Gastroenterological Endoscopy. Группа авторов

by providing adequate power during the entire resection process (

Fig. 7.3). On the other hand, narrow power-to-impedance curves are characteristic of bipolar or monopolar outputs designed for contact hemostasis, with peak power delivered between 30 and 250 ohms (Ω) (

Fig. 7.4). Tissues with resistances within this range, such as blood vessels, are targeted to receive the maximum power output that optimizes coagulation and, as tissue impedance increases with ongoing heating, the power output falls significantly so that the tissue does not overly desiccate and cause the accessory to stick. Although the manufacturer’s suggested power settings for specific procedures are a good starting point for a particular ESU, adjustments can be made based on the operator’s preference, technique, and desired outcome.

      One variable that is entirely under the control of the operator is duration (time) of current application, which can impact the final outcome to a large degree. Taking into consideration that the total amount of energy (J) delivered equals power (W) ×time (s), how fast or slow the power is being deposited becomes important. The outcome of delivering 50 W of power for 2 seconds is quite different than that of delivering 20 W for 5 seconds, even though the total amount of heat energy delivered (100 J) is the same. Furthermore, if the time of application and power settings remain the same, either a continuous or modulated waveform will deliver an identical amount of total energy but with a very different outcome (see the section Electrosurgical Units and Waveforms).

      Fig. 7.3 Broad power-to-resistance curves suitable for snare polypectomy.

      Fig. 7.4 Narrow power-to-resistance curves suitable for bipolar hemostasis.

      7.2.2 Monopolar versus Bipolar Circuit

      The terms monopolar and bipolar refer to the manner in which the electrosurgical circuit is completed. In a monopolar circuit, RF current oscillates from the active electrode (e.g., polypectomy snare) through the patient’s body via the path of least resistance to the inactive or dispersive electrode (electrosurgical pad), and returns to the ESU to complete the circuit (

Fig. 7.5). The term dispersive electrode conveys an understanding that the energy exiting the patient through the pad is so much less concentrated that the risk of a burn injury at the pad site is reduced. Placement of the dispersive electrode close to the target site is recommended to keep the circuit as short as possible.

      In a bipolar circuit, the active and return electrodes are in close proximity to each other, as illustrated by the bipolar hemostatic probe (e.g., Gold Probe, Boston Scientific Inc., Natick, MA) (

Fig. 7.6). Current travels from the active to return electrodes through only a small volume of tissue in contact with the tip of the probe. An electrosurgical pad is not required for bipolar accessories. Except for hemostasis applications, bipolar devices are far less commercially available for endoscopic use than their monopolar counterparts.

      Fig. 7.5 Schematic of monopolar circuit. Current flows from the active electrode (e.g., snare) through the patient’s body to the dispersive electrode (pad) placed on the patient’s skin.

      Fig. 7.6 Schematic of bipolar circuit. Bipolar hemostasis probe with active and return electrodes closely spaced at the probe’s tip (inset).

      7.3 Electrosurgical Units and Waveforms

      ESUs enable the user to select from a menu of current waveforms a particular output, such as “Cut,” “Coag,” and “Blend.” An understanding of the waveform selected for a given ESU is important since manufacturer’s labeling of the outputs is not standardized and can be misleading (

Table 7.2). For instance, an output labeled “Coag” with voltage spikes above 200 V is quite capable of electrosurgical cutting.

      ESUs produce current outputs ranging from low-voltage, continuous sinusoidal waves to interrupted (modulated) waveforms with much higher voltages (

Fig. 7.7). When voltage is maintained below 200 Vp, continuous waveforms produce superficial contact coagulation without electrosurgical cutting. These coagulation current outputs can be recognized by particular labels, such as “Soft Coag” or “TouchSoft” when used with monopolar devices and “BiCap” or “Bipolar” when used with bipolar accessories. With voltages between 200 and 600 Vp, continuous waveforms promote maximum cutting effects since they can rapidly produce high current densities along the active electrode to vaporize (cut) cells. Cells that are situated further away from the active electrode do not heat fast enough to vaporize, so they coagulate. Thus, even if a waveform is named “Pure Cut,” some coagulation will always be present along the margins of the cut. Although higher voltages deepen the coagulation spread along the cut margin, tissue charring becomes problematic when voltage is increased above 600 Vp for these continuous waves.

      Fig. 7.7 Schematic of current waveforms (see text for details). (Repoduced with permission from Macmillan publishers Ltd: Am J Gastroenterol, copyright 2009.³)

      Modulated or interrupted waveforms heat tissue more slowly than continuous waves and are designed for outputs intended to produce varying amounts of coagulation, ablation, and hemostasis. By interrupting the current flow, tissue has a chance to cool, and the proportion of cells that desiccate without bursting increases. These modulated waveforms are recognized by particular labels, such as “Blend,” “Forced Coag,” and “Coag.” Peak voltages for modulated waveforms range from well over 200 Vp to 4,500 Vp, and even higher for noncontact fulguration modes. High-voltage modulated waveforms are used to ionize argon gas during noncontact argon plasma coagulation (APC).

      One should not assume that a specific label, such as “Blend,” from one ESU corresponds to the same waveform output from another ESU with the same name. When reporting ESU settings in research studies and publications, quantitative descriptions of the waveforms based on the duty cycle and/or crest factor should be used to enable comparison and categorization of the qualitative waveform labels from different ESUs (

Table 7.3). The duty cycle relates the percentage of time that the current is actually on during