James G. Whayne

Three-dimensional finite element analysis of current density and temperature distributions during radio-frequency ablation

This study analyzed the influence of electrode geometry, tissue-electrode angle, and blood flow on current density and temperature distribution, lesion size, and power requirements during radio-frequency ablation. The authors used validated three-dimensional finite element models to perform these analyses. They found that the use of an electrically insulating layer over the junction between electrode and catheter body reduced the chances of charring and coagulation. The use of a thermistor at the tip of the ablation electrodes did not affect the current density distribution. For longer electrodes, the lateral current density decreased more slowly with distance from the electrode surface. The authors analyzed the effects of three tissue-electrode angles: 0, 45, and 90/spl deg/. More power was needed to reach a maximal tissue temperature of 95/spl deg/C after 120 s when the electrode-tissue angle was 45/spl deg/. Consequently, the lesions were larger and deeper for a tissue-electrode angle of 45/spl deg/ than for 0 and 90/spl deg/. The lesion depth, volume, and required power increased with blood flow rate regardless of the tissue-electrode angle. The significant changes in power with the tissue-electrode angle suggest that it is safer and more efficient to ablate using temperature-controlled RF generators. The maximal temperature was reached at locations within the tissue, a fraction of a millimeter away from the electrode surface. These locations did not always coincide with the local current density maxima. The locations of these hottest spots and the difference between their temperature and the temperature read by a sensor placed at the electrode tip changed with blood flow rate and tissue-electrode angle.<<ETX>>

Microwave catheter ablation of myocardium in vitro. Assessment of the characteristics of tissue heating and injury.

BACKGROUND Radiofrequency (RF) catheter ablation lesion size has been limited by the small volume of tissue directly heated by the RF electrode. Microwave (MW) energy has been proposed as an alternative energy source to generate larger lesions because of its increased volume of direct tissue heating. To further characterize MW ablation of myocardium, we studied the temperature-versus-distance profiles during MW ablation in an in vitro model of perfused and superfused porcine right ventricular free wall. METHODS AND RESULTS Radial tissue temperatures in 19 isolated porcine right ventricles were measured and recorded with four fluoroptic thermometry probes placed within the myocardium at 2.5-mm radial increments from the catheter. The MW antenna catheters used were monopolar and helical-coil antennas resonating at 915 and 2450 MHz. Durations of energy delivery for a 915-MHz MW monopolar antenna (60 to 600 seconds) and a 4-mm-tip RF electrode (60 and 300 seconds) were varied to compare time courses of lesion formation. For each lesion, the temperature at the lesion border zone (the isotherm of irreversible tissue injury) was determined. Similar lesion size and temperature profiles were observed for 915- versus 2450-MHz MW antennas and monopolar versus helical-coil MW antennas. Lesion depth for the 915-MHz monopolar antenna increased monoexponentially with a half-time of 170 seconds. The isotherms for all MW antenna designs were not significantly different. The mean isotherm of irreversible tissue injury for MW lesions was not significantly different from the mean isotherm for RF lesions (54.4 degrees C versus 53.6 degrees C, respectively). CONCLUSIONS Microwave ablation has the potential to directly heat a greater volume of tissue than RF ablation but only with efficient MW antennas. The primary mechanism of tissue injury for both MW and RF ablation appears to be thermal.

Effects of radiofrequency catheter ablation on regional myocardial blood flow. Possible mechanism for late electrophysiological outcome.

BACKGROUND We postulated that the late electrophysiological effects of radiofrequency (RF) ablation may be related to microvascular injury extending beyond the region of acute coagulation necrosis. METHODS AND RESULTS Eighteen RF lesions created in the left anterior descending coronary artery (LAD) perfusion bed of seven open chest anesthetized dogs were studied. The ablation electrode and surrounding myocardium were imaged using high-resolution two-dimensional echocardiography at x 4 magnification. After 60 seconds of RF delivery, sonicated albumin microbubbles (mean size, 4.3 microns) were injected into the LAD to measure regional myocardial perfusion, and time-intensity plots were generated from simultaneously acquired two-dimensional echocardiography images. The regions with persistent contrast effect on two-dimensional echocardiography were larger than the pathological lesions (mean cross-sectional area, 48.3 +/- 6.3 versus 19.3 +/- 4.7 mm2, respectively; P < .0001). The mean contrast transit rate in the area corresponding to the pathological lesion was 25 +/- 12% of that in the normal myocardium, but it was also reduced beyond the lesion, being 48 +/- 27% and 82 +/- 28% of normal, respectively, in the 3-mm and 3- to 6-mm circumferential rims surrounding the pathological lesion (P < .05). Electron microscopy performed in two additional dogs with similar lesions demonstrated the presence of ultrastructural damage to the microvascular endothelium well beyond the pathological lesion edge. CONCLUSIONS RF catheter ablation not only results in a marked reduction in blood flow within the acute pathological lesion but also causes reduced flow beyond the borders of the acute lesion because of microvascular endothelial cell injury. The progression or resolution of tissue injury within the region beyond the border of the pathological lesion may explain the late electrophysiological effects of RF ablation.

