Electrosurgery in Therapeutic Endoscopy

Variables That Impact Tissue Effect, Microprocessor Controls, and Waveforms

Current density is the defining variable in determining specific tissue effects in electrosurgery; yet it is the sum of the effects of all the other variables (see Fig. 6.1 ).

Current density is the measure of current concentration or, by definition, the current per unit area. The rate of heat generation, and therefore the resulting therapeutic effect, is a function of the current density. It is a measure of intensity. Mathematically, the temperature rises as a square of the current density. Current that will boil water on a square millimeter area will not even feel warm on a square-centimeter area. The dramatic difference in surface area between the active electrode and the dispersive (grounding) pad is perhaps the best-known example of this principle. Current density depends on the applied voltage, current, and type of waveform, as well as the tissue impedance, the size of the electrode, and the time that current is flowing. Understanding the different characteristics of tissue heating under different circumstances is essential to understanding electrosurgery.

The physical law that relates all the electrosurgical variables is summarized by Ohm in the equation:

where P is power, I is current, and R is resistance (for our purposes, resistance and impedance are used synonymously). The P = V x I derivation of this equation relates the type of waveform to total power ( P ). Power combines both voltage ( V ) and current ( I ). For example, if power is to remain constant as voltage increases, then current must decrease. Ohm's Law defines another principle crucial to a clinical understanding of electrosurgery: as impedance in tissue rises, power (either current or voltage or both) decreases. If “nothing is done” as the therapy produces coagulation, and therefore the tissue impedance rises, the generator power must drop (stall).

To address this problem, research turned in the late 1980s to using microprocessors to measure and respond to changing impedance. First patented in 1986, microprocessor monitoring allows an ESU to track a resistance baseline and subsequent changes in the resistance during the electrosurgical activation. Using millions of data points, the generator's software programming (algorithm) tells the generator how to regulate the current and/or voltage (which together form power) being delivered during the activation. A graphic representation of how a particular output is designed to react to changes in impedance is called a Power to Impedance Curve or Power Curve . Sometimes marketers will use the informal term power dosing to explain this. Power curves associated with each output selection are so important to understanding the operation of the ESU that regulatory bodies require these graphs to be included in every user manual.

Power to impedance curves tend to be either “narrow” or “broad.” By convention, power is shown on the vertical axis with increasing resistance progressing to the right on the horizontal axis ( Fig. 6.2 ). The horizontal axis is also likely to correlate with time as tissue resistance increases when tissue becomes coagulated with increasing time of application.

The power curve dictates that the power setting displayed on an ESU is not the power that will be delivered over the entire activation. Depending on the algorithm defining the curve, the power setting displayed may be a more or less close approximation of the actual or average power delivered, or simply a target maximum power. In all cases, the average power delivered will be less than the maximum or peak power delivered.

Outputs with narrow type power curves have been long utilized in flexible endoscopy for contact coagulation using very familiar bipolar endostasis probes. Matched with low-voltage continuous outputs and a narrow power curve, this bipolar method is a frequent and effective choice for hemostasis and control of bleeding.

In Fig. 6.2 , 25 watts have been selected by the user and will be displayed on the ESU. To the ESU's algorithm, this selected power represents only the maximum (or target) power to be delivered. The ESU is programmed to ramp up quickly in the low-resistance situation characterized by frank bleeding. The target maximum is reached, but the power drops quickly as soon as the resistance reaches approximately 100 to 300 Ohms, which correlates well with the appearance of the characteristic whitish eschar, indicating adequate coagulation. This power curve delivers exactly what is desired: rapid onset of hemostasis with self-limiting depth of injury. With the low voltage of the output and this power curve, the tissue tends not to overly dry out, which would cause the bipolar accessory to stick. This well-matched output mode, power curve, and appropriate accessory provides the desired clinical effect of reliable and quick hemostasis.

