Principles of Electrosurgery

Chapter 10 Principles of Electrosurgery



Petros Benias, David L. Carr-Locke


Electrosurgery harnesses electricity with the intent of creating various thermal effects such as resection, incision, hemostasis, and devitalization of target tissues. The therapeutic basis of all electrosurgery is the production of thermal energy at the cellular level, typically as a result of a high-frequency alternating current created by an electrosurgery generator or unit (ESU).


Heat generated by this process is the result of resistance or impedance to flow of electricity within the tissue. The current must alternate (i.e., change direction between positive and negative) at a frequency of more than 100,000 times per second (100,000 Hz) to avoid the neuromuscular responses and shocks that occur with 60 Hz household current. However, the process is not electrocautery, as this is a misnomer referring merely to the ability to “burn” with electricity. Electrosurgery provides both cutting and coagulation, making it the ideal technology for producing therapeutic coagulation, resection, and tissue ablation throughout the gut. When the current density is sufficient within the targeted tissue, cellular water is rapidly heated, resulting in boiling and bursting of cellular membranes. When this energy is directed along a blade or wire, the result is electrosurgical cutting. At lower current densities, a less intense reaction results in tissue coagulation and desiccation without cutting.1–4


Electrosurgery has had widespread use in multiple endoscopic applications such as polypectomy, hemostasis, and tissue resection. The advent of flexible duodenoscopes and miniaturized electrosurgical tools allowed electrosurgical applications to be applied to endoscopic retrograde cholangiopancreatography (ERCP), permitting sphincterotomy, tumor ablation, and intracorporeal stone destruction. Present and future applications require a thorough understanding of electrosurgery.



A Brief History of Electrosurgery and ERCP


Electrosurgery was introduced in Europe in 1923 by ERBE Elektromedizin GmbH and in the United States in 1926 by William Bovie and Harvey Cushing. In the 1960s and 1970s, ESUs became a mainstay in medical care, but, without formal education regarding their use, many physicians experienced the catastrophic potential of an inadequately understood technology. Return pad and alternate site burns were not uncommon. While burns cannot ever be totally eliminated when using ESUs, the current isolated systems work with safety systems in the generator to help prevent such injuries. They also have preprogrammed modes and microprocessors allowing for intelligent control of the current.4


Electrosurgical technologies were first introduced to the field of ERCP in 1974 when Kawai and Classen independently published case series of endoscopic sphincterotomy with successful stone extraction. Classen described the use of “a special high-frequency diathermy knife,” essentially a miniaturized electrosurgical tool with cutting properties. The field was young but the benefits of endoscopy with electrosurgical potential were immediate.5


ESUs have become more complex but also more intelligent and arguably safer. For years it was difficult to account for all of the electrical variables and achieve consistently reproducible results. However, the introduction of regulated electrosurgery in the 1980s by the ERBE Company (ERBE Elektromedizin GmbH, Tuebingen, Germany) was a significant advance. Modern ESUs continuously monitor current and voltage, calculate parameters such as power and tissue resistance from these data, and analyze these findings in milliseconds. Depending on the desired effect, these parameters are kept constant or modulated by the ESU. Electrosurgery therefore has become widespread and safe in its current form. However, the potential for danger is still present and arises from a poor understanding of the technology, especially when the desired tissue effect is not achieved.2,3



Basics of Electricity as Applied to Electrosurgery



Basics of Electricity


Basic laws of physics govern the behavior of electricity and, as such, its behavior is predictable. There are four variables that can be used to describe a circuit and that are entirely interdependent: resistance (R), voltage (V), current (I), and power (P). In its simplest form a circuit must include a power source, a resistive element, and a path for the flow of current. Electrical current is defined as the flow of electrons, as measured in amperes or amps, through a circuit in response to an applied electromotive force termed voltage. Resistance or impedance represents the obstacle to the flow of current and is measured in ohms. The flow of current through a conductor is governed by Ohm’s law, which ties together current (I), voltage (V), and resistance (R):



image



It states simply that current increases as voltage increases for a constant resistance and that current decreases as resistance increases for a constant voltage. The relationship is predictable. Another simple relationship is represented by:



image



where P is the power generated in a circuit. Power is the rate of transfer of energy and is measured in watts. The ability of a current to do work is a result of the energy potential stored in a circuit, which is then dissipated at specific points, usually at the site of a resistor. In our human circuit, the tissue acts as the resistor and the power used is dissipated as thermal energy. The rise in temperature is governed by Joule’s law:



image



where Q is the heat generated by a constant current (I) flowing through a conductor of electrical resistance (R) for a time (t). When electrosurgery is applied to a tissue, the effect—whether it is cutting or coagulation—depends directly on Q.1



