Electrosurgery
Encyclopedia
Electrosurgery is the application of a high-frequency electric current to biological tissue as a means to cut, coagulate, desiccate, or fulgurate
tissue. (These terms are used in specific ways for this methodology—see below). Its benefits include the ability to make precise cuts with limited blood loss. Electrosurgical devices are frequently used during surgical operations helping to prevent blood loss in hospital operating rooms or in outpatient procedures.
In electrosurgical procedures, the tissue is heated by an electric current
. Although electrical devices may be used for the cauterization
of tissue in some applications, electrosurgery is usually used to refer to a quite different method than electrocautery. The latter uses heat conduction
from a probe heated to a glowing temperature by a direct current (much in the manner of a soldering iron). This may be accomplished by direct current from dry-cells in a penlight-type device. Electrosurgery, by contrast, uses alternating current to directly heat the tissue itself. When this results in destruction of small blood vessels and halting of bleeding, it is technically a process of electrocoagulation, although "electrocautery" is sometimes loosely and nontechnically used to describe it.
Often electrosurgery is mistakenly referred to as diathermy
. Unlike Ohmic heating by electric current passing through the conductive tissue in conventional electrosurgery, diathermy
means dielectric heating, produced by rotation of molecular dipoles in high frequency alternating electric field. This effect is most widely used in microwave oven
s which operate at gigahertz frequencies.
Electrosurgery is commonly used in dermatological, gynecological, cardiac, plastic, ocular, spine, ENT, maxillofacial, orthopedic, urological, neuro- and general surgical procedures as well as certain dental procedures.
Electrosurgery is performed using an electrosurgical generator (also referred to as power supply or waveform generator) and a handpiece including one or several electrodes, sometimes referred to as an RF Knife. The apparatus when used for cutting or coagulation in surgery is still often referred to informally by surgeons as a "Bovie," after the inventor.
. The first use of an electrosurgical generator in an operating room occurred on October 1, 1926 at Peter Bent Brigham Hospital
in
Boston
, Massachusetts
. The operation—removal of a mass from a patient’s head—was performed by Harvey Cushing. The low powered Hyfrecator
for office use was introduced in 1940.
which exerts force on charged particles. A flow of free charge carriers – electron
s and ion
s - is called electric current
. In metals and semiconductors the charge carriers are primarily electrons, whereas in liquids the charge is carried predominantly by ions. Electrical conduction in biological tissues is primarily due to the conductivity of the interstitial fluids, and thus is predominantly ionic. Transition between the electronic and ionic conduction is governed by electrochemical processes
at the electrode–electrolyte interface. Value of electric current, I, is determined by the applied voltage, V, and material’s resistance, R, according to Ohm's law
:
Electric current of a constant polarity is referred to as direct current (DC). A current of alternating polarity is referred to as alternating current
(AC). Its frequency
is measured in cycles per second or hertz
(Hz).
Current flowing through a resistor causes the generation of Joule heating
. In other words, the resistance of the tissue converts the electric energy
of the voltage source into heat (thermal energy) which causes the tissue temperature to rise.
The deposited electric power
(energy per time) can be calculated using:
where P represents the electric power, typically measured in watt
s.
In absence of heat conduction
, the rate of temperature
rise, dT/dt, in a heated object is proportional to the deposited power P, and inversely proportional to which is in turn proportional to the mass
m of the object and its specific heat capacity c:
Larger amount of heat is required to increase the temperature of a heavier object. Thus when heat is generated in a small region of an object, the temperature of that localized region will rise much faster than if the same amount of heat is evenly dispersed over the entire object.
