Principles of Electricity
An electrical circuit is any pathway that allows the uninterrupted flow of electrons. Electrical current is the flow of electricity (the number of electrons) in a given circuit over a constant period of time and is measured in amperes (A). Current can be supplied either as direct current (DC) with constant positive and negative terminals or as alternating current (AC) with constantly reversing poles. The electromotive force, or voltage, is a measurement of the force that propels the current of electrons and is related to the difference in potential energy between two terminals. The resistance is the tendency of any component of a circuit to resist the flow of electrons and applies to DC circuits. The equivalent of this tendency in an AC circuit is known as impedance. Any electromagnetic wave, from household electricity to radio broadcasts to visible light, can be described by three components: speed, frequency, and wavelength. Because all electromagnetic waves travel at the speed of light, which is a constant, these waves depend on the relationship between their frequency and wavelength. Since these three characteristics are defined by the equation:
c = f λ (where c is the speed of light, 2.998 × 108 m/s)
frequency (f) and wavelength (λ) are inversely related; that is, as frequency increases, wavelength decreases, and vice versa. The ability to pass high-frequency current through the human body without causing excess damage makes electrosurgery possible.
Electrosurgery is often incorrectly termed electrocautery, which is a separate technique. Electrocautery is a closed-circuit DC device in which current is passed through an exposed wire offering resistance to the current (Figure 7–1). The resistance causes some of the electrical energy to be dissipated as heat, increasing the temperature of the wire, which then heats tissue. In true electrocautery, no current passes through the patient. Electrocautery is primarily applied for microsurgery, such as ophthalmologic procedures, where a very small amount of heat will produce the desired effect or where more heat or current may be dangerous.
In electrocautery, current passes through a wire loop and heats it. This heat cauterizes tissue. No current passes through the patient.
Principles of Electrosurgery
True 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. While 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. While other scientists and engineers made significant contributions to the development of this new technology, it was Bovie who 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 and returns to the wall outlet. By convention, charge is depicted as moving from positive (cathode) to negative (anode) despite that the particles that are actually moving are electrons, which have a negative charge. These descriptions are based on that convention, following the flow of positive charge.
The electrosurgical circuit consists of four primary parts: the electrosurgical generator, the active electrode, the patient, and 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 importance of the waveform is discussed in later sections.) 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 electrosurgical generator by a return electrode, which allows the charge delivered by the electrosurgical probe to pass through the patient, exerting its effect, and back to the generator, completing the circuit. In reality, the term monopolar circuit is incorrect, as there are in fact two poles (the active and return electrodes); it is distinguished from bipolar electrosurgery in which both electrodes are under the surgeon’s direct control (Figure 7–2A).
A. In monopolar electrosurgery, current from an electrosurgical generator passes from an active electrode (the “Bovie” tip) through the patient to a return electrode of greater area. B. In bipolar electrosurgery, the active and return electrodes are in the handpiece, and current only flows through the surgical site.
The essential components of the bipolar electrosurgical circuit are the same as those in the monopolar circuit; however, in this system, the active and return electrodes are in the same surgical instrument. In this technique, high-frequency current is passed through the active electrode and through the patient to heat and disrupt tissue. In this arrangement, however, the return electrode is in the handpiece, as the opposite pole of the active electrode. This method enables the surgeon to heat only a discrete amount of tissue (Figure 7–2B).
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 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 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, can produce 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 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 that offers less resistance to the flow of current, and if it also comprises a relatively small surface area, then the patient could be in danger of suffering an electrosurgical burn. Similarly, it is possible that if the return electrode were to be damaged, or if contact was not maintained, a burn could occur in this area. 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 that assesses the completeness of contact (by maintaining a smaller, secondary circuit) and automatically disables power if full contact of the pad is lost (as could be caused by tripping over a wire and tearing the return pad).
All types of electrosurgery exert their effects via the localized production of heat and the subsequent changes in the heated tissue. Therefore, the different effects produced by electrosurgical instruments are created by altering the manner in which this heat is produced and delivered. Adjustment is made possible by altering the wave pattern of the current.
Cutting depends on the production of a continuous sine wave of current (Figure 7–3A). Compared with coagulation current (discussed later), cutting current has a relatively low voltage and 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. In this technique, 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 patient’s tissue at a very small site produces intense heat, vaporizing water and exploding the cells in the immediate vicinity of the current. Thus, cutting occurs with minimal coagulum production and consequently 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.
Electrosurgical waveforms. A. Cutting current. B. Coagulation current.
