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“Electrosurgery Basics” Download the PDF version: While it is not necessary to be an electrical engineer to perform laparoscopic or hysteroscopic surgery with radiofrequency (RF) energy, it is important to know some basic tenets of electricity as it applies to surgical instruments. Basic Principles of Electricity Electrosurgery is performed using RF alternating current (AC), derived from a wall source and modified by the electrosurgical generator or unit (ESU) to an output with a frequency of about 500 Kilohertz, in the range provided by am radio. Whereas all RF electrosurgery requires two poles interacting with the patient (and is therefore "bipolar"), surgical instrumentation is designed as being either monopolar (unipolar) or bipolar. Bipolar instruments possess both electrodes and only the tissue interposed between the two electrodes is included in the circuit. In contrast, monopolar instruments comprise only the "active" electrode, while the inactive or dispersive electrode is located remotely with the entire patient interposed between the two electrodes (Figure 1). Alternating currents result from a source of alternating polarity and therefore do not "flow" like direct currents so concepts such as "return" electrode do not apply. Effect of RF Current on Cells and Tissue How does RF electricity cause a tissue effect? The tissue effects of RF current are caused by an increase in intracellular temperature. With the application of RF current, the rapidly alternating polarity introduced across the cell causes intracellular cations and anions to rapidly oscillate. This mechanical energy is converted by friction into thermal energy thereby elevating intracellular temperature (Figure 2). If the intracellular temperature reaches about 60° to 90°C, both cellular dehydration and protein denaturation occur. When the tissue cools, molecular bonds reform in a random fashion turning the impacted tissues into a homogenous, undifferentiated mass. If, instead, the cytoplasmic temperature quickly reaches or exceeds 100°C, the intracellular water boils, and the subsequent formation of gas (steam) and intracellular expansion results in explosive vaporization of the cell. What determines the amount of power transferred to the cells and tissue? Power density is expressed in watts/cm² (Figure 3) and is largely determined by the shape and size of the electrode. A very small electrode, such as a needle shaped device, or the edge of a blade, will concentrate current so that the power density is high, allowing for the rapid elevation of cellular temperature and the creation of a narrow zone of vaporization. Given the same power, a wider, larger electrode, in contact with tissue, will dilute the power density preventing elevation of intracellular temperature to the boiling point, instead causing coagulation and desiccation. At the extreme, a very large electrode prevents cellular and tissue heating altogether—this is the principle around the dispersive electrode. What are the tissue effects that can be created with RF electricity? Cutting and Vaporization. Vaporization of tissue is best achieved with a continuous, low voltage ("cutting") current, using a pointed or thin edged (e.g., a blade) monopolar electrode held near to but not in contact with the tissue, thereby creating a zone of high power density (Figure 2). When vaporization occurs, the ionic products create a localized cloud of low impedance, sometimes called the steam envelope, within which the surgeon should seek to maintain the electrode tip. Electrosurgical cutting is really a process of linear vaporization whereby the electrode is advanced in the desired direction, always maintaining the tip or edge within the continuously propagated steam envelope. To move too quickly results in contact with the tissue, a resulting reduction in the power density, and consequent propensity to coagulate and not vaporize. Properly performed, collateral damage to adjacent cells is minimal. Desiccation and Coagulation. Low voltage outputs (usually labeled cutting and blend) are preferred to high voltage modulated outputs, usually labeled coagulation (Figure 3). There exist a number of potential reasons for this apparent paradox. First, the highly modulated (interrupted) nature of the high voltage outputs contributes to uneven protein bonding that may prevent, for example, complete and secure occlusion of a blood vessel. In addition, such current may cause the more superficial layers of the tissue to become rapidly coagulated, increasing impedance, thereby inhibiting further transmission of current to the deeper layers. Finally, the effect near the electrode causes rapid heating and adherence of tissue, allowing the eschar to be pulled off with removal of the electrode, a process that facilitates bleeding. To perform coaptive coagulation of a vessel, it is necessary to have effective vascular compression, usually with forceps or a clamp, thereby preventing the heat sink effect of flowing blood, and allowing the broken molecular bonds to reform across the vessel lumen thereby creating a strong tissue seal. In addition, low voltage current is desirable to foster the creation of a homogenous seal of the walls of the target blood vessel, not achieved with the modulated, high voltage "coagulation" waveform. Fulguration. This non-contact technique results in superficial coagulation of tissue and is used to treat oozing of narrow caliber blood vessels. The "coagulation" output is selected, set relatively high (80 watts) and the relatively large surface area electrode (e.g., ball shape) is held millimeters away from the tissue, but close enough to allow the discharge to traverse the gap between electrode and target (Figure 3). Following activation, the "spraying" coagulates and carbonizes tissue and raises tissue impedance, thereby preventing the current from continuing to heat the deeper layers of the tissue, limiting the depth to about 0.5 mm. Another way that laparoscopic fulguration may be achieved is via the Argon Beam Coagulator which is, in essence, an RF device that works on the principle of fulguration propagated by argon gas to allow a greater distance between electrode and tissue. Electrosurgical Complications. Complications of electrosurgery involve inadvertent tissue injury and result from inadvertent or excessive active electrode use, or from diversion of the current secondary to insulation failure, direct coupling (shorting) or capacitative coupling. Capacitative coupling is a particular concern at endoscopic surgery and may be minimized by using low voltage waveforms (minimizing use of the "coagulation" current) and avoiding mixing of metal access cannulas and plastic anchoring systems. _____________________ Reprinted with permission from NewsScope. Copyright 2006 by the AAGL, Advancing Minimally Invasive Gynecology Worldwide, www.aagl.org. |
Electrosurgery Basics