The Electrosurgical Unit


In an endoscopic circuit, the electrosurgical generator serves as a voltage source. The active electrode, such as a sphincterotome, conducts electrons to the patient. The patient acts as a resistive element. Electrons then return via the patient return electrode to the ESU. The power setting on the electrosurgical generator allows control of the power it supplies. This power is a representation of the amount of work the circuit will do at the point of contact. As noted above, since the power is set as a constant and the resistance is inherent to the human tissue, the generator will intelligently try to control the current and the voltage accordingly.3,6


Electrosurgery uses high-frequency alternating current, which may alternate polarity or direction up to 500,000 times a second. The cutting and coagulation effects that are desired in electrosurgery occur when the frequency is in the lower radio frequency (RF) range, 300,000 to 1 million Hz. Modern ESUs contain microprocessors that not only control the frequency, voltage, and current but also are able to calculate impedance of the tissue in contact with the electrode. These ESUs have at least one selection that attempts to hold power constant as closely as possible to the selected watts over a broad range of impedances. As tissue desiccates and fulgurates, impedance increases. An ESU that can dynamically adjust for changing impedance within a tissue can also control for unwanted effects. For example, constant and consistent power during polypectomy helps to reduce snare entrapment as the snare begins to close and the current density increases. In sphincterotomy, as the wire shortens and the area of tissue contact may diminish, constant power allows for a controlled cut rather than a “zipper cut.”


In addition, modern ESUs are “isolated” and keep current flow within the contained circuit, attempting always to capture the current through the return plate. If the circuit is broken, no current will flow at any point within the system. An isolated ESU has a transformer that causes the current to return only to the generator and not use alternate pathways to return to its source. If this is not possible, the generator will shut down. An isolated ESU prevents alternate site burns but not patient return electrode burns.



Monopolar versus Bipolar Circuits


Generators typically use one of two types of circuit: monopolar or bipolar. Monopolar circuits use the body between the active electrode and the grounding pad to complete the circuit back to the ESU. Bipolar circuits are complete within the electrosurgical tool itself by containing both electrodes in close proximity. Both monopolar and bipolar circuits have specific uses and advantages in endoscopy.


In monopolar circuits, the return plate, dispersive pad, grounding pad, or neutral electrode is essential because it collects the electrosurgical energy from the patient and returns it safely to the generator. Without a return plate there is no circuit and the electrosurgical device will do no work. Additionally, the return plate, which is situated externally on the patient’s skin, becomes an active part of the circuit, which in the past created potential for return site burns. The energy returned, however, is of low current density, minimizing or eliminating this effect, but the potential still remains if the plate is poorly sited.


The benefit of a monopolar device is the ability to achieve high levels of thermal effect with the versatility of being able to cut and coagulate. Examples of the monopolar mode in endoscopy are the polypectomy snare, sphincterotome, needle knife, and argon plasma coagulation. While the bipolar or multipolar mode does not require a grounding pad, the thermal effect is localized only to the tissue in direct contact with the target electrode. The advantage of this mode is the precise delivery of intense energy into a small space, such as electrohydraulic lithotripsy.


Both types of circuit are similar, in that their result depends directly on the current density achieved by the tool at the site of the targeted tissue. Current density is the result of several variables but in essence represents the density of energy within a given electrical field. Given a constant amount of energy being generated, as a sphincterotomy wire shortens or a snare closes, the density increases. Current density is lower when spread over a greater volume of tissue and the resulting effect will be slower heating. Energy spread over a ball tip or flat forceps jaw promotes coagulation by reducing the current density, as opposed to concentrating current along a snare or sphincterotome wire that promotes cutting.

Related posts:

Endoscopic Ultrasound–Assisted Access to the Pancreatic Duct Chronic Pancreatitis: Stones and Strictures Pancreatoscopy Access (Precut) Papillotomy Biliary Surgery Adverse Events Including Liver Transplantation ERCP for Acute and Chronic Adverse Events of Pancreatic Surgery and Pancreatic Trauma

Stay updated, free articles. Join our Telegram channel
Tags:
Mar 11, 2017 | Posted by in GASTROENTEROLOGY | Comments Off on Principles of Electrosurgery

Full access? Get Clinical Tree
Get Clinical Tree app for offline access