Radiofrequency ablation uses monopolar alternating electric current delivered directly into the target tissue, where the native impedance of the tissue leads to heat generation. Where heating is sufficient, lethal temperatures are produced and the tissue is ablated. Specifically, a radiofrequency electrode is positioned into the target tissue and a grounding pad placed on the body. A computer-controlled generator applies alternating electrical current with a frequency within the radio segment of the electromagnetic spectrum. This current flows from the probe to the grounding pad. The radiofrequency voltage creates an electric field that exerts a force on the ions within the tissue fluid adjacent to the electrode causing them to vibrate as the current alternates polarity. Frictional dissipation of this ionic current causes heating of the tissues as an inverse function of tissue impedance. Heat is not directly supplied by the probe itself (19). The heating of tissue decreases with distance from the probe (heat = length/radius) and relies on the thermal conductance properties of the treated tissue (20,21). The treatment zone is that area that achieves temperatures sufficient for cell death by either of these two mechanisms. Overall, heat distribution in the tissue surrounding the probe is a function of tissue impedance, native tissue temperature, thermal conductivity, and heat loss through the circulation.
The exact mechanism of cell death caused by radiofrequency ablation is not completely understood. Most investigators believe thermal effects are the source of tissue injury, although the direct effect of the electrical field on the tissues is unknown (22). Heating of tissues to temperatures greater than 60°C routinely leads to desiccation and coagulative necrosis (23). Exposure to high temperatures also has direct effects on cellular components including the cell membrane, cytoskeleton, and nucleus. At supraphysiologic temperatures fluidity of the cell membrane increases, altering the kinetics of membrane proteins. These temperatures also denature the proteins that make up the cytoskeleton effecting cellular architecture. In the nucleus, high temperatures significantly impair deoxyribonucleic acid replication, in addition, cellular metabolism and electrophysiology are adversely affected by the high temperatures created during ablation (22). radiofre-quency ablation also has a direct effect on tissue perfusion by causing microvascular and arteriolar occlusion within the ablated zone leading to ischemia of treated tissues (24). The overall effect is a predictable zone of necrosis surrounding the radiofrequency probe.
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