Current density, j is a measure of the concentration of electric current. A higher current density results in a higher concentration of Joule heating
. Power density generated by electric current in the material, p is proportional to the square of the current density, and to the material's resistivity
, g:
In absence of heat conduction
, the rate of local temperature rise is proportional to the power density, p, produced in that region of tissue, and inversely proportional to its specific heat capacity and density
.
cells are electrically-excitable, i.e. they can be stimulated by electric current. In human patients such stimulation may cause acute pain, muscle spasms, and even cardiac arrest
. Sensitivity of the nerve and muscle cells to electric field is due to the voltage-gated ion channel
s present in their cell membrane
s. Stimulation threshold does not vary much at low frequencies (so called rheobase
-constant level). However, the threshold starts increasing with decreasing duration of a pulse (or a cycle) when it drops below a characteristic minimum (so called chronaxie
). Typically, chronaxie
of neural cells is in the range of 0.1–10 ms, so the sensitivity to electrical stimulation (inverse of the stimulation threshold) decreases with increasing frequency in the kHz range and above. (Note that frequency of the alternating electric current is an inverse of the duration of a single cycle).
To minimize the effects of muscle and neural stimulation, electrosurgical equipment typically operates in the radio frequency
(RF) range of 100 kHz to 5 MHz.
Operation at higher frequencies also helps minimizing the amount of hydrogen and oxygen generated by electrolysis of water
. This is especially important consideration for applications in liquid medium in closed compartments, where generation of gas bubbles may interfere with the procedure. For example, bubbles produced during an operation inside an eye may obscure a field of view.
In bipolar configuration the voltage is applied to the patient using a pair of similarly-sized electrodes. For example, special forceps
, with one tine connected to one pole of the AC generator and the other tine connected to the other pole of the generator. When a piece of tissue is held by the forceps, a high frequency electric current flows from one to the other forceps tine, heating the intervening tissue.
In monopolar configuration the patient is attached to the return electrode, a relatively large metal plate or a flexible metalized plastic pad which is connected to the return electrode of the AC source. The surgeon uses a pointed electrode to make contact with the tissue. The electric current flows from the active electrode, through the body to the return electrode, and then back to the electrosurgical generator. Since electric current spreads from the pointed electrode as it enters the body the current density is rapidly (quadratically) decreasing with distance from the electrode. Since the rate of heating is proportional to the square of current density, the heating occurs in a very localized region, only near the probe tip.
On an extremity such as a finger, there is limited cross-sectional area for the return current to spread across, which might result in higher current density and some heating throughout the volume of the extremity.
There is also a common intermediate configuration, when both electrodes are located on the same probe, but the return electrode is much larger than the active one. Since current density is higher in front of the smaller electrode, the heating and associated tissue effects take place only (or primarily) in front of the active electrode, and exact position of the return electrode on tissue is not critical. Sometimes such configuration is called sesquipolar, even though the origin of this term in Latin (sesqui) means a ratio of 1.5 .
of the patient's body (which is between the patient's body and the machine's return potential or ground) is large enough to allow the resulting displacement current
to act as a return path.
One example of such a machine is called a hyfrecator
. This term began in 1940 as a Birtcher Corporation brandname Hyfrecator® for "High Frequency Eradicator", but now serves generically to describe a general class of single-electrode, non-isolated (earth-referenced) low-powered electrosurgical machines intended mainly for office use. An accidental additional return path through an earth-ground provides a danger of a burn at a site far away from the probe electrode, and for this reason single-electrode devices are used only on conscious patients who would be aware of such complications, and only on carefully insulated tables.
In such a setting, hyfrecators are not used to cut tissue, but to destroy relatively small lesions, and also to stop bleeding in surgical incisions made by blade instruments under local anesthesia.
Coagulation is performed using waveforms with lower average power, generating heat insufficient for explosive vaporization, but producing a thermal coagulum instead.
Electrosurgical desiccation
occurs when the electrode touches the tissue open to air, and the amount of generated heat is lower than that required for cutting. The tissue surface and some of the tissue more deep to the probe dries out and forms a coagulum (a dry patch of dead tissue). This technique may be used for treating nodules under the skin where minimal damage to the skin surface is desired.
