Experimental relationship of radiofrequency electrical current and lesion size for application to percutaneous cordotomy

✓ Using the brains of anesthetized dogs, a study was carried out to evaluate the role played by differences in electrode size and inaccuracies in electrode placement during percutaneous cordotomy. It was determined that if the bare, active-electrode actually is entirely inside neural tissue, an effective lesion can be created with a minimum of current. For example, by using an electrode 3 mm in length and 0.4 mm in diameter, a radiofrequency current level of 50 mA (1.3 W) for 60 sec will create a lesion 5.5 mm long and 5 mm wide. If this electrode is properly positioned within neural tissue, it is impossible to achieve current levels of 75 mA or more, for the power density becomes so high that the local tissue is boiled and fried, which creates a gaseous and/or organic insulation around the electrode causing the current to drop to zero. The same phenomenon occurs at lower current levels if a smaller electrode is used due to the higher power density caused by putting the current through a smaller electrode-tissue interface. If all or any part of the bare, active electrode is outside neural tissue and in contact with cerebrospinal fluid, up to 100 to 300 mA of current may be needed to effect an adequate lesion since most of the current supplied is then shunted proportionately away from the high impedance neural tissue and into the lower impedance CSF. The heat spreading to the leptomeninges probably accounts for the severe neck pain experienced.