A nearly identical narrow power curve and voltage cap, matched with monopolar accessories, is usually named SoftCoag (Erbe) or TouchSoft (Genii). These soft coagulation outputs are suggested for use with monopolar hot biopsy forceps (HBF) or newer monopolar contact coagulation accessories such as the Olympus Coagrasper or the Genii TouchSoft Coagulator. One study described successful use of a snare tip with a soft coagulation output for touch-ups during the resection of large colonic lesions. These monopolar counterparts do require the use of a return dispersive electrode (grounding pad) and require higher initial power settings to overcome the higher resistance in the monopolar circuit. TouchSoft or soft coagulation power maximums are usually set at 40, 50, or 60 watts versus the 15 to 25 watts that are very common for bipolar contact applications.

For snare polypectomy, sphincterotomy, or other procedures that depend on a smooth advance of an electrode, the narrow power curve would not be suitable, as stalling would result. Instead, a broad power curve ( Fig. 6.3 ) where cutting with various levels of concurrent coagulation (depending upon the waveform and accessory selected) is the choice. These outputs usually ramp more slowly toward an effective power and maintain the effectiveness over a broad range of impedances. The result is a smooth and reliable transection. Again, in all cases the average power delivered over the activation will be less than the maximum or peak power. A marketing term called constant power is sometimes applied to broad curves but is mostly inaccurate. The power delivered is dynamically driven by the algorithm of the ESU as it responds to impedance/resistance measures. Rigid terms such as constant , always , and never are to be avoided in scientific and medical use for good reason. It is similarly not correct to refer to one entire ESU as a constant power generator . Every ESU in current use today has some outputs of both curve types included in its available selections. It is the physician's good understanding of the available outputs and their associated power curves that allow matching to suitable accessories to produce desired and optimal clinical results.

Broad power curves are ideal for most applications that require at least some cutting because the transection is done smoothly without stalling. The generator overcomes the Ohm's law directed drop in power by automatically adjusting the voltage and/or current to keep the cut progressing smoothly over a wide (broad) range of impedances. For ESUs intended for flexible endoscopy, it is now very common to also provide an additional microprocessor or software control layer over some of the broad curve output choices in order to interrupt or fractionate the cut. Named EndoCut (Erbe), Pulsed Cut (Genii), Pulsed Blend Cut (Genii), etc., these types of outputs have multiple options for adding more or less coagulation to the cutting advance. Multiple studies have shown a sole but important benefit of the pulsing or interruption of the advance: a significant reduction in uncontrolled or “zipper” cuts in sphincterotomy.

Microprocessor control on modern generators is very precise in reading dynamic impedance and holding current and/or voltage (power) changes tightly to the algorithm design demanded by the power curve. For this reason, often the “watts” shown on the generator display may be absent or only an indication of a maximum power target. In any case, and with every power curve, the average power delivered over the total activation will be less than the peak power delivered. Often the generator requires a certain lag time to reach the programmed maximum power, may on occasion even slightly overshoot it, and will ramp down from the peak depending on the impedance. It is customary for record keepers to chart only the maximum selected power shown on the generator display.

As noted, the controlling concept of the final tissue effect is the current density at the treatment site. But many variables affect the current density. Some are under the physician's control, and some are not. Only a few are functions of the ESU (see Fig. 6.1 ).

A defining variable that is a selection on the electrosurgery generator is the high-frequency waveform, or output mode. In solid-state generators (the only type now produced), continuous sinusoidal waveforms with fairly low voltage increasingly give way to a higher-voltage waveform that is more and more interrupted (modulated) to take the tissue effect from one of mostly cutting to one of mostly coagulation ( Fig. 6.4 ).

A continuous high-frequency waveform with a peak voltage of at least 200Vp will produce an intensity of current sufficient to create micro electric sparks between the active electrode and the target tissue. Such high-current density along the leading edge of the electrode causes cells to literally explode, separating the tissue as if it were cut. As this cell vaporization continues, a micro steam layer is formed, which helps to propagate the cutting effect.

Along the edges of the cut, there will always be a margin of cells whose distance from the active electrode allows them to heat more slowly. These cells simply coagulate. This is why it is often said, “Only a cold cut is a pure cut.” It is better to use the term cut rather than pure cut , but the literature does not yet reflect this new standard. The depth of the coagulated margin along the cut is related to the height of the peak voltage in the cutting waveform, with higher voltages leaving a thicker margin of coagulation. The coagulation margin is also influenced by the thickness of the electrode. A thin wire leaves less coagulation than a flat blade or a thicker endoscopic dissection wire or knife.