In fulguration
mode, the electrode is held away from the tissue, so that when the air gap between the electrode and the tissue is ionized, an electric arc
discharge develops. In this approach the burning to the tissue is more superficial, because the current is spread over the tissue area larger than the tip of electrode. Under these conditions, superficial skin charring or carbonization is seen over a wider area than when operating in contact with the probe, and this technique is therefore used for very superficial or protrusive lesions such as skin tags. Ionization of an air gap requires voltage in the kV range.
Besides the thermal effects in tissue, electric field can produce pores in the cellular membranes - a phenomenon called electroporation
. This effect may affect cells beyond the range of thermal damage.
Cut/Coag Most wet field electrosurgical systems operate in two modes: "Cut" causes a small area of tissue to be vaporized, and "Coag" causes the tissue to "dry" (in the sense of bleeding being stopped). "Dried" tissues are killed (and will later slough or be replaced by fibrotic tissue) but they are temporarily physically intact after electrosurgical application. The depth of tissue death is typically a few millimeters near the contact of the electrode.
Cut If the voltage level is high enough, the heat generated can generates a vapour pocket. The vapour pocket typically reaches temperatures of approximately 400 degrees Celsius, which vaporizes and explodes a small section of soft tissue, resulting in an incision.
Coag When the system is operating in "coag mode" the voltage output is usually lower than in cut mode and less power is delivered. This therefore generates less heat and a vapour pocket is not generated. Tissue remains grossly intact, but cells are destroyed at the point of contact, and smaller vessels are destroyed and sealed, stopping capillary and small-arterial bleeding.
s can be used for different electrosurgical procedures. For cutting, a continuous single frequency sine wave
is often employed. Rapid tissue heating leads to explosive vaporization of interstitial fluid
. If the voltage is sufficiently high (> 400 V peak-to-peak) the vapor sheath is ionized, forming conductive plasma
. Electric current continues to flow from the metal electrode through the ionized gas into the tissue. Rapid overheating of tissue results in its vaporization, fragmentation and ejection of fragments, allowing for tissue cutting. In applications of a continuous wave the heat diffusion typically leads to formation of a significant thermal damage zone at the edges of the lesion. Open circuit voltage in electrosurgical waveforms is typically in the range of 300–10,000 V peak-to-peak.
Higher precision can be achieved with pulsed waveforms . Using bursts of several tens of microseconds in duration the tissue can be cut, while the size of the heat diffusion zone does not exceed the cellular scale. Heat accumulation during repetitive application of bursts can also be avoided if sufficient delay is provided between the bursts, allowing the tissue to cool down .
The proportion of ON time to OFF time can be varied to allow control of the heating rate. A related parameter, duty cycle
, is defined as the ratio of the ON time to the period (the time of a single ON-OFF cycle). In the terminology of electrical engineering
, this process of altering an amplitude of a periodic waveform is called modulation
.
For coagulation, the average power is typically reduced below the threshold of cutting. Typically, sine wave is turned on and off in a rapid succession. The overall effect is a slower heating process, which causes tissue to coagulate. In simple coagulation/cutting mode machines, the lower duty cycle typical of coagulation mode is usually heard by the ear as a lower frequency and a rougher tone than the higher frequency tone typical of cutting mode with the same equipment.
Many modern electrosurgical generators provide sophisticated waveforms with power adjusted in real time, based on changes of the tissue impedance.
To prevent unintended burns, the skin is cleaned and a conductive gel is used to enhance contact with the return electrode. Proper electrical grounding practices must be followed in the electrical wiring of the building. It is also recommended to use a modern electrosurgical unit that includes a return electrode monitoring system that continuously tests for reliable and safe patient contact. These systems interrogate the impedance of a split or dual-pad return electrode and will alarm out, disabling further generator output in case of fault. Prior generators relied on single pad return electrodes and thus had no means of verifying safe patient connection. Return electrodes should always have full contact with the skin and be placed on the same side of the body and close to the body part where the procedure is occurring.
