lesion size without increasing the incidence of cratering and reduces coagulum formation of the tip.     

Electrode radius predicts lesion radius during radiofrequency energy heating. Validation of a proposed thermodynamic model

Myocardial heating by transcatheter delivery of radiofrequency (RF) energy has been proposed as an effective means of arrhythmia ablation. A thermodynamic model describing the radial temperature gradient at steady state during RF-induced heating is proposed. If one assumes that RF power output is adjusted to maintain a constant electrode-tissue interface temperature at all times, then this thermodynamic model predicts that the radius of the RF-induced lesion will be directly proportional to the electrode radius. A total of 76 RF-induced lesions were created in a model of isolated canine right ventricular free wall perfused and superfused with oxygenated Krebs-Henseleit buffer. Electrode radius was varied between 0.75 and 2.25 mm. RF energy (500 kHz) was delivered for 90 seconds, and the power output was adjusted to maintain a constant electrode-tissue interface temperature of 60 degrees C. A strong linear correlation was observed between electrode radius and lesion radius in two dimensions: transverse (p = 0.0001, r = 0.85) and transmural (p = 0.0001, r = 0.89). With these data, the temperature correlation with irreversible myocardial injury in this model was calculated at 46.6-48.8 degrees C. Therefore, the proposed thermodynamic model closely predicts the observed relation between electrode radius and lesion size during RF myocardial heating.     

Impedance monitoring during catheter ablation of atrial fibrillation

BACKGROUND: Delivery of radiofrequency energy in proximity of a pulmonary vein can cause vein stenosis. A sudden decrease in impedance as the catheter is moved from the vein into the left atrium (LA) has been used to define the pulmonary vein-LA transition during ablation procedures. OBJECTIVES: The purpose of this study was to define the variables affecting impedance measurement. METHODS: In vitro analysis of impedance was performed in a saline bath using sheaths and a plastic stereolithographic model of the LA. Impedance was continuously monitored during a calibrated pullback from the pulmonary vein into the LA in 37 veins of 10 patients referred for catheter ablation. Location of the catheter was confirmed by the following imaging modalities: intracardiac echocardiography, contrast venography, electroanatomic mapping, and computed tomography/magnetic resonance imaging (offline) in all patients. RESULTS: Larger cross-sectional areas containing the catheter correlated with lower impedance in an exponential manner both with respect to sheath size (R(2) = 0.99) and in the stereolithographic model (R(2) = 0.91). In vivo, the impedance in the pulmonary veins decreased in an exponential manner as the catheter was pulled back into the LA. However, impedance at the vein orifice was not significantly higher than the LA. A defined cutoff value for defining the pulmonary vein-LA transition could not be identified. CONCLUSION: The primary determinant of impedance is the cross-sectional area of the space containing the catheter. Impedance monitoring alone does not guarantee a catheter tip position outside the pulmonary vein. Intraprocedural imaging confirmation should be considered to avoid radiofrequency application within pulmonary veins.     

Cooled needle catheter ablation creates deeper and wider lesions than irrigated tip catheter ablation

OBJECTIVES: To design and test a catheter that could create deeper ablation lesions. BACKGROUND: Endocardial radiofrequency (RF) ablation is unable to reliably create transmural ventricular lesions. We designed an intramural needle ablation catheter with an internally cooled 1.1-mm diameter straight needle that could be advanced up to 14 mm into the myocardium. The prototype catheter was compared with an irrigated tip ablation catheter. METHODS: Ablation lesions were created under general anesthesia in 14 male sheep (weight 44 +/- 7.3 kg) with fluoroscopic guidance. Each of the catheters was used to create two ablation lesions at randomly allocated positions within the left ventricle. The irrigation rate, target temperature, and maximum power were: 20 mL/min, 85 degrees C, 50 W for the intramural needle catheter and 20 mL/min, 50 degrees C, 50 W for the irrigated tip catheter, respectively. All ablations were performed for 2 minutes. After the last ablation, blue tetrazolium (12.5 mg/kg) was infused intravenously. The heart was removed via a left thoracotomy after monitoring the sheep for one hour. RESULTS: There was no evidence of cardiac tamponade in any sheep. The intramural needle catheter lesions were significantly wider (10.9 +/- 2.8 mm vs 10.1 +/- 2.4 mm, P = 0.01), deeper (9.6 +/- 2.0 mm vs 7.0 +/- 1.3 mm, P = 0.01), and more likely to be transmural (38% vs 0%, P = 0.03). CONCLUSIONS: Cooled intramural needle ablation creates lesions that are significantly deeper and wider than endocardial RF ablation using an irrigated tip catheter in sheep hearts. This technology may be useful in treating ventricular tachycardia resistant to conventional ablation techniques.     

Direct imaging of transvenous radiofrequency cardiac ablation using a steerable fiberoptic infrared endoscope.

