Sidney D. Fleischman

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>>

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.

Transcatheter Subendocardial Infusion: A Novel Technique for Mapping and Ablation of Ventricular Myocardium

BACKGROUND Catheter ablation with radiofrequency energy is feasible in a limited subset of patients with ventricular tachycardia. The purpose of this study was to evaluate a technique for mapping and ablation of ventricular myocardium with the use of transcatheter subendocardial infusion. METHODS AND RESULTS A needle-tipped deflectable electrode catheter was used to deliver reagents to endocardial target sites. This was equipped with two central lumens to allow sequential administration of mapping and ablation injectants with minimal admixture. The mapping injectant consisted of a mixture of lidocaine, iohexal, and glycerin; the ablation injectant contained ethanol, iohexal, and glycerin. Infusion of the mapping injectant (1 cm3 over 3 or 5 seconds, n = 14) produced a stain on fluoroscopy and increased local capture threshold by 61%. No lesions resulted from mapping infusions. Infusion of the ethanol-containing injectant (n = 48) produced discrete lesions, with a mean volume ranging from 0.6 to 1.5 cm3. There was a direct relationship between infusion volume, infusion duration, and resultant lesion volume. Fibrosis in a region of healed myocardial infarction did not impair diffusion of the injectant or affect lesion dimensions. Microscopic analysis of chronic lesions showed a sharply demarcated border zone between fibrotic and normal myocardium. CONCLUSIONS Transcatheter subendocardial infusion can be used to reversibly impair local excitability and mark an injection site fluoroscopically. Subendocardial injection of ethanol can predictably ablate a large volume of ventricular myocardium. Additional study of this system in an arrhythmia model will help to define its potential for mapping and ablation of hypotensive ventricular tachycardia.

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.

Coil electrodes for RF ablation of atrial fibrillation

Long and contiguous lesions may cure AF. We analyzed a multielectrode steerable catheter capable of creating such lesions. For optimal temperature control purposes, based on analytical and experimental data, one temperature sensor was placed at each end of the long coil electrodes. In vitro ablations were performed to analyze the catheter performance. At set temperatures of 70/spl deg/C, 85% of the lesions were contiguous. All lesions created at set temperatures of 80 and 90/spl deg/C were contiguous. No sudden impedance rises occurred. The lesion length increased linearly with the number of electrodes. Power to reach set temperature was larger and the impedance decreased as more electrodes were connected in the ablation circuit.