Comparison of In Vivo Tissue Temperature Profile and Lesion Geometry for Radiofrequency Ablation With a Saline-Irrigated Electrode Versus Temperature Control in a Canine Thigh Muscle Preparation

Abstract

Background It is thought that only a thin layer of tissue adjacent to the electrode is heated directly by electrical current (resistive heating) during radiofrequency ablation. Most of the thermal injury is thought to result from conduction of heat from the surface layer. The purpose of this study was to determine whether lesion depth could be increased by producing direct resistive heating deeper in the tissue with higher radiofrequency power, allowed by cooling the ablation electrode with saline irrigation to prevent the rise in impedance that occurs when the electrode-tissue interface temperature reaches 100°C. Methods and Results In 11 anesthetized dogs, the thigh muscle was exposed and bathed with heparinized canine blood (36°C to 37°C). A 7F catheter, with a central lumen, a 5-mm tip electrode with six irrigation holes, and an internal thermistor, was positioned perpendicular to the thigh muscle and held at a constant contact weight of 10 g. Radiofrequency current was delivered to 145 sites (1) at high constant voltage (66 V) without irrigation (CV group, n=31), (2) at variable voltage (20 to 66 V) to maintain tip-electrode temperature at 80°C to 90°C without irrigation (temperature-control group, n=39), and (3) at high CV (66 V) with saline irrigation through the catheter lumen and ablation electrode at 20 mL/min (CV irrigation group, n=75). Radiofrequency current was applied for 60 seconds but was terminated immediately in the event of an impedance rise ≥10 Ω. Tip-electrode temperature and tissue temperature at depths of 3.5 and 7.0 mm were measured in all three groups (n=145). In 33 CV irrigation group applications, temperature was also measured with a separate probe at the center (n=18) or edge (n=15) of the electrode-tissue interface. In all 31 CV group applications, radiofrequency energy delivery was terminated prematurely (at 11.6±4.8 seconds) owing to an impedance rise associated with an electrode temperature of 98.8±2.1°C. All 39 temperature-control applications were delivered for 60 seconds without an impedance rise, but voltage had to be reduced to 38.4±6.1 V to avoid temperatures >90°C (mean tip-electrode temperature, 84.5±1.4°C). In CV irrigation applications, the tip-electrode temperature was not >48°C (mean, 38.4±5.1°C) and the electrode-tissue interface temperature was not >80°C (mean, 69.4±5.7°C). An abrupt impedance rise with an audible pop and without coagulum occurred in 6 of 75 CV irrigation group applications at 30 to 51 seconds, probably owing to release of steam from below the surface. In the CV and temperature-control group applications, the temperatures at depths of 3.5 (62.1±15.1°C and 67.9±7.5°C) and 7.0 mm (40.3±5.3°C and 48.3±4.8°C) were always lower than the electrode temperature. Conversely, in CV irrigation group applications, electrode and electrode-tissue interface temperatures were consistently exceeded by the tissue temperature at depths of 3.5 mm (94.7±9.1°C) and occasionally 7.0 mm (65.1±9.7°C). Lesion dimensions were smallest in CV group applications (depth, 4.7±0.6 mm; maximal diameter, 9.8±0.8 mm; volume, 135±33 mm³), intermediate in temperature-control group applications (depth, 6.1±0.5 mm; maximal diameter, 11.3±0.9 mm; volume, 275±55 mm³), and largest in CV irrigation group applications (depth, 9.9±1.1 mm; maximal diameter, 14.3±1.5 mm; volume, 700±217 mm³; P<.01, respectively). Conclusions Saline irrigation maintains a low electrode-tissue interface temperature during radiofrequency application at high power, which prevents an impedance rise and produces deeper and larger lesions. A higher temperature in the tissue (3.5 mm deep) than at the electrode-tissue interface indicates that direct resistive heating occurred deeper in the tissue (rather than by conduction of heat from the surface).