BACKGROUND: Direct imaging through blood has been achieved in vivo using fiberoptics and infrared wavelength technology. OBJECTIVES: The purpose of this study was to determine the feasibility of using a percutaneous, steerable, fiberoptic infrared endoscope to identify and characterize the electrode-tissue interface during transvenous cardiac ablation. METHODS: Infrared endoscopy was performed during 24 catheter ablation attempts in 10 mongrel dogs. Infrared imaging was performed through a transparent dome located at the tip of a 7Fr steerable endoscope using an imaging wavelength of 1,620 nm. Radiofrequency ablation was performed using a 4-mm-tip electrode catheter. Attempts were made to identify the electrode-endocardial interface at each ablation site and to characterize any signal changes during ablation. RESULTS: The electrode-tissue interface could be identified at 19 of the 24 ablation sites. Changes at the electrode-tissue interface were observed during ablation at 14 sites, which included a gradual increase in the tissue signal intensity at 12 sites. Small lucencies near the ablation electrode were observed at six sites. There was no interference during energy delivery. Endocardial features identified by endoscopy correlated with the postmortem appearance. CONCLUSION: Direct imaging of intracardiac structures and the electrode-tissue interface can be achieved through blood during transvenous catheter ablation with infrared endoscopy using a steerable, fiberoptic, infrared endoscopic catheter. Ablation lesion formation can be seen as a gradual increase in signal intensity. Fiberoptic infrared endoscopy appears to be a promising new tool for guiding catheter ablation.     

Electrochemical potentials during radiofrequency energy delivery: a new method to control catheter ablation of arrhythmias.

AIMS: Thermal injury of subendocardial tissue leads to a release of electrolytes and free radicals from the intracellular site creating a change in electrochemical potential (eP) between the distal and the proximal catheter tip electrodes. The aim of the study was to verify the detection of ablation-induced release of electrolytes and free radicals and to assess the suitability of control-line energy delivery at ablation by measuring eP. METHODS AND RESULTS: In vitro tests under constant flow conditions were performed in a 101 bath of physiological saline solution or bovine blood. Endomyocardial preparations of fresh bovine hearts were used. Closed-loop temperature-controlled, irrigated and closed-loop eP-controlled ablations were performed. In vivo animal investigations were performed in six anaesthetized and ventilated pigs. The existence of the eP was established in the tank model and was confirmed in animal investigations. High correlations were found between eP and catheter tip temperature (r=0.87) and between maximum eP and induced lesion size (r=0.85). Also a high correlation (r=0.85, P<0.001) was found between eP and lesion volume. CONCLUSIONS: Control of energy delivery during RF ablation by the measurement of eP is feasible. In comparison with temperature controlled RF ablation, ablation guided by eP-measurement revealed a superior correlation with induced lesion size. Especially during cooled radiofrequency catheter ablation eP is the only parameter for control of energy delivery.     

Comparison of radiofrequency ablation in normal versus scarred myocardium

INTRODUCTION: Reentrant circuits causing ventricular tachycardia are closely associated with previously scarred myocardium. The presence of scar has been blamed for the poor success rate of radiofrequency ablation (RFA) in that context. This article investigates the in vivo effects of radiofrequency ablation in myocardium scarred from acute myocardial infarction. METHODS AND RESULTS: Anterior myocardial infarction was induced in five dogs by ligating the left anterior descending artery. The mean left ventricular ejection fraction after infarction was 38%. At a mean of 15 weeks following myocardial infarction, 50 RFA lesions were created in random order, 25 in scarred and 25 in normal myocardium using a needle electrode (21 gauge, 5 mm in length) introduced from the epicardium of the left ventricle at thoracotomy. During unipolar temperature-controlled RFA (90 degrees C for 60 seconds), intramural temperatures were measured by thermistors at distances of 1, 2, 3, 4, and 5 mm from the ablating electrode. The margins of the lesions were clearly discernible in scar at histological examination in 64% of ablations where the scarring was patchy. There were no significant differences between lesion sizes, intramural temperatures at different distances, total energy required for ablation, or mean impedance during ablation of normal versus scarred myocardium. CONCLUSIONS: Scar does not affect lesion size or intramural temperature profile during RFA if electrode size, tissue contact, and tip temperature are controlled. More radiofrequency energy is not required to maintain tip temperature at 90 degrees C in scar compared to normal myocardium.     

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.     

Robotically Assisted Ablation Produces More Rapid and Greater Signal Attenuation Than Manual Ablation

Introduction: Robotic remote catheter ablation potentially provides improved catheter‐tip stability, which should improve the efficiency of radiofrequency energy delivery. Percentage reduction in electrogram peak‐to‐peak voltage has been used as a measure of effectiveness of ablation. We tested the hypothesis that improved catheter‐tip stability of robotic ablation can diminish signals to a greater degree than manual ablation. Methods: In vivo NavX? maps of 7 pig atria were constructed. Separate lines of ablation were performed robotically and manually, recording pre‐ and postablation peak‐to‐peak voltages at 10, 20, 30, and 60 seconds and calculating signal amplitude reduction. Catheter ablation settings were constant (25W, 50°, 17 mL/min, 20–30 g catheter tip pressure). The pigs were sacrificed and ablation lesions correlated with NavX maps. Results: Robotic ablation reduced signal amplitude to a greater degree than manual ablation (49 ± 2.6% vs 29 ± 4.5% signal reduction after 1 minute [P = 0.0002]). The mean energy delivered (223 ± 184 J vs 231 ± 190 J, P = 0.42), power (19 ± 3.5 W vs 19 ± 4 W, P = 0.84), and duration of ablation (15 ± 9 seconds vs 15 ± 9 seconds, P = 0.89) was the same for manual and robotic. The mean peak catheter‐tip temperature was higher for robotic (45 ± 5°C vs 42 ± 3°C [P < 0.0001]). The incidence of >50% signal reduction was greater for robotic (37%) than manual (21%) ablation (P = 0.0001). Conclusion: Robotically assisted ablation appears to be more effective than manual ablation at signal amplitude reduction, therefore may be expected to produce improved clinical outcomes.