If there is any metal in the body of the patient, the return electrode is placed on the opposite side of the body from the metal and be placed between the metal and the operation site. This prevents current from passing selectively through metal on the way to the return electrode. For example, for a patient who has had a right sided hip replacement who is scheduled for surgery, the return electrode is placed on the left side of the body on the lateral side of the lower abdomen, which places the return electrode between the location of the metal and the surgical site and on the opposite side from the metal. If there is metal on both sides of the body, the return electrode is placed between the metal and the procedure site when possible. Common return electrode locations include lateral portions of the outer thighs, abdomen, back, or shoulder blades.
The use of the bipolar option does not require the placement of a return electrode because the current only passes between tines of the forceps or other bipolar output device.
Electrosurgery should only be performed by a physician who has received specific training in this field and who is familiar with the techniques used to prevent burns.
Fulguration
Fulguration, also called electrofulguration, is a procedure to destroy tissue using a high-frequency electric current applied with a needlelike electrode.- External links :...
tissue. (These terms are used in specific ways for this methodology—see below). Its benefits include the ability to make precise cuts with limited blood loss. Electrosurgical devices are frequently used during surgical operations helping to prevent blood loss in hospital operating rooms or in outpatient procedures.
In electrosurgical procedures, the tissue is heated by an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
. Although electrical devices may be used for the cauterization
Cauterization
The medical practice or technique of cauterization is the burning of part of a body to remove or close off a part of it in a process called cautery, which destroys some tissue, in an attempt to mitigate damage, remove an undesired growth, or minimize other potential medical harmful possibilities...
of tissue in some applications, electrosurgery is usually used to refer to a quite different method than electrocautery. The latter uses heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
from a probe heated to a glowing temperature by a direct current (much in the manner of a soldering iron). This may be accomplished by direct current from dry-cells in a penlight-type device. Electrosurgery, by contrast, uses alternating current to directly heat the tissue itself. When this results in destruction of small blood vessels and halting of bleeding, it is technically a process of electrocoagulation, although "electrocautery" is sometimes loosely and nontechnically used to describe it.
Often electrosurgery is mistakenly referred to as diathermy
Diathermy
In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine.-Surgical uses:...
. Unlike Ohmic heating by electric current passing through the conductive tissue in conventional electrosurgery, diathermy
Diathermy
In the natural sciences, the term diathermy means "electrically induced heat" and is commonly used for muscle relaxation. It is also a method of heating tissue electromagnetically or ultrasonically for therapeutic purposes in medicine.-Surgical uses:...
means dielectric heating, produced by rotation of molecular dipoles in high frequency alternating electric field. This effect is most widely used in microwave oven
Microwave oven
A microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
s which operate at gigahertz frequencies.
Electrosurgery is commonly used in dermatological, gynecological, cardiac, plastic, ocular, spine, ENT, maxillofacial, orthopedic, urological, neuro- and general surgical procedures as well as certain dental procedures.
Electrosurgery is performed using an electrosurgical generator (also referred to as power supply or waveform generator) and a handpiece including one or several electrodes, sometimes referred to as an RF Knife. The apparatus when used for cutting or coagulation in surgery is still often referred to informally by surgeons as a "Bovie," after the inventor.
History
Development of the first commercial electrosurgical device is credited to William T. Bovie, who developed the first electrosurgical device while employed at Harvard UniversityHarvard University
Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...
. The first use of an electrosurgical generator in an operating room occurred on October 1, 1926 at Peter Bent Brigham Hospital
Brigham and Women's Hospital
Brigham and Women's Hospital is the largest hospital of the Longwood Medical and Academic Area in Boston, Massachusetts. It is directly adjacent to Harvard Medical School of which it is the second largest teaching affiliate with 793 beds...
in
Boston
Boston
Boston is the capital of and largest city in Massachusetts, and is one of the oldest cities in the United States. The largest city in New England, Boston is regarded as the unofficial "Capital of New England" for its economic and cultural impact on the entire New England region. The city proper had...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
. The operation—removal of a mass from a patient’s head—was performed by Harvey Cushing. The low powered Hyfrecator
Hyfrecator
A hyfrecator is a low-powered medical apparatus used in electrosurgery on conscious patients, usually in an office setting. It is used to destroy tissue directly, and to stop bleeding during minor surgery. It works by emitting low-power high-frequency A.C. electrical pulses, via an electrode...
for office use was introduced in 1940.
