On mechanisms of interaction in electrosurgery

Electrosurgery is broadly used in a wide variety of surgical procedures, yet its underlying mechanisms of interaction are poorly characterized. Fundamentals of electrosurgery have not changed much since the 1930s—cutting is still performed using continuous RF waveforms, leaving a collateral damage zone of hundreds of micrometers in depth. Pulsed waveforms with variable duty cycle are used mostly for tissue coagulation. Recently, we have demonstrated that electrosurgery with microsecond bursts applied via microelectrodes can provide cellular precision in soft tissue dissection. This paper examines dynamics of pulsed electrical discharges in conductive medium, and accompanying phenomena, such as vaporization, cavitation and ionization. It is demonstrated that ionization of the vapor cavity around the electrode is essential for energy delivery beyond the vaporization threshold. It is also shown that the ionization threshold voltage and resistance of the plasma- mediated discharge are much lower in the negative phase of the discharge than in the positive one. Capacitive coupling of the ac waveform to the electrode compensates for this asymmetry by shifting the medium voltage on the electrode, thus increasing the positive and decreasing the negative amplitudes to achieve charge balance in the opposite phases. With planar insulated electrodes having exposed edges of 12.5µm in width and bursts of 40µs in duration even tough biological tissues can be dissected with cellular precision. For example, cartilage dissection is achieved with pulse energy of 2.2mJ per millimeter of length of the blade, and leaves a thermal damage zone of only 5-20µm in width.