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

Background It is thought that only a thin layer of tissue adjacent to the electrode is heated directly by electrical current (resistive heat ing) during radiofrequency ablation. Most of the thermal injury is tho ught to result from conduction of heat from the surface layer. The pur pose of this study was to determine whether lesion depth could be incr eased 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 w hen the electrode-tissue interface temperature reaches 100 degrees C. Methods and Results In 11 anesthetized dogs, the thigh muscle was expo sed and bathed with heparinized canine blood (36 degrees C to 37 degre es C). A 7F catheter, with a central lumen, a 5-mm tip electrode with six irrigation holes, and an internal thermistor, was positioned perpe ndicular to the thigh muscle and held at a constant contact weight of 10 g. Radiofrequency current was delivered to 145 sites (1) at high co nstant voltage (66 V) without irrigation (CV group, n=31), (2) at vari able voltage (20 to 66 V) to maintain tip-electrode temperature at 80 degrees C to 90 degrees C without irrigation (temperature-control grou p, 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 gro up, n=75). Radiofrequency current was applied for 60 seconds but was t erminated immediately in the event of an impedance rise greater than o r equal to 10 Ohm. Tip-electrode temperature and tissue temperature at depths of 3.5 and 7.0 mm were measured in all three groups (n=145). I n 33 CV irrigation group applications, temperature was also measured w ith a separate probe at the center (n=18) or edge (n=15) of the electr ode-tissue interface. In all 31 CV group applications, radiofrequency energy delivery was terminated prematurely (at 11.6+/-4.8 seconds) owi ng to an impedance rise associated with an electrode temperature of 98 .8+/-2.1 degrees C. All 39 temperature-control applications were deliv ered for 60 seconds without an impedance rise, but voltage had to be r educed to 38.4+/-6.1 V to avoid temperatures >90 degrees C (mean tip-e lectrode temperature, 84.5+/-1.4 degrees C). In CV irrigation applicat ions, the tip-electrode temperature was not >48 degrees C (mean, 38.4/-5.1 degrees C) and the electrode-tissue interface temperature was no t >80 degrees C (mean, 69.4+/-5.7 degrees C). An abrupt impedance rise with an audible pop and without coagulum occurred in 6 of 75 CV irrig ation group applications at 30 to 51 seconds, probably owing to releas e of steam from below the surface. In the CV and temperature-control g roup applications, the temperatures at depths of 3.5 (62.1+/-15.1 degr ees C and 67.9+/-7.5 degrees C) and 7.0 mm (40.3+/-5.3 degrees C and 4 8.3+/-4.8 degrees C) were always lower than the electrode temperature. Conversely, in CV irrigation group applications, electrode and electr ode-tissue interface temperatures were consistently exceeded by the ti ssue temperature at depths of 3.5 mm (94.7+/-9.1 degrees C) and occasi onally 7.0 mm (65.1+/-9.7 degrees 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 mm(3)), intermediate in temperature-control group applications (depth, 6.1+/-0.5 mm; maximal diameter, 11.3+/-0.9 mm; volume, 275+/-55 mm(3)), and largest in CV irrigation group applic ations (depth, 9.9+/-1.1 mm; maximal diameter, 14.3+/-1.5 mm; volume, 700+/-217 mma; P<.01, respectively). Conclusions Saline irrigation mai ntains a low electrode-tissue interface temperature during radiofreque ncy application at high power, which prevents an impedance rise and pr oduces deeper and larger lesions. A higher temperature in the tissue ( 3.5 mm deep) than at the electrode-tissue interface indicates that dir ect resistive heating occurred deeper in the tissue (rather than by co nduction of heat from the surface).