Tissue heating by electric current
When voltage is applied across the material it produces electric fieldElectric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
which exerts force on charged particles. A flow of free charge carriers – electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s and ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s - is called electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
. In metals and semiconductors the charge carriers are primarily electrons, whereas in liquids the charge is carried predominantly by ions. Electrical conduction in biological tissues is primarily due to the conductivity of the interstitial fluids, and thus is predominantly ionic. Transition between the electronic and ionic conduction is governed by electrochemical processes
Electrochemistry
Electrochemistry is a branch of chemistry that studies chemical reactions which take place in a solution at the interface of an electron conductor and an ionic conductor , and which involve electron transfer between the electrode and the electrolyte or species in solution.If a chemical reaction is...
at the electrode–electrolyte interface. Value of electric current, I, is determined by the applied voltage, V, and material’s resistance, R, according to Ohm's law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
:
Electric current of a constant polarity is referred to as direct current (DC). A current of alternating polarity is referred to as alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC). Its frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
is measured in cycles per second or hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
(Hz).
Current flowing through a resistor causes the generation of Joule heating
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
. In other words, the resistance of the tissue converts the electric energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
of the voltage source into heat (thermal energy) which causes the tissue temperature to rise.
The deposited electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
(energy per time) can be calculated using:
where P represents the electric power, typically measured in watt
Watt
The watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s.
In absence of heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
, the rate of temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
rise, dT/dt, in a heated object is proportional to the deposited power P, and inversely proportional to which is in turn proportional to the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
m of the object and its specific heat capacity c:
Larger amount of heat is required to increase the temperature of a heavier object. Thus when heat is generated in a small region of an object, the temperature of that localized region will rise much faster than if the same amount of heat is evenly dispersed over the entire object.
Current density, j is a measure of the concentration of electric current. A higher current density results in a higher concentration of Joule heating
Joule heating
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat. It was first studied by James Prescott Joule in 1841. Joule immersed a length of wire in a fixed mass of water and measured the temperature...
. Power density generated by electric current in the material, p is proportional to the square of the current density, and to the material's resistivity
Resistivity
Electrical resistivity is a measure of how strongly a material opposes the flow of electric current. A low resistivity indicates a material that readily allows the movement of electric charge. The SI unit of electrical resistivity is the ohm metre...
, g:
In absence of heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
, the rate of local temperature rise is proportional to the power density, p, produced in that region of tissue, and inversely proportional to its specific heat capacity and density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
.
Electrical stimulation of neural and muscle cells
Neural and muscleMuscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
cells are electrically-excitable, i.e. they can be stimulated by electric current. In human patients such stimulation may cause acute pain, muscle spasms, and even cardiac arrest
Cardiac arrest
Cardiac arrest, is the cessation of normal circulation of the blood due to failure of the heart to contract effectively...
. Sensitivity of the nerve and muscle cells to electric field is due to the voltage-gated ion channel
Voltage-gated ion channel
Voltage-gated ion channels are a class of transmembrane ion channels that are activated by changes in electrical potential difference near the channel; these types of ion channels are especially critical in neurons, but are common in many types of cells....
s present in their cell membrane
Cell membrane
The cell membrane or plasma membrane is a biological membrane that separates the interior of all cells from the outside environment. The cell membrane is selectively permeable to ions and organic molecules and controls the movement of substances in and out of cells. It basically protects the cell...
s. Stimulation threshold does not vary much at low frequencies (so called rheobase
Rheobase
In neuroscience, rheobase is the minimal current amplitude of indefinite duration that results in the depolarization threshold of the cell membranes being reached In neuroscience, rheobase is the minimal current amplitude of indefinite duration (practically, a few hundred milliseconds) that...
