Principles of 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 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):

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 P = VI = I ² R, where P is the power generated in a circuit. Power is the transfer of energy and is measured in watts and rate per second as expressed in joules (watt-seconds). 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:

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.

The Electrosurgical Unit

In a typical monopolar endoscopic circuit, the electrosurgical generator serves as a voltage source. An active electrode, in this case a sphincterotome, conducts electrons to the patient. The patient acts as a resistive element. Electrons come back via the patient return electrode to the ESU. A power setting is present on the electrosurgical generator. This power is a representation of the amount of work the circuit will do at the point of contact. As noted above, because 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.

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 radiofrequency range of 300,000 to 1 million Hz. Modern ESUs contain microprocessors that not only control the frequency, voltage, and current but also calculate the 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 and 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 reduce against snare entrapment as the snare begins to close and the current density increases. During sphincterotomy, as the wire shortens, the area of tissue contact may diminish and impedance rises as tissue desiccates; 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 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 ( Fig. 11.1 ). 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, and neutral electrode are essential because they collect the electrosurgical energy from the patient and return 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 it can still potentially occur if the plate is poorly sited.

The benefit of a monopolar device is its ability to achieve high levels of thermal effect with the versatility to cut and coagulate. Examples of the monopolar mode in endoscopy are polypectomy snares, sphincterotomes, needle knifes, ESD tools, and argon plasma coagulation. Although the bipolar mode does not require a grounding pad, the thermal effect is localized only to the tissue in direct contact with the device electrode. The advantage of this mode is the precise delivery of intense energy into a small space, such as in a bipolar hemostasis probe or electrohydraulic lithotripsy fiber.

Both types of circuits 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 space. Given a constant amount of energy being generated, as when a sphincterotomy wire shortens or a snare closes, the current 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.