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:
(where c is the speed of light, 2.998 × 108 m/s)
frequency (f) and wavelength (λ) are inversely related; ie, 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
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
The Electromagnetic Spectrum and 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
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”—ie, 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.
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
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.
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.
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.