Capacitance is the ability of a conductor to establish an electrical current in an unconnected nearby circuit. An electrical field is established around the shaft of any activated unipolar instrument (including the cord), a circumstance that creates a potential capacitor. This field is harmless if the circuit is completed through a dispersive, low-power density pathway (Fig. 23.21). For example, if capacitative coupling occurs between a monopolar laparoscopic instrument and a metal cannula positioned in the abdominal wall, the current is harmlessly dispersed in the abdominal wall at the point where it connects with the dispersive electrode (Fig. 23.21A). However, if the metal cannula is anchored to the skin by a nonconductive plastic retaining sleeve or “gripper” (a hybrid system), the current cannot access the abdominal wall because the sleeve acts as an insulator (Fig. 23.21B). Instead, the capacitor will have to “look” elsewhere to complete the circuit. The bowel or any other nearby conductor can become the target of a relatively high-power density discharge (Fig. 23.21C). This situation can occur when a unipolar instrument is inserted through an operating laparoscope that, in turn, is passed through a nonconductive plastic laparoscopic cannula. In this configuration, the plastic port acts as the insulator. If the electrode capacitively couples with the metal laparoscope, nearby bowel will be at risk for significant thermal injury (115). The circumstance occurs with the relatively new “single port” systems where the laparoscope and hand instruments, including monopolar instruments, are passed through the same concentrated array of ports. This situation is identical to that of the “operating” laparoscope with an increased risk that the laparoscope or other instruments function as capacitors.
The risk of capacitive coupling-related complications can be reduced in a number of ways. First, it is important to avoid the use of hybrid laparoscope-cannula systems that contain a mixture of conductive and nonconductive elements. Instead, the use of all-plastic or all-metal cannula systems is preferred. It may be best to avoid or at least minimize the use of monopolar instruments
Figure 23.20 Current diversion secondary to insulation defects and direct coupling. These events may occur with the use of monopolar instrumentation when there is a defect in the insulation (A) or, classically, to contact a conductive instrument that, in turn, touches other intraperitoneal structures. In the example depicted (B), the active electrode is touching the laparoscope, and current is transferred to bowel through a small enough contact point that thermal injury results. Another common target of such coupling is to noninsulated hand instruments.
using operating laparoscopes or multiport, single site access systems. If an operating laparoscope is to be used, all-metal cannula systems should be the rule unless there is no intent to use unipolar electrosurgical instruments through the operating channel. Finally, minimizing the use of high voltage, modulated current (“coagulation” current) will reduce the risk of capacitative coupling.
Modern electrosurgical units are designed with isolated circuits and impedance monitoring systems that shut down the machine if dispersive electrode (“patient pad”) detachment occurs. The use of isolated-circuit electrosurgical generators with dispersive electrode monitors has virtually eliminated dispersive electrode–related thermal injury. Dispersive electrode monitoring is accomplished by measuring the impedance in the dispersive electrode, which should always be low because of the large surface area. Without such devices, partial detachment of the dispersive
Figure 23.21 Capacitative coupling. A: All activated monopolar electrodes emit a surrounding charge, proportional to the voltage of the current. This makes the electrode a potential capacitor. B: Generally, as long as the charge is allowed to disperse through the abdominal wall, no sequelae result. However, if the “return” to the dispersive electrode is blocked by insulation, such as a plastic anchor (C), the current can couple to a conductive cannula or directly to bowel.