Electrode-tissue Interface Temperature Research Articles

Renal Artery Branch Denervation: Evaluation of Lesion Characteristics Using a Thermochromic Liquid Crystal Phantom Model

Lately, combined main vessel and branch ablation has been recommended during radiofrequency (RF) renal artery denervation. Utilising a validated renal artery phantom model, we aimed (1) to determine thermal injury extent (lesion depth, width and circumferential coverage) and electrode-tissue interface temperature for branch renal artery ablation, and (2) to compare the extent of thermal injury for branch versus main vessel ablation using the same RF System. We employed a gel based renal artery phantom model simulating variable vessel diameter and flow, which incorporated a temperature sensitive thermochromic-liquid-crystal (TLC) film for assessing RF ablation thermodynamics. Ablations in a branch renal artery model (n = 32) were performed using Symplicity Spyral (Medtronic, Minneapolis, MN, USA). Lesion dimensions defined by the 51 °C isotherm, circumferential injury coverage, and electrode-tissue interface temperature were measured for all ablations at 60 seconds. Lesion dimensions were 2.13 ± 0.13 mm and 4.13 ± 0.18 mm for depth and width, respectively, involving 23% of the vessel circumference. Maximum electrode-tissue interface temperature was 68.31 ± 2.29 °C. No significant difference in lesion depth between branch and main vessel ablations was found (Δ = 0.02 mm, p = 0.60). However, lesions were wider in the branch (Δ=0.49 mm, p < 0.001) with a larger circumferential coverage compared to main vessel (arc angle of 82.02±3.27° versus 54.90±4.36°, respectively). In the phantom model, branch ablations were of similar depth but had larger width and circumferential coverage compared to main vessel ablations. Concerning safety, no overheating at the electrode-tissue interface was observed.

Computational Modeling of Open-Irrigated Electrodes for Radiofrequency Cardiac Ablation Including Blood Motion-Saline Flow Interaction.

Radiofrequency catheter ablation (RFCA) is a routine treatment for cardiac arrhythmias. During RFCA, the electrode-tissue interface temperature should be kept below 80°C to avoid thrombus formation. Open-irrigated electrodes facilitate power delivery while keeping low temperatures around the catheter. No computational model of an open-irrigated electrode in endocardial RFCA accounting for both the saline irrigation flow and the blood motion in the cardiac chamber has been proposed yet. We present the first computational model including both effects at once. The model has been validated against existing experimental results. Computational results showed that the surface lesion width and blood temperature are affected by both the electrode design and the irrigation flow rate. Smaller surface lesion widths and blood temperatures are obtained with higher irrigation flow rate, while the lesion depth is not affected by changing the irrigation flow rate. Larger lesions are obtained with increasing power and the electrode-tissue contact. Also, larger lesions are obtained when electrode is placed horizontally. Overall, the computational findings are in close agreement with previous experimental results providing an excellent tool for future catheter research.

The biophysics of renal sympathetic denervation using radiofrequency energy

Renal sympathetic denervation is currently performed in the treatment of resistant hypertension by interventionists who otherwise do not typically use radiofrequency (RF) energy ablation in their clinical practice. Adequate RF lesion formation is dependent upon good electrode-tissue contact, power delivery, electrode-tissue interface temperature, target-tissue impedance and the size of the catheter's active electrode. There is significant interplay between these variables and hence an appreciation of the biophysical determinants of RF lesion formation is required to provide effective and safe clinical care to our patients. In this review article, we summarize the biophysics of RF ablation and explain why and how complications of renal sympathetic denervation may occur and discuss methods to minimise them.

Abstract 1545: Insulating an Open Irrigated Ablation Electrode Improves Electrode Temperature Feedback for Radiofrequency Lesion Control

Open irrigation (Irrig) cools the electrode and electrode-tissue interface (ETI), preventing thrombus. Improving electrode temperature (ET) feedback by insulating the Irrig electrode internally may help prevent excessive tissue heating. The purpose was to compare an insulated Irrig electrode to a conventional Irrig electrode in ET response and lesion formation using a canine thigh muscle preparation. Methods In 11 dogs, the skin over the thigh muscle was incised and raised to form a cradle which was superfused with heparinized blood at 37°C. The 7F insulated catheter has 6 Irrig holes proximal to a 2.5mm tip electrode (spray around electrode and ETI) and one thermally insulated Irrig channel through the electrode (only external cooling, Ext Cool, St Jude Medical). The Ext Cool and a conventional 7F, 2.5mm open Irrig catheter (external and internal electrode cooling, Ext/Int Cool, St Jude Medical) were held perpendicular or parallel to thigh muscle. RF was applied for 60 sec (n=176) at 10, 15, 20, or 25W with saline Irrig at 17ml/min in low blood flow (&lt;0.1 m/sec) to favor thrombus. ET, ETI and tissue temperatures (3 and 7mm depths) were measured. After each RF, blood was removed to examine for thrombus. Results Table and Fig Conclusions Ext Cool had: greater correlation between ET and both lesion depth (r=0.84 vs 0.54) and steam pop (ET≥51°C vs ET 32– 48°C); lower ETI temperature and less thrombus.

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.

Tissue rupture and bubble formation during radiofrequency catheter ablation: "echoes of a pop".

The popping phenomenon during radiofrequency catheter ablation is well known to experienced electrophysiologists. When the electrode-tissue interface temperature increases rapidly above the boiling point, it can vaporize blood, thus causing a mini-explosion and an audible pop. Evaporation may occur intramurally, leading to gas bubble formation within the tissue. With continued energy application, this bubble expands and erupts through the weakest path, cleaving the tissues. Whether the intramural vaporization and explosion can lead to shock-wave production, with consequent cavitation and microbubble formation akin …

Why a large tip electrode makes a deeper radiofrequency lesion: effects of increase in electrode cooling and electrode-tissue interface area.