Percutaneous Multielectrode Endocardial Mapping During Ventricular Tachycardia in the Swine Model

BACKGROUND Identification of critical areas within the ventricular tachycardia circuit is a prerequisite for catheter ablation. Currently, mapping during ventricular tachycardia, usually performed with standard catheters, is difficult and time-consuming and can be used only in patients with hemodynamically stable tachycardia. METHODS AND RESULTS A total of 43 pigs underwent closed-chest induction of myocardial infarction. A basket-shaped catheter carrying 64 electrodes was deployed in the left ventricle during normal sinus rhythm. Unipolar pacing at 3 mA was successful in 78% of the basket catheter electrodes, demonstrating good electrode-tissue contact. Hemodynamic and echocardiographic measurements did not reveal any significant interference with myocardial or valvular function during or after catheter deployment. One hundred eighteen episodes of monomorphic ventricular tachycardia were induced in 28 pigs through right ventricular stimulation, 81 of which were mapped and analyzed. Ventricular tachycardia mapping was rapid, requiring only several beats and < 10 seconds to complete. Presystolic potentials, a possible target for ablation, were identified in 58% of the tachycardia episodes mapped. Pathological examination revealed only minor valvular and endocardial catheter-induced lesions immediately after mapping and none a month later. CONCLUSIONS The multielectrode catheter enables rapid and safe percutaneous endocardial mapping of ventricular tachycardia in the swine model. Exploration of the clinical potential of the multielectrode catheter seems warranted.

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.

Intracoronary ethanol ablation in swine: Effects of ethanol concentration on lesion formation and response to programmed ventricular stimulation

Intracoronary Ethanol Ablation. Introduction: Intracoronary ethanol ablation has been successfully used as arrhythmia therapy, hut the dose response of ventricular function and arrhythmogenesis to varying ethanol concentrations is undefined.

Radiofrequency multielectrode catheter ablation in the atrium.

We developed a temperature-controlled radiofrequency (RF) system which can ablate by delivering energy to up to six 12.5 mm long coil electrodes simultaneously. Temperature feedback was obtained from temperature sensors placed at each end of coil electrodes, in diametrically opposite positions. The coil electrodes were connected in parallel, via a set of electronic switches, to a 150 W 500 kHz temperature-controlled RF generator. Temperatures measured at all user-selected coil electrodes were processed by a microcontroller which sent the maximum value to the temperature input of the generator. The generator adjusted the delivered power to regulate the temperature at its input within a 5 degrees C interval about a user-defined set point. The microcontroller also activated the corresponding electronic switches so that temperatures at all selected electrodes were controlled within a 5 degrees C interval with respect to each other. Physical aspects of tissue heating were first analysed using finite element models and current density measurements. Results from these analyses also constituted design input. The performance of this system was studied in vitro and in vivo. In vitro, at set temperatures of 70 degrees C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90 degrees C were contiguous. The lesion length increased almost linearly with the number of electrodes. Power requirements to reach a set temperature were larger as more electrodes were driven by the generator. The system impedance decreased as more electrodes were connected in the ablation circuit and reached a low of 45.5 ohms with five coil electrodes in the circuit. In vivo, right atrial lesions were created in eight mongrel canines. The power needed to reach 70 degrees C set temperature varied between 15 and 114 W. The system impedance was 105+/-16 ohms, with one coil electrode in the circuit, and dropped to 75+/-12 ohms when two coil electrodes were simultaneously powered. The length and the width of the lesion set varied between 17.6+/-6.1 and 59.2+/-11.7 mm and 5.9+/-0.7 and 7.1+/-1.2 mm respectively. No sudden impedance rises occurred and 75% of the lesions were contiguous. From the set of contiguous lesions, 90% were potentially therapeutic as they were transmural and extended over the entire target region. The average total procedure and fluoroscopy times were 83.4 and 5.9 min respectively. We concluded that the system can safely perform long and contiguous lesions in canine right atria.

Contiguous lesions by radiofrequency multielectrode ablation

Radiofrequency catheter ablation can cure cases of atrial fibrillation. Multielectrode ablation catheters that can create long and contiguous lesions may benefit the physician by both increasing the predictability of lesion profiles and by reducing procedure time. This study presents an analysis of thermal profiles in tissue under a steerable 8F catheter capable of creating contiguous lesions. The catheter carried three 3-mm electrodes spaced 3.4 mm apart.

Temperature distribution under cooled electrodes during radiofrequency catheter ablation

Larger lesions were created when the electrode of an RF ablation system was actively cooled below blood temperature. To keep the maximal tissue temperature at about 90/spl deg/C, significantly more RF power had to be delivered. Because the hottest tissue region moved farther from the electrode surface, it was more difficult to predict the adequate tissue heating. Therefore, the probability of overheating the tissue and producing micro-explosions increased.

Effects of temperature sensor placement on performance of temperature-controlled ablation [cardiac arrhythmias application]

The significant changes in power with tissue-electrode angle suggested that the lesions were more predictable when ablating with temperature-based RF generators, rather than non-temperature systems. The maximal tissue temperature was reached at locations within the tissue a fraction of a millimeter away from the ablation electrode surface. The errors in tissue temperature prediction introduced by temperature sensors placed inside the electrode changed with blood flow rate and tissue-electrode angle. This study shows that to reduce tissue temperature prediction errors, during temperature-controlled ablation it was better to use a temperature sensor placed at the tip rather than at the center of the ablation electrode.