by Scott Rohlf
The safe use of electrosurgical energy has been demonstrated in open laparotomy over the last 50 years. However, the potential problems associated with the laparoscopic use of electrosurgical energy are not well appreciated. Passing electrosurgical energy through cannulas and long insulated active electrodes significantly changes the physics surrounding the use of high frequency electrosurgical energy.
Monopolar electrosurgical energy can be safely used during laparoscopic procedures. It is a highly effective and versatile power source. However, if patient injuries are to be avoided, clinicians must be aware of the unique environment posed by minimally invasive procedures. The potential hazards presented by this environment can be largely avoided through an understanding of recommended practices.
The potential problems documented during minimally invasive surgery (MIS) procedures include insulation failure, direct coupling of current, and capacitively coupled energy. Each situation will be discussed from the perspective of how a problem may occur and how to avoid it.
Insulation failure can occur from damage to the insulation such as breaks or holes caused by reprocessing or damage created during use. Insulation failure can also occur during surgery through the use of high voltage coagulation waveform. Breaks in the insulation caused by trauma during use or reprocessing provide an alternate pathway for the current to leave the electrode as it completes the circuit to the patient return electrode. If, while the generator is being activated, the portion of the electrode with defective insulation encounters adjacent tissue, the current can complete the circuit by arcing from the electrode through the insulation break to adjacent tissue (Figure 1). If this point of exit is small, current density can be high enough to produce significant tissue damage. Often this problem can occur outside the surgeon’s field of vision. Consequently, this occurrence can go undetected.
Although rare, it is possible to create insulation failure during the procedure. In other words, insulation that was intact before the procedure may become compromised during the procedure. This is most likely to occur when the surgeon selects the coagulation waveform, especially if the insulation is already damaged in any way. The coagulation waveform was designed primarily to fulgurate. Fulguration allows the surgeon to “spray” coagulation waveform through the high impedance of air. To do this, the coagulation waveform has a very high voltage, sometimes more than 10,000 volts. It is important to remember that voltage is the “push” or “force” that drives electrons through the circuit. In a circuit, the high voltage coagulation waveform can push the current (blow holes) through the otherwise intact insulation. If this point of breakage is touching a small amount of tissue, significant current densities may be present. The result of high current density may be full thickness burns of the bowel or other vital structures.
Open circuit activation occurs when you activate the generator but no electrical activity is visible at the tip of the active electrode. The impedance between the active electrode and the target tissue is too great for the current to complete the circuit. Consequently, no electrical activity is observed. However, the generator is being activated. The generator is prepared to and is trying to complete the circuit. The high impedance created by the open circuit is interpreted by the generator as a need for maximum voltage. The generator is trying to provide enough voltage to push the current to the intended target tissue and thus complete the circuit. Maximum voltage is built up throughout the length of the active electrode. While in open circuit, if the insulation of the active electrode is touching tissue, the voltage may be high enough to blow a hole through the insulation. By doing so, the current has found a way to complete the circuit to the patient return electrode. Unfortunately, the surgeon has no way of controlling the current density of this exit point. If the hole in the insulation is small enough and the point of tissue contact is also small, current concentration may be high enough to do significant inadvertent damage.
How can we avoid insulation failure? It is imperative that the insulation be carefully inspected for small cracks and defects before use, even on new disposable electrodes. Use the cut waveform whenever possible. The cut waveform is significantly lower in voltage and can be used to coagulate when in the desiccation mode (direct electrode contact with tissue). Due to the lower voltages, the cutting waveform is less likely to blow holes in insulation. Last, but most importantly, do not activate the generator in open circuit. To make sure that the current is able to complete the circuit through the target tissue, the generator should not be activated until the electrode is in near proximity to or touching the target tissue. This will make “blowing holes” in insulation next to impossible, and, even when holes are present, it will be less likely that the current will find these defects to be the path of least resistance.
DIRECT COUPLING OF CURRENT
Direct coupling of current is defined as the unintended contact of the energized active electrode with another metal instrument or object within the surgical field (Figure 2). When this occurs, it may occur out of site of the telescope and is usually operator error. The surgeon should never activate the electrode while it is touching or in close proximity to another metal object.
