Principles of electrosurgery

electrosurgeryTrue electrosurgery, colloquially referred to as the “Bovie” (following its inventor, William T. Bovie, engineer and collaborator of Harvey Cushing), is perhaps the most ubiquitous power source in surgery. Although the principle of using heat to cauterize bleeding wounds dates back to the third millennium BC, the directed use of electrical current to produce these effects is a far more recent development. Other scientists and engineers made significant contributions to the development of this technology, but Bovie refined the electrical generator and made it practical and applicable to everyday surgery. At the most fundamental level, electrosurgery uses high-frequency (radiofrequency) electromagnetic waves to produce a localized heating of tissues, leading to localized tissue destruction. The effect produced (cutting vs. coagulation) depends on how this energy is supplied.

A useful exercise to understand the way electrosurgery works is to follow the flow of current from the power outlet as it travels through the patient, returns to the wall outlet. By convention, charge is depicted as moving from positive (cathode) to negative (anode) even though the particles that are actually moving are of course electrons, which have a negative charge. The following descriptions are based on that convention, following the flow of positive charge.

Monopolar Circuits

The electrosurgical circuit consists of four primary parts: the electrosurgical generator, the active electrode, the patient, the return electrode. Current flows from the electrosurgical generator after it is modulated to a high-frequency, short-wavelength current, and where multiple waveforms can be produced. The current flows from the machine, through the handpiece, out the tip of the device, to the patient. If the patient were not connected in some way either to a negative terminal or to ground, no current would flow, as there would be no way to complete the circuit, hence nowhere for the charge to go. However, the patient is always connected to the generator by a return electrode. This allows the charge delivered by the electrosurgical probe to pass through the patient, exerting its effect and back to the generator. In reality, the term “monopolar” circuit is incorrect, as there are in fact two poles. However, monopolar electrosurgery is distinguished from bipolar electrosurgery, in which both electrodes are under the surgeon’s direct control.

Bipolar Circuits

The essential components of the bipolar electrosurgical circuit are the same as those in the monopolar circuit; however, in this system, the active electrodes and return electrodes are in the same surgical instrument. In this technique, high-frequency current is passed through the active electrode, through the patient to heat and disrupt tissue. In this arrangement, however, the return electrode is included in the handpiece, as the opposite pole of the active electrode. This allows heating of only a discrete amount of tissue.

The Electromagnetic Spectrum & Tissue Effects

The current that powers the electrosurgical generator is supplied at a frequency of 60 Hz. This type of electromagnetic energy can indeed cause very strong (potentially lethal) neuromuscular stimulation, making it unsuitable for use in its pure form. Muscle and nerve stimulation, however, ceases at around 100000 cycles per second (100 kHz). Current with a frequency above this threshold can be delivered safely, without the risk of electrocution. The outputs of electrosurgical generators deliver current with a frequency greater than 200000 cycles per second (200 kHz). Current at this frequency is known as radiofrequency (RF); it is in the same portion of the spectrum as some radio transmitters. This level of RF, released from a radio antenna, is capable of producing serious RF burns if the proper precautions are not taken.

Applying electrosurgical current to a patient produces localized tissue destruction via intense heat production, yet, barring a mishap, no other lesions are produced during application of this technique. The reason that the effect is exerted only at the site where the surgeon is operating — and not at the site of the return electrode — is that the surface area by which the charge is delivered is much smaller than that to which it returns. Thus, there is a far greater density of charge at the site of the handpiece (“active” electrode) contact than there is at the site of return. If there is another connection between the patient and ground, and if it also comprises a relatively small surface area, the patient could be in danger of suffering an electrosurgical burn, if this pathway offers less resistance to the flow of current. The possibility of a burn at the site of the return electrode is eliminated in most modern machines by the presence of a monitoring system. The monitoring system assesses the completeness of contact (by maintaining a smaller, secondary circuit) and automatically disables power if full contact of the pad (such as could be caused by tripping over a wire and tearing the return pad) is lost.

Types of Electrosurgery

All types of electrosurgery exert their effects via the localized production of heat and the resultant effects on the tissues subjected to it. Therefore, the different effects produced by electrosurgical instruments are created by altering the manner in which this heat is produced and delivered. Adjusting this heat is made possible by altering the wave pattern by which the current is delivered.

Cutting

Cutting depends on the production of a continuous sine wave of current. Compared with coagulation current, cutting current has a relatively low voltage. It also has a relatively high crest factor, which is the ratio of the peak voltage to the mean (root-mean-square) voltage of the current. Additionally, it has a relatively high “duty cycle”—that is, once the current is applied, the current is actively flowing during the entire application.

The tip of the electrode is held just slightly off the surface of the tissue. The flow of the high-frequency current through the resistance of the tissue at a very small site produces intense heat, vaporizing water, exploding the cells in the immediate vicinity of the current. Thus, cutting occurs with minimal coagulum production, and consequentially, minimal hemostasis. A combination of coagulation and cutting can be produced by setting the electrosurgical generator to “blend,” which damps down a portion of the waveform, allowing greater formation of a coagulum and consequently more control of local bleeding.

