RADIOFREQUENCY TISSUE ABLATION - IMPORTANCE OF LOCAL TEMPERATURE ALONG THE ELECTRODE TIP EXPOSURE IN DETERMINING LESION SHAPE AND SIZE

Rationale and Objectives. We determined whether heat distribution alon g a radiofrequency (RF) electrode would be uniform when longer tip exp osures are used and whether local temperature effects would influence the shape of induced tissue coagulation. Methods. Thermistors were emb edded within 18-gauge RF electrodes at both ends and in the middle of the exposed tip, The length of tip exposure p varied from 1 to 7 cm, R F was applied in vitro to pig liver for G rlin using a constant tip te mperature, which was varied in 10 degrees C increments from 60 degrees C to 110 degrees C. Experiments were performed in triplicate, The 3- and 5-cm probes were used at a 30 degrees C tip temperature to create lesions in live pig liver and muscle using similar parameters. Tempera ture was measured throughout the procedure. Observable coagulation nec rosis was measured at the end of the treatment, Regression analysis wa s used to evaluate the local temperature-lesion diameter relationship. Results. Temperatures were not uniform along the tip exposure for any given trial, Temperature variation increased with higher tip temperat ures and longer tip exposures. The diameter of local coagulation necro sis was a function of the local mean temperature. For in vitro trials, no coagulation was seen when the local temperature was less than 50 d egrees C. Temperatures above this threshold resulted in progressively greater lesion diameter, with a minimum of 1 cm Of necrosis occurring at 71 degrees C. Additional increases in lesion diameter (1.4-1.6 cm) were observed at approximately 90 degrees C. Mathematical modeling dem onstrated a best-fit curve: lesion diameter (in cm) = (1.4 + 0.03 (tip exposure)] {1 - e([-0.067(local temp - 49.5 degrees C)])}, r(2) = .98 6, SD = 0.14 cm for each curve. In living tissue, less uniformity in t he shape of coagulation necrosis was seen around the electrodes, Local temperature-lesion diameter data fit the same logarithmic relation, b ut the threshold for coagulation necrosis was 8.5 degrees C higher tha n for in vitro specimens. Conclusion. Using a single-probe technique f or RF-induced tissue necrosis, the diameter of tissue coagulation may be predicted by the local temperature along tile exposed electrode. Th e uniformity of temperature decreases with increased tip exposures. Th is effect may be partially corrected by creating lesions at higher tip temperatures, where necrosis diameter is increased, Because effects a re more pronounced in vivo, uniform volumes of tissue necrosis are lim ited to tip exposures of 3 cm or less.