Three potential problems arise from direct coupling of current. Metal to metal sparking can cause tremendous heat production. Sparking to metal clips for example could cause necrosis of underlying tissue. This could cause the clip to fall off the vessel as the tissue begins to slough. Metal to metal sparking can also cause frequency demodulation. In other words, high frequency current generated by the electrosurgical generator may become demodulated (lower frequency) through the action of metal to metal sparking. This demodulation can result in neuromuscular stimulation of surrounding muscles, which is noticed as muscular twitching.
Third, metal to metal sparking may cause current to flow to unintended sites. For example, if the electrode is activated while touching the laparoscope, the metal laparoscope may become energized. The energy will want to complete the circuit to the patient return electrode. If the current dissipates through the relatively large surface area between the laparoscope and the abdominal wall, there is little likelihood for damage. However, if the laparoscope is insulated from the abdominal wall by a plastic collar, the current will be forced to find another point of exit from the laparoscope to complete the circuit. If it finds a point of tissue contact with the laparoscope, the current can complete its circuit at that point of contact. Like insulation failure, the determining factor regarding the amount of tissue damage (if any) will be the amount of current density present. A large point of contact will have low current density and, therefore, no tissue damage would be expected. However, if the laparoscope is touching a small amount of tissue, high current density can occur and tissue damage is possible.
To avoid direct coupling, the surgeon should never activate the electrosurgical generator while the electrode is touching or in near proximity to another piece of metal.
CAPACITIVELY COUPLED CURRENT
The phenomenon of capacitively coupled currents is more difficult to understand. It is the inducement of current through the intact insulation of active electrodes to surrounding cannulas or instruments.
Note: The term induced/inducement technically relates to inductive coupling, not capacitive coupling. However, the term will be used in this paper for the purpose of teaching the concept of the transfer of current through a nonconductor.
What is capacitance? It is a stored electrical charge. Capacitors are defined as two electrical conductors separated by an insulator. During minimally invasive procedures incorporating the passage of electrosurgical energy into the abdomen, a capacitor can be created. The active electrode (a conductor) is surrounded by insulation (a nonconductor) which is often passed through a metal cannula (conductor). A capacitor has been created. A capacitor can induce an electrical current into the metal cannula through the process of capacitance. This capacitively coupled current wants to complete the circuit by finding a pathway to the patient return electrode. The electrical charge will be stored in the metal cannula until either the generator is deactivated or a pathway to complete the circuit presents itself. As we begin our discussion of capacitively coupled current, it is important to remember that capacitively coupled current is increased with a high voltage coagulation waveform, as opposed to a low voltage cutting waveform. Additionally, activating the generator in open circuit greatly increases the level of capacitance that will occur. Also, by isolating the outer conductor (metal cannula or instrument) from the abdominal wall by a plastic nonconductor, the possibility for injury is increased. The level of capacitance is increased when the cannula is small (i.e. 5 mm vs. 10 mm) and the electrode is long.
The amount of electrical energy capacitively coupled onto the metal cannula will be dependent on a number of variables--first, when the generator is activated in open circuit. This, as previously described, causes the generator to go to maximum voltage. The higher the voltage, the more current that will be produced. Conversely, capacitance will be minimal if the generator is activated in closed circuit. This means that the active electrode is touching or in near proximity to the target tissue. This will help ensure that the generator’s output will travel through the target tissue on its way to the patient return electrode. Voltages will remain lower and the amount of capacitance will likewise remain low.
The second variable is the waveform selected by the surgeon. Since the cut waveform uses significantly less voltage, the amount of capacitive current is less. The cutting waveform can be used for both vaporization (cutting) and desiccation. The high voltage coagulation waveform should be reserved for fulguration. When using the coagulation waveform, the surgeon should be careful not to activate the generator in open circuit.
The type of cannula/trocar system chosen also plays an important role in the amount of capacitance produced and whether this current can be safely dispersed. An all metal system is appropriate in that any capacitive currents will be safely dispersed through the greater surface area provided by the chest or abdominal wall, thereby reducing current density. In normal circumstances, this surface area will be adequate to safely dissipate any current buildup on the cannula without significant heat production or tissue damage.
An all plastic cannula system is also appropriate. When using an all plastic system the definition of a capacitor has been eliminated. Instead of two conductors separated by a nonconductor, there is now the conductive electrode, covered by nonconductive insulation, surrounded by the nonconductive cannula. A capacitor no longer exists and, therefore, concerns over the capacitively coupled current have also been eliminated.