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. 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 degrees C; (2) same except the surrounding blood was stationary; (3) perpendicular electrode position, maintaining tissue temperature (3.5-mm depth) at 90 degrees C; and (4) electrode parallel to the muscle, maintaining tissue temperature at 90 degrees 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. With a larger electrode, both increased cooling and increased electrode-tissue interface area increase volume of resistive heating and lesion depth.

Temperature-controlled radiofrequency catheter ablation of manifest accessory pathways.

The primary objectives of this study were to assess the feasibility of temperature-controlled radiofrequency catheter ablation of left and right sided manifest accessory pathways in patients with Wolff-Parkinson-White syndrome and to gain more insights into biophysical aspects of temperature-controlled catheter ablation in humans. The electrode-tissue interface temperature and other biophysical parameters are among important variables determining the efficacy and safety of radiofrequency ablation of accessory pathways. Experimental studies have shown that radiofrequency-induced tissue necrosis can be accurately predicted by monitoring of catheter tip temperature. 38 consecutive patients (14 f, 24 m; aged 42 +/- 12 years) with anterograde conducting accessory pathways (left sided: n = 22; right sided: n = 16) underwent temperature-controlled radiofrequency ablation (HAT 200S, Dr Osypka, Germany). The electrode temperature was monitored via a thermistor embedded into a 4 mm catheter tip. Power output was adjusted automatically during energy delivery in a closed loop system (preselected temp.: 70.1 +/- 5.8 degrees C). Accessory pathway conduction was successfully abolished in all patients after the delivery of 2.3 +/- 2.1 radiofrequency pulses (range: 1-9, median: 2). Interruption of the accessory pathway as evidenced by loss of preexcitation occurred after 5.9 +/- 5.4 s. At the time of the interruption of the accessory pathway the catheter tip temperature measured 54.2 +/- 11.2 degrees C in patients with left and 44.9 +/- 5.0 degrees C in patients with right sided accessory pathways, respectively (P < 0.008). Higher temperature levels during left sided applications did not shorten the time it took for the effect to appear (left sided accessory pathway: 7.5 +/- 6.3 s, right sided accessory pathway: 3.7 +/- 2.9 s; ns). The catheter tip temperature was significantly higher during left compared to right sided applications after 5 (52.1 +/- 3.1 degrees C vs 47.2 +/- 4.3 degrees C) and 10 s (61.5 +/- 6.2 degrees C vs 52.7 +/- 4.2 degrees C) following initiation of the impulse (P < 0.005). Power output and delivered energy did not differ significantly at the time of accessory pathway abolition. Peak values of delivered power (45.1 +/- 10.9 W vs 41.3 +/- 10.6 W; P < 0.05) and total delivered energy (2452 +/- 1335 J vs 1392 +/- 762 J; P < 0.02) were significantly higher in the group of right sided pathways compared to left sided applications. The peak temperature measured 77.1 +/- 13 degrees C during effective and 69.9 +/- 14 degrees C during ineffective energy applications (P < 0.05). The time it took for the effect to appear was significantly longer in transiently effective pulses (10.4 +/- 7.2 s) compared to permanently effective applications (5.9 +/- 5.4 s; P < 0.02). Despite temperature control, an abrupt rise in impedance was observed in 10 of 89 (11%) energy applications. No procedure-related complications occurred. Temperature-controlled radiofrequency ablation of manifest accessory pathways is highly effective and safe. The temperature response is faster and significantly higher in left-sided energy applications compared to right-sided pulses. Peak temperature levels measured at the electrode tip are significantly higher during effective than ineffective pulses. Sudden rises in impedance are not completely prevented during temperature-controlled radiofrequency ablation of accessory pathway, although no procedure-related complications were noted in this patient cohort.

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

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. 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 degrees C in 35% of the perpendicular orientations, 1.9 degrees C in 82% of the 45 orientations, and 5 degrees 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 degrees C and 7.6 degrees 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 degrees C. The normal to extended tip thermistor configurations analysis showed similar relationships between lesion size and temperature. Accuracy of a single tip thermistor was found to be dependent upon catheter-tissue 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 degrees C, a single flush-mounted tip thermistor is probably adequate for temperature monitoring of lesion formation and avoidance of impedance rises.

Ultrastructural Observations in the Myocardium Beyond the Region of Acute Coagulation Necrosis Following Radiofrequency Catheter Ablation

We hypothesized that myocardial injury following radiofrequency (RF) catheter ablation may extend beyond the region of acute coagulation necrosis as defined by histochemical staining. Five RF lesions were created in vivo in the left ventricle of two dogs using a 4-mm tipped ablation electrode in which RF power was adjusted to maintain an electrode-tissue interface temperature of 85 degrees C for 60 seconds. The lesions were bisected; one half of the lesions were stained with nitroblue tetrazolium (NBT) and the other half processed for electron microscopy. Three zones of interest were identified extending 0-3 mm, 3-6 mm, and > 6 mm from the visible pathologic lesion border. The degree of ultrastructural injury to the myocardium was scored for each zone. Electron microscopy demonstrated the presence of significant abnormalities of the plasma membrane, mitochondria, sarcomeres, sarcoplasmic reticulum, and gap junctions of myocytes, as well as damage to the microvasculature extending up to 6 mm beyond the pathologic lesion edge. The plasma membrane and gap junctions of myocytes and the microvasculature appeared particularly sensitive to thermal injury, whereas the intercalated discs were relatively thermally resistant. RF catheter ablation results in ultrastructural damage to the myocardium extending up to 6 mm beyond the acute pathologic RF lesion border as defined by NBT histochemical staining.

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.

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.