Even with a continuous waveform, if the voltage never goes above 200Vp, no cutting can occur. Such outputs lack the intensity to initiate the initial sparking necessary to produce the cutting effect. A superficial coagulation results instead. These waveforms have been described earlier in the discussion about bipolar and monopolar soft coagulation output choices with narrow power curves.

To produce deeper thermal injury with less electrosurgical cutting and an increasingly greater proportion of coagulated cells, the continuous cut waveform is interrupted or modulated. Modulating the waveform delivers the energy, even at equivalent power settings, more slowly. To increase the depth of the coagulation, the voltage spikes are increased. This is necessary because along the margins of the electrode path, the impedance is rising as the tissue is coagulated. The higher voltage spikes force the current through the desiccated layer along the cut margin, increasing the depth of the coagulation.

The modulated output that for a particular generator provides the deepest coagulation (hemostasis) with only some cutting may be named either coag or forced coag . Users often erroneously call these outputs pure coag . The term is erroneous because as long as the voltage spikes (at least sometimes) over 200Vp, some electrosurgical cutting is expected. This common but imprecise use of terminology can promote confusion. The most common coag waveform type (which often has approximately a 6% duty cycle and crest factor of approximately six) has been a first choice for polypectomy for decades; this is because the cutting action along the snare wire allows a clean transection, whereas the coagulation remaining promotes reliable hemostasis. With the advance of ESU technology offering a greater number of output choices on many ESUs, waveforms with a bit more cutting action and comparatively less thermal spread have gained nearly equal use. These waveforms may have names such as blend coag or swift coag . Robust, evidence-based data as to what waveform should be preferred is currently absent. Therefore, an optimal guideline for waveform choice is not available, leaving this to the physician's discretion, preference, and experience.

Many ESUs are equipped with a varying number of modulated waveforms that fall between the extremes of nearly all cut or nearly all coag. These are typically named the blended currents . This is perhaps the most descriptive name, as it correctly implies that some of the cells will be vaporized, or cut, whereas some will be coagulated. Increasingly modulated waveforms with increasingly high voltages will produce tissue effects with increasing hemostasis and decreasing cutting.

Methods for Quantifying Waveforms

The qualitative descriptions of waveforms can be quantified by assigning them a duty cycle or a crest factor. The duty cycle is the numerical percentage of the time the waveform is on (spiking) and the time it is off (interrupted). A waveform with a 6% duty cycle is on 6% of the time and off 94% of the total time. The common coag or forced coag types discussed previously are those with approximately a 6% duty cycle. Any continuous waveform has a 100% duty cycle. It will be the peak voltage that will determine if a 100% duty cycle waveform produces pure (intense) electrosurgical cutting or only soft coagulation. Crest factor is similarly used to quantify the degree to which a waveform is modulated, but is more informative for modulated waves in that it includes information about the peak voltage. Modulated waveforms with low duty cycles and high crest factors produce more and deeper coagulation than waveforms with low crest factors and high duty cycles ( Table 6.2 ). Either duty cycle or crest factor information can be found in every generator's user manual.

TABLE 6.2

Quantified Outputs for Several Commonly Used Generator Models

Data published or provided by the manufacturers: Conmed Corp., Utica, NY; EndoStat distributed by Boston Scientific Inc., Natick, MA; Erbe USA Inc., Marietta, GA; Genii Inc., St. Paul, MN; Meditron, a division of Cooper Surgical, Trumbull, CT; Valleylab, Medtronic, Dublin, Ireland. Not all generators listed may be currently marketed.

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*Beamer Polyp Pulse Cut 1,2 and Papilla Pulse Cut 1,2 CF not available.

This table lists duty cycles and/or crest factors to aid endoscopists in quantifying different electrosurgery generator unit (ESU) modes. Each ESU will have additional waveform properties that contribute to tissue effect. Please refer to the operator's manual for each ESU or contact the manufacturer for further information.

A physician with a good knowledge of the principles of current density, the waveform choices on the ESU at hand, and its microprocessor controlled power curves is in a very good position to control, avoid, or compensate for all the other non-ESU variables that enter into the final tissue effect (see Fig. 6.1 ).

