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 (

Fig. 7.1). At these higher frequencies, there is insufficient time for cellular depolarization to occur before the current alternates again and undesirable neuromuscular stimulation (electric shock) is therefore avoided. Since the frequencies employed in electrosurgery are in the range of amplitude-modulated radio broadcasts, the term radiofrequency (RF) current is also used.

      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 (

Fig. 7.2). Many of these variables are operator-dependent, such as selection of the ESU settings, type of accessory utilized, and technique and duration of application, so that the proportion of cells cut to those coagulated can be controlled to achieve the desired tissue effect.^2,3 An understanding of the basic electrosurgical properties and the interplay of these variables on the final tissue outcome are paramount for the safe and effective use of electrosurgery during endoscopy. Herein, the fundamental principles of electrosurgery and practical recommendations regarding utilization of electrosurgical devices for commonly performed procedures, such as polypectomy, sphincterotomy, and hemostasis, are highlighted.

      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 (

Table 7.1). The terms resistance and impedance are applicable to direct current and AC, respectively.

      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 (

Table 7.1). Through feedback tissue sensing, modern ESUs have outputs that are able to maintain power relatively constant over a range of measured impedances during current activation. These outputs with broad power-to-impedance curves are useful during certain procedures, such as polypectomy, where automatic