Gastroenterological Endoscopy. Группа авторов
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7 Electrosurgical Principles for Endoscopy
Louis M. Wong Kee Song and Michael B. Wallace
7.1 Introduction
Electrosurgery is an integral part of many therapeutic applications in gastrointestinal endoscopy. Electrosurgical units generate high-frequency alternating current (AC), which connects to accessories, such as polypectomy snares and sphincterotomes, for the purpose of cutting and/or coagulation.1 An electrosurgical unit (ESU) converts low-frequency AC from a household electrical outlet (60 Hz in North America; 1 hertz [Hz] = 1 cycle per second) to > 300 KHz (
The amount and rate of heat produced at the cellular level through passage of RF current through tissue determine the end result. A high current concentration or density (measure of current applied per unit area) delivered by an electrosurgical knife or wire rapidly boils and vaporizes cells along the cleavage line, resulting in electrosurgical cutting. At low current density, tissue coagulation occurs since cells are heated more slowly and desiccate without any cutting. Several variables affect the current density and, consequently, the final tissue outcome (
The term cautery is often erroneously used during electrosurgery.4,5 An example of a cautery device is the Heat Probe Unit (HPU-20, Olympus Corp., Tokyo, Japan), which uses an electrically heated probe that is then applied to tissue for coagulation (hemostasis) without any cutting (Video 7.1). The procedure is similar to hot iron branding and, unlike electrosurgery, there is no passage of electrical current through tissue.
7.2 Electrosurgical Principles
7.2.1 Electrical and Tissue Variables
The three interacting electrical properties of current (I), voltage (V), and resistance or impedance (R) affect temperature rise in tissue and are governed by Ohm’s law (
Impedance is influenced by the water and electrolyte content of tissue. Tissues with high water content, such as blood vessels, pose less resistance to current flow than dehydrated tissues, such as bone and fat. Consequently, a lipomatous lesion is more difficult to transect with a snare relative to a nonfatty polyp at similar electrosurgical settings. Fibrosis and scarring also increase tissue impedance, which may necessitate adjustments in power and/or current waveform to achieve the desired effect. The buildup of charred tissue at the tip of a hemostatic probe or RF ablation catheter impedes current flow, hence the need to clean the electrodes intermittently during the procedure for efficient contact coagulation.
Fig. 7.1 (a) Household alternating current versus (b) high-frequency (radiofrequency) alternating current. Vp is peak voltage amplitude. Vpp is peak-to-peak voltage amplitude.
Fig. 7.2 Factors that impact the current density and final tissue outcome.
Table 7.1 Electrosurgical variables and equations
| Variables | ||
| Variable | Unit | Definition |
| Current (I) | Ampere (A) | Flow of electric charge (electrons) in a circuit per unit time |
| Voltage (V) | Volt (V) | Force that pushes an electric charge through resistance along the circuit |
| Resistance or impedance (R) | Ohm (Ω) | Measure of impediment to current flow |
| Power (P) | Watt (W) | Work, or amount of energy per unit time |
| Energy (Q) | Joule (J) | Capacity to do work |
| Equations | ||
| Ohm’s law | I = V / RV = I × RR = V / I | |
| Power | P = V × IP = V2 / RP = I2 × R | |
| Energy | P × t (s) | |
Voltage is the force that drives current through tissue. Voltage spikes must be greater than 200 peak volts (Vp) in order to generate adequate current density for electrosurgical cutting. Below 200 Vp, only tissue coagulation occurs regardless of the power setting. ESU outputs that maintain voltage constantly below the 200-Vp threshold are typical of bipolar applications for hemostasis, as well as the monopolar soft coagulation mode found in certain ESUs.
Current density ultimately determines the end result at the treated site and impacts the design or selection of the active electrode to suit a specific clinical purpose. Current density is lower when the RF energy extends over a greater volume of tissue, resulting in slower heating. Hence, the energy spread over the jaws of a flat hemostatic forceps (e.g., Coagrasper, Olympus Corp., Tokyo, Japan) or probe (e.g., TouchSoft, Genii Inc., St. Paul, MN) promotes coagulation as opposed to the high current density delivered along the thin wire of a sphincterotome that promotes cutting. Moreover, the operator can affect the current density by controlling the contact area between tissue and the active electrode.
Another operator-dependent variable is the power setting, which in turn impacts current density. As tissue heats, impedance rises, which decreases current flow and, consequently, power (