Characteristics and mode of action of radiofrequency lesions

Abstract

When making a radiofrequency (RF) lesion the temperature at the tip of the electrode is commonly measured, but the temperature gradients away from the electrode tip are determined by a number of variables. A computer model is described that calculates the influence of these variables in a plane perpendicular to the electrode.The computer model indicates that the duration of the initial phase of rising tip temperature has little influence on the equilibration time and no influence on the ultimate size of the lesion. Variations in the conductivity to heat influence the size of the lesion but not the equilibration time.Variations in the vascularity of the surrounding tissue influence both the size of the lesion and the equilibration time.Variations in the diameter of the electrode are an important determinant of the size of the lesion.The model could also calculate the effects of a dishomogeneous zone away from the electrode. It was found that a fall in conductivity, an increased impedance, and a decrease in vascularity in such a zone each have an (additive) effect.The effect is most marked when such a zone is at a distance of 3 to 5 mm away from the electrode tip.While making an RF lesion the tissue that surrounds the electrode tip is also exposed to a strong RF electric field.To examine the clinical effects of this exposure, RF lesions were made with a tip temperature not exceeding 42° C.This can be achieved either by limiting the generator output or by applying RF with a pulsed time cycle of 2 x 20 msecond/second, in which case the silent phase allows for the circulation and the conductivity to heat or eliminate the heat that has been produced during the active phase. It turns out that pulsed RF has a definite clinical effect while continuous RF with a low generator output does not.The clinical effect of RF lesions may therefore be entirely or in part due to the electric field. Pulsed RF merits further investigation since the method has the following advantages: it is a painless procedure, since RF is far outside the physiologic range, so that depolarization does not take place; a neuritis-like reaction such as often follows a conventional RF lesion does not occur; there are no side effects; and as far as could be observed clinically it is a nondestructive method.