Effects of Heating with Radiofrequency Power on Myocardial Impulse Conduction: Is Radiofrequency Ablation Exclusively Thermally Mediated?

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Abstract
Although it is generally accepted that radiofrequency (RF) ablation causes exclusively thermally mediated effects, it has never been proved. In a previous report, temperatures required to induce conduction block in superfused canine epicardial ventricular myocardium were identified by exposure to heated superfusate: 50.3 degrees +/- 1.1 degrees C and 53.6 degrees +/- 0.6 degree C for transient and permanent block, respectively. In the present study, heating was performed using RF power in an otherwise identical model. Nine preparations from four dogs were used. A 1-cm diameter electrode was placed beneath the center of each preparation for RF delivery. Incisions were made to create a conductive isthmus over the ablation electrode. Preparations were paced to one side of the isthmus and electrograms recorded from the center of the isthmus and to either side. Temperature was measured using a miniature thermocouple located just below the epicardial surface, adjacent to the recording electrode in the heated zone. RF was delivered for 30 seconds at 5-minute intervals with increments in power per episode causing increments in temperature of approximately 2 degrees C. Temperature during pulses at which transient block occurred was 50.7 degrees +/- 3.0 degrees C; temperature at 30 seconds of heating in pulses leading to permanent block was 58.0 degrees +/- 3.4 degrees C. These findings provide evidence suggesting that the electrophysiologic effects of RF ablation are exclusively thermally mediated and are otherwise unrelated to the dissipation of high-frequency curren