One of these non-ESU variables is the operator's technique, which includes the tension applied to the accessory. Some degree of tissue tension is necessary for progression of an incision (e.g., polypectomy or sphincterotomy). However, overzealous tension may result in poor control of cutting, leading to a rapid incision with inadequate vessel coagulation and increased risk of hemorrhage. A slow advance can promote more coagulation. Highly skilled endoscopists performing ESD procedures are able to control small bleeding episodes simply by slowing the cut advance markedly. This holding staunches the bleeding with no change whatsoever in the accessory, the waveform output, or the power selection.

Resistance in an entire monopolar circuit or in the tissue itself varies before and during the incision. Highly fibrotic or scarred tissue is more resistant (and harder to cut) than tissue with a high water content. Tissue resistance is a dynamic variable that alters as tissue desiccates during the incision. The entire resistance between an argon probe tip and the dispersive electrode plays a role in the length of the plasma arc that is generated during argon coagulation procedures. Blood flow is a variable that may help dissipate heat at the site of the tissue. Similarly, submucosal saline injection may act as a “heat sink” and help dissipate energy and prevent transmural injury during polypectomy and other procedures.

Monopolar and Bipolar Circuits

The most common method for completing the electrical circuit in gastroenterology procedures is to use a monopolar accessory and a remotely placed dispersive (grounding) pad. The current flows from the ESU and is concentrated at the treatment site by the accessory (e.g., a snare, sphincterotome, HBF, or needle-knife). The concentrated energy at the site has high current density which promotes tissue heating. The energy then flows through the patient's tissue following a path of least resistance to the dispersive electrode on the skin and hence back to the generator, completing an electrical circuit ( Fig. 6.5 ). Because the surface area of the pad is so much larger than the active accessory, the current density is very low at the pad site. Proper pad placement is important to evenly disperse the energy and avoid any increase in skin temperature under the pad. Most modern ESUs have dispersive electrode safety alarms available when used in conjunction with “split” or “dual” pads. Patented in 1983, this technology advance is inexpensive and highly recommended, as it markedly reduces the risk of a patient skin burn under the pad.

Bipolar (multipolar) accessories are different from monopolar electrodes in that both active and return functions are included in the tip of the instrument. Current flows through a relatively small area of tissue, and there is no need for a dispersive electrode on the patient's skin ( Fig. 6.6 ). The most common flexible endoscopic use for this technology is for contact coagulation using a flexible bipolar hemostasis probe that is supplied by many manufacturers ( Fig. 6.7 ). Bipolar hemostasis probes have been shown to be effective devices for hemostasis for bleeding peptic ulcers, arteriovenous malformations, and other hemorrhagic lesions. Optimal results are obtained by using moderate to firm tamponade in the case of active bleeding. Power settings of 15 to 25 watts are most common. These probes are best used with generator outputs that have continuous low-voltage outputs paired with a narrow power curve in which the microprocessor control design allows the energy to ramp up quickly in the low-resistance case of frank blood, but to limit the power delivery automatically as the tissue becomes coagulated. Flexible endoscopic bipolar snares, biopsy forceps, and sphincterotomes have also been developed but have not been successfully or widely adopted.

Radio Frequency Ablation Devices

A newer, important use of bipolar current for thermal therapy is radiofrequency ablation employed to ablate Barrett's esophagus. In this situation, the current moves from one adjacent electrode on a balloon catheter to another. This energy passes via the mucosa and is sufficient to ablate the mucosa, while being superficial enough (< 1 mm depth of ablation) to prevent significant submucosal damage, and minimizing stricture formation.

Contact and Noncontact Applications

High-frequency electrosurgical energy can be applied to the tissue in either of two ways. The first and far most common method in flexible endoscopy is to have the accessory in direct contact with the surface of the tissue. A well-recognized list of common flexible endoscopic therapies and accessories employ this method: polypectomy and EMR/snares; sphincterotomy/sphincterotomes and needle-knives; contact coagulation/bipolar endostasis probes; coagraspers; and contact coagulators.

In the second way, by using especially high-voltage waveforms with or without the addition of argon (or other) gas to the treatment field, the energy can travel to the tissue surface without the accessory being in contact with the surface.