The big problem with capacitance occurs when a metal cannula is held in place by a plastic anchor (hybrid trocar/cannula system). (This complies with the definition of a capacitor through the metal portion of the cannula.) However, the coupled current is not able to disperse safely through the chest or abdominal wall because the cannula is held in place by a nonconductive plastic anchor.
Consequently, the current that is coupled onto the metal cannula can only complete the circuit by discharging to tissue that it may encounter within the cavity (Figure 3). The unknown variable is the amount of tissue that is in proximity to the metal cannula. If it is large, it is unlikely that the discharging current will cause any damage due to the principle of low current density. However, a small contact area will create high current density and potentially a significant injury. Often this occurrence is outside the field of vision and, therefore, is not detected as a problem until several days postoperatively.
ACTIVELY ELECTRODE MONITORING (AEM)
Unfortunately, it is not always possible to completely avoid complications during MIS procedures. The best technique may still be rendered inadequate. For example, it is not humanly possible to detect small breaks in insulation prior to a procedure or to detect insulation that has been compromised during the procedure. Reducing the likelihood of capacitance through the use of the cut waveform is not always possible. There are times when the coagulation waveform is needed. Yet, to use it increases the chance of capacitance. Open circuit activation can be difficult to avoid considering the visual limitations presented by laparoscopic surgery.
To assist in the elimination of these risks, Active Electrode Monitoring (AEM) was developed. The technology constantly monitors and actively shields the active electrodes during use. This is accomplished through the use of an Electroshield® that is placed over the entire length of the active electrode. The Electroshield® is an electrically conductive metal tube surrounded by insulation through which most laparoscopic active accessories may be inserted. The Electroshield® is connected through the Active Electrode Monitor to the return electrode and harmlessly conducts stray currents away from the surgical site. The Active Electrode Monitor measures the currents flowing in the shield and detects faults in insulation. When insulation failure or capacitance is detected, the Active Electrode Monitor interrupts the contact quality circuit of the return electrode on the generator, thereby shutting the generator off. This deactivation of the generator happens before an injury can occur.
Recently, integrated electrodes have been introduced. Rather than relying on a separate shield through which the active electrode must be inserted, integrated electrodes have the shielding device incorporated into the assembly of the instrument. Integrated electrodes do not require the use of a separate outer shield. The advantage is that the integrated electrodes can be used with 5 mm cannulas. Conventional electrodes that require use of a separate shield must be used with 8 mm cannulas.
Advantages of Active Electrode Monitoring (AEM) include the following:
-Continuously shields against accidental burns
caused by capacitive coupling.
- Actively monitors for insulation failure.
- Deactivates the generator when stray currents are
detected. Visual and audible alarms alert the
O.R. staff that a fault has occurred.
- Integrated electrodes allow for the use of 5 mm
- Compatible with most electrosurgical generators
with a contact quality circuit.
In review, monopolar electrosurgery is a commonly used power source during MIS procedures. This is due to its effectiveness, versatility, and relative safety. The surgeon using electrosurgery can feel confident of its safety if he/she will follow the guidelines listed below:
- Inspect all insulation for defects.
- Use the lowest power setting that achieves the
desired results. Lower power settings (for both
cut and coag) reduce the likelihood of insulation
failure, capacitance and injury.
- Use low voltage waveform whenever appropri-
ate. The cutting waveform uses far lower volt-
ages and thereby reduces the likelihood of “blow-
ing” holes in insulation. Low voltage waveforms
also reduce the amount of capacitively coupled
energy that can be produced.
- Use brief activation vs. prolonged activation.
- Avoid metal to metal sparking (direct coupling).
Do not activate the electrode until it is touching
or in close proximity to the target tissue.
- Avoid open circuit activation. Activation when
the electrode is touching or in close proximity to
the target tissue helps ensure that the target tissue
will be the path of least resistance. Capacitance
will be minimal during closed circuit activation
and even with defective insulation the current will
most likely find the target tissue to be the path-
way of choice.
- Use bipolar electrosurgery when appropriate.
- Never use a hybrid trocar/cannula system. An all
metal cannula is the safest choice in the operative
channel. All metal systems will allow for safe dis-
persal of capacitively coupled current through the
chest or abdominal wall.
- Consider Active Electrode Monitoring (AEM) tech-
nology in order to help eliminate concerns over
insulation failure and capacitively coupled currents.
By following these recommended practices, the surgeon practicing minimally invasive procedures can safely utilize electrosurgical energy for effective cutting and hemostasis of tissue.