Coagulation

In contrast with cutting currents, coagulation currents do not produce a constant waveform. Rather, they rely on spikes of electric wave activity. Although these currents produce less heat overall than the direct sine wave, enough heat is produced to disrupt the normal cellular architecture. Because the cells are not instantly vaporized, however, the cellular debris remains associated with the edge of the wound, the heat produced is enough to denature the cellular protein. This accounts for the formation of a coagulum, a protein-rich mixture that allows sealing of smaller blood vessels and control of local bleeding. Compared to cutting, coagulation currents have a higher crest factor and a shorter duty cycle (94% off, 6% on). In part, the increased voltage is necessary to overcome the impedance of air during the process of arcing current to the tissues.

Coagulation can be accomplished by using desiccation or fulguration.

Desiccation

With desiccation the conductive tip is placed in direct contact with the tissue.

Fulguration

In fulguration, the tip of the active electrode is not actually brought into contact with the tissues, but rather is held just off the surface and, following activation, the current arcs through the air to the target.

Disadvantages & Potential Hazards

Alternate Site Burns

Early electrosurgical generators used a ground referenced circuit design. In this type of construction, grounded current from the wall outlet was directly modulated, it was assumed that it would return to the generator via the return electrode. With this type of system, however, any path of low resistance to ground can complete the circuit, including metal instruments, ECG leads, and other wire and conductive surfaces. This situation presented a relatively high hazard for alternate site burns when current was not distributed over a great enough area to dissipate the current.

Modern electrosurgical units use isolated generator technology. The isolated generator separates the therapeutic current from ground by referencing it within the generator circuitry. In an isolated electrosurgical system, the circuit is completed by the generator, and electrosurgical current from isolated generators will not recognize grounded objects as pathways to complete the circuit. Isolated electrosurgical energy recognizes the patient return electrode as the preferred pathway back to the generator. Since the ground is not the reference for completion of the circuit, the potential for alternate site burns is greatly reduced.

Surgical Fires

In any setting with high heat sources and an ample supply of oxygen, vigilance against combustion is essential. Potential risks for ignition include drapes, gowns, gas (particularly in bowel surgery or cases involving the upper airway), and hair. Careful application of electrosurgery, use of a protective holster to store the electrode while not in use are important to minimize these risks.

Minimally Invasive Surgery

Several safety concerns are unique to minimally invasive surgery, given the limited and relatively tight environment in which operations occur. One potential danger is that of direct coupling between the electrode and other conductive instruments, leading to inadvertent tissue damage. Additionally, with the use of high-voltage currents (especially those used for coagulation), there is a risk of breakdown in the insulation, resulting in arcing from an exposed conductor to adjacent tissue that may lead to unwanted tissue damage. The likelihood of such damage can be reduced by using cutting current to lower the voltage used.

One unique hazard is the potential for creating a capacitor with the cannulae used. A capacitor is any conductor separated from another conductor by a dielectric. The conductive electrode separated from either a metal cannula or the abdominal wall (both good conductors) can induce capacitance in either of these structures.

Principal Applications for Electrosurgery

Electrosurgery is ubiquitous in its presence within the modern operating room. In its earliest use by Dr. Cushing, it allowed surgery on previously inoperable vascular tumors in neurosurgery. Today, electrosurgery is an essential component of all types of surgery. Applications include dissection in all types of general and vascular surgery, allowing tissue to be resected with minimal blood loss. Additionally, use in urology facilitates transurethral resection of the prostate and other procedures. In gynecologic practice, electrosurgical instruments are essential in cervical resections and biopsies.

Argon Beam Coagulation

Argon beam coagulation is closely related to basic application of electrosurgery. Argon beam coagulation uses a coaxial flow of argon gas to conduct monopolar RF current to the target tissue. Argon is an inert gas that is easily ionized by the application of an electrical current. When ionized, argon gas becomes far more conductive (less impedance) than normal air and provides a more efficient pathway for transmitting current from the electrode to tissues.

The current arcs along the pathway of the ionized gas, which is heavier than both oxygen and nitrogen, and thereby displaces air. Whereas current can sometimes follow unpredictable pathways while arcing through the air, the argon gas allows more accurate placement of current flow. Once the current arrives at the tissue, it produces its coagulating effect in the same manner as conventional electrosurgery. Argon beam coagulation devices can operate only in two modes: pinpoint coagulation and spray coagulation. They do not cut even the most delicate tissue.

Advantages of electrosurgery

There are multiple advantages to this type of electrosurgical current delivery. First, it allows use of the coagulation mode without contact of the electrode. This prevents buildup of eschar, which diminishes electrode efficiency, on the electrode tip. Second, there is generally decreased smoke and a reduced odor from coagulating with this type of current. Third, there is decreased tissue loss and reduced tissue damage when the current is more accurately targeted. Fourth, because the argon gas is delivered at room temperature, there is less of a danger of the instrument igniting gowns or drapes. Finally, the beam of coagulation generally improves coagulation and reduces blood loss, the risk of rebleeding.

Disadvantages of electrosurgery

One disadvantage of this type of electrosurgery is that it cannot be used to produce a cutting effect as other types of electrosurgical equipment. Second, the nozzle for gas delivery can become clogged, reducing its efficiency, and just as with other electrosurgical instruments.

Applications

Argon beam coagulation is especially useful for procedures in which the doctor needs to rapidly and efficiently coagulate a wide area of tissue. It is especially suited to dissecting very vascular tissues and organs, such as the liver. Its efficiency at delivering a consistent current load and its inability to become occluded with eschar are advantageous when there is a risk of hemorrhage.

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