B. Coagulation: Desiccation and Fulguration
In contrast with cutting currents, coagulation currents do not produce a constant waveform. Rather, they rely on spikes of electric wave activity (Figure 7–3B). 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, and 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 in one of two ways. With desiccation, the conductive tip is placed in direct contact with the tissue. Direct contact of the electrode with tissue reduces the concentration of the current; less heat is generated, and no cutting action occurs. A relatively low power setting is used, resulting in a limited area of tissue ablation with coagulation. Desiccation is achieved most efficiently with the cutting current. The cells dry out and form a coagulum rather than vaporize and explode.
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. Again, this process disrupts normal cellular protein to form a coagulum; the tissue is charred, and a black eschar forms at the site of operation. It is possible to cut with the coagulation current and, conversely, to coagulate with the cutting current by holding the electrode in direct contact with tissue. It may be necessary to adjust power settings and electrode size to achieve the desired surgical effect. The benefit of using the cutting current is that far less voltage is needed, an important consideration during minimally invasive procedures.
Just as the power setting and the waveform affect the results of the current application, any change in the circuit that influences the impedance of the system will influence the tissue effect. These include the size of the electrode, the position of the electrode, the type of tissue, and the formation of eschar.
The smaller the electrode, the higher the current concentration. Consequently, the same tissue effect can be achieved with a smaller electrode, even though the power setting is reduced. At any given setting, the longer the generator is activated, the more heat is produced. The greater the heat, the farther it will travel to adjacent tissue (thermal spread). (See various electrodes, Figure 7–4.)
A. Knife electrode. B. Ball electrode. C. Needle electrode. D. Loop electrode. E. Wire electrode.
Placement of the electrode
Placement can determine whether vaporization or coagulation occurs. Which one occurs is a function of current density and the heat produced while sparking to tissue versus holding the electrode in direct contact.
Tissues vary widely in resistance.
Eschar is relatively high in resistance to current. Electrodes should be kept clean and free of eschar, maintaining lower resistance within the surgical circuit.
D. Disadvantages and Potential Hazards
Early electrosurgical generators used a ground referenced circuit design. In this type of construction, grounded current from the wall outlet was directly modulated, and 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, including metal instruments, EKG leads, and other wire and conductive surfaces, can complete the circuit. The ground referenced circuit design 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. However, if the return electrode were to become partially disconnected, a burn could occur at the site of the return electrode if the area was too small to distribute the current widely enough to prevent heating of the tissue or if the impedance was too high. It is important to place the return electrode over a well-vascularized tissue mass, not over areas of vascular insufficiency or over bony prominences where contact might be compromised. Therefore, some electrosurgical generators use a monitoring system that assesses the quality of the contact between the return electrode and the patient by monitoring impedance, which is related to surface area. Any loss of contact between the electrode and the generator results in interruption of the circuit and deactivation of the system.
In any setting with high heat sources and an ample supply of oxygen, vigilance against combustion is essential. Drapes, gowns, gas (particularly in bowel surgery and cases involving the upper airway), and hair, for example, are flammable and must be kept away from heat sources. Careful application of electrosurgery and use of a protective holster to store the electrode while not in use are important to minimize the risk of fire.
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. Another is the risk, with the use of high-voltage currents (especially those used for coagulation), of breakdown in the insulation, resulting in arcing from an exposed conductor to adjacent tissue, again, causing unwanted tissue damage. The risk can be reduced by using cutting current instead of coagulation current to lower the voltage used.
Yet another unique hazard is the potential for creating a capacitor with the cannula. 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. For maximum safety, an all-metal cannula (by which current can escape to the rest of the body) rather than a combination of metal and plastic should be used, and vigilance must be maintained at all times.
E. 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 general and vascular surgery, allowing tissue to be resected with minimal blood loss. Additionally, use in urology facilitates transurethral prostatectomy (TURP) and other procedures. In gynecologic practice, electrosurgical instruments are essential in cervical resections and biopsies.
Argon beam coagulation is closely related to basic 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 (has less impedance) than normal air and provides a more efficient pathway for transmitting current from the electrode to tissues (Figure 7–5). 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. The method does not cut even the most delicate tissue.
Ionized argon gas facilitates the flow of current from the handpiece to the tissue.
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 less smoke and less odor from coagulating with this type of current. Third, tissue loss and tissue damage are reduced when the current is more accurately targeted. Fourth, because the argon gas is delivered at room temperature, there is less danger of the instrument igniting gowns or drapes. Finally, the beam of coagulation generally improves coagulation and reduces blood loss and the risk of rebleeding.
Argon beam coagulation cannot be used to produce a cutting effect in the same manner as other types of electrosurgical equipment. Also, the nozzle for gas delivery can become clogged, reducing its efficiency, and just as with other electrosurgical instruments, if it is used for a prolonged period of time, it may overheat and cause inadvertent damage when set aside.
Argon beam coagulation is especially useful for procedures in which the surgeon must 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 efficient delivery of a consistent current load and its inability to become occluded with eschar are advantageous for operation with a significant risk of hemorrhage.