Department of Anaesthesia, Royal Free Hospital, London, UK
High-frequency electric current has been in use for a long time for treating pain. The nerves are coagulated by passing a high-frequency alternating current (100,000–500,000 Hz) so that pain signal transmission is blocked.
51.1 Radiofrequency Ablation
Radiofrequency is applied through an electrode which is most of the time a Sluijter Mehta kit (SMK) system. It is a 22G disposable cannula with a fine probe inside. The distal non-insulated part is called the active tip. There is formation of heat at the electrode tip which heats up the tissues in the immediate vicinity. Radiofrequency generator system creates a voltage gradient between the active electrode and ground plate. This leads to generation of current which creates an electric force on ions. Movement of ions produces heat which is dissipated because of conduction and blood circulation (heat washout). It is expressed in watts/degree Celsius. The heat formation is greatest at the site where current density is largest. The heat propagates parallel to the active tip and not ahead of the tip. Electrode ideally should be placed parallel to the nerve for maximum efficacy. A temperature greater than 80° is dangerous and temperature should be increased gradually at the rate of 1 °C/s. The target is to achieve a current of 200 mA. The growth of the lesion is maximum at 90 s. The procedure coagulates nervous tissue and creates a barrier to the transmission of nociceptive impulses. The most common side effects include numbness, hyperaesthesia, infection, haematoma and allergy.