Jeffrey D. Mosko, MD and Gyanprakash A. Ketwaroo, MD, MSc
Endoscopic resection has come a long way since the advent of injection-assisted endoscopic mucosal resection (EMR) in the 1950s.1 With improved technique has come the ability to treat and cure many precancerous and cancerous lesions of the gastrointestinal (GI) tract in a minimally invasive manner. Such innovation could not have been possible without the simultaneous evolution of the electrosurgical generator unit (ESU), which provides the high-frequency, alternating electrical current necessary to perform these procedures. ESUs, now with advanced microprocessors that facilitate the creation of multiple waveforms, use endoscopic accessories to deliver electrical current through a circuit to a specified target. The result is electrosurgical cutting and/or coagulation. This is in direct contrast to electrocautery whereby an electrode applies heat when it comes in contact with tissue.
GI endoscopy remains one of the more challenging applications of the ESU. In addition to working in the narrow lumens of hollow organs, endoscopic resection has steadily moved deeper in an attempt to achieve en bloc resection in a safe and effective manner. To achieve all of this, it is imperative to have a full understanding of the principles of electrosurgery and the devices used to deliver energy to our therapeutic targets.
Current (I, measured in amps) is defined as the flow of electrons through the electrical circuit. A complete circuit involves the flow of electrons from a positive/active electrode through the target tissue to a negative/return electrode.2 Impedance (R, measured in ohms) is a measure of any tissue’s opposition to electron flow. This resistance to current results in the production of heat. Voltage (V, measured in volts) represents the electrical force required to push current through varying levels of tissue resistance in the circuit. Ohm’s law describes the relationship between the above variables and helps predict how the ESU will interact with tissues (I = V/R or V = I × R). Voltage peak is an important variable determining the desired end effect on tissue, particularly the extent of thermal injury.
Endoscopic devices utilized with any ESU can be divided into monopolar and bipolar based on the type of electrical circuit they complete. In monopolar circuits, current travels from the ESU through an active electrode (endoscopic device) into the target tissue and back through the return/neutral electrode, the path of least resistance, to the ESU (Figure 12-1). Pads, often incorrectly referred to as grounding pads, are used as the dispersive electrode. They are optimally placed facing the operating field on the patient’s flank (a well-vascularized area [not the buttock]), thereby creating the shortest circuit possible. Examples of monopolar devices include snares used for polypectomy and EMR, knives used for endoscopic submucosal dissection (ESD), sphincterotomes used for sphincterotomy during endoscopic retrograde cholangiopancreatography, and coagulation graspers/forceps used for hemostasis. Bipolar circuits do not require dispersive electrodes (“grounding” pads) as both the active and return electrodes are contained within the working tip of the device. Current travels from the ESU to the active electrode, into the target tissue, back into the neutral electrode, and then back to the ESU (see Figure 12-1). An example of a bipolar device is the bipolar probe used for hemostasis.
Understanding the concept of current density is integral to the safe and effective delivery of electrosurgical energy during endoscopic resection.3 It can be adjusted by the endoscopist during a procedure to achieve the desired tissue effect. Current density is the amount of current concentrated on any given area of tissue.3 It is higher when focused on a smaller area as in the cutting that occurs with a thin snare or sphincterotome wire. In contrast, current density is low when spread over a larger area like the coagulation occurring with the use of hemostatic forceps.
There are many variables affecting the desired tissue effect during endoscopic resection. Those of particular importance include voltage, power, current density, tissue impedance, amount of tissue targeted, electrode size/type, waveform used, and endoscopist technique4 (Figure 12-2). Some of these variables are within the control of the operator while some are not. The resistance/impedance of any tissue is directly proportional to its water content. As tissue is progressively desiccated from electrosurgical energy, its resistance increases. Submucosal fibrosis, often the result of prior biopsies and/or attempted resection, also has high tissue resistance, therefore (remember I = V/R) requiring higher voltage to ensure a desired cutting effect.
|Bovie Medical Corporation||Bovie Generator|
|US Endoscopy, Inc||Gi4000|
Electrosurgical Generator Unit Types
As a crucial tool for endoscopic intervention, the electrosurgical unit has undergone dramatic advancement. Until recently, endoscopists were required to select a desired power output. Conventional ESUs would then deliver that power regardless of the changing properties of the surrounding tissue (power control). This resulted in significant fluctuations in voltage thereby varying tissue effects.5 Newer-generation ESUs contain microprocessors that are able to detect changes in the voltage delivered. Even as a circuit’s tissue impedance changes, the ESU keeps the voltage constant while varying the current (voltage control). This results in a more consistent target tissue effect. Such control is essential for resections requiring high levels of precision. Table 12-1 lists many of the current ESU manufacturers and units available. The most widely used in endoscopy units performing advanced endoscopic resection, based on the variety of output modes available and microprocessor control, are the Erbe VIO and Olympus ESG systems.