-constant level). However, the threshold starts increasing with decreasing duration of a pulse (or a cycle) when it drops below a characteristic minimum (so called chronaxie
Chronaxie
In the mathematical description of the functioning of the nervous system, the chronaxie is the minimum time over which an electric current, double the strength of the rheobase, needs to be applied, in order to stimulate a muscle fiber or nerve cell.The terms "chronaxie" and "rheobase" were coined...
). Typically, chronaxie
Chronaxie
In the mathematical description of the functioning of the nervous system, the chronaxie is the minimum time over which an electric current, double the strength of the rheobase, needs to be applied, in order to stimulate a muscle fiber or nerve cell.The terms "chronaxie" and "rheobase" were coined...
of neural cells is in the range of 0.1–10 ms, so the sensitivity to electrical stimulation (inverse of the stimulation threshold) decreases with increasing frequency in the kHz range and above. (Note that frequency of the alternating electric current is an inverse of the duration of a single cycle).
To minimize the effects of muscle and neural stimulation, electrosurgical equipment typically operates in the radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) range of 100 kHz to 5 MHz.
Operation at higher frequencies also helps minimizing the amount of hydrogen and oxygen generated by electrolysis of water
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas due to an electric current being passed through the water.-Principle:...
. This is especially important consideration for applications in liquid medium in closed compartments, where generation of gas bubbles may interfere with the procedure. For example, bubbles produced during an operation inside an eye may obscure a field of view.
Common electrode configurations for ground-return-pad devices
There are several commonly used electrode configurations or circuit topologies:In bipolar configuration the voltage is applied to the patient using a pair of similarly-sized electrodes. For example, special forceps
Forceps
Forceps or forcipes are a handheld, hinged instrument used for grasping and holding objects. Forceps are used when fingers are too large to grasp small objects or when many objects need to be held at one time while the hands are used to perform a task. The term 'forceps' is used almost exclusively...
, with one tine connected to one pole of the AC generator and the other tine connected to the other pole of the generator. When a piece of tissue is held by the forceps, a high frequency electric current flows from one to the other forceps tine, heating the intervening tissue.
In monopolar configuration the patient is attached to the return electrode, a relatively large metal plate or a flexible metalized plastic pad which is connected to the return electrode of the AC source. The surgeon uses a pointed electrode to make contact with the tissue. The electric current flows from the active electrode, through the body to the return electrode, and then back to the electrosurgical generator. Since electric current spreads from the pointed electrode as it enters the body the current density is rapidly (quadratically) decreasing with distance from the electrode. Since the rate of heating is proportional to the square of current density, the heating occurs in a very localized region, only near the probe tip.
On an extremity such as a finger, there is limited cross-sectional area for the return current to spread across, which might result in higher current density and some heating throughout the volume of the extremity.
There is also a common intermediate configuration, when both electrodes are located on the same probe, but the return electrode is much larger than the active one. Since current density is higher in front of the smaller electrode, the heating and associated tissue effects take place only (or primarily) in front of the active electrode, and exact position of the return electrode on tissue is not critical. Sometimes such configuration is called sesquipolar, even though the origin of this term in Latin (sesqui) means a ratio of 1.5 .
Dedicated non-ground-return machines
Relatively low-powered high frequency electrosurgery can be performed on conscious outpatients with no return electrode at all . Safely Operating at low currents with no return electrode is possible, because at the medium RF frequencies (usually 100 - 500 kHz) that the machines generate, the self-capacitanceCapacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
of the patient's body (which is between the patient's body and the machine's return potential or ground) is large enough to allow the resulting displacement current
Displacement current
In electromagnetism, displacement current is a quantity that is defined in terms of the rate of change of electric displacement field. Displacement current has the units of electric current density, and it has an associated magnetic field just as actual currents do. However it is not an electric...
to act as a return path.
One example of such a machine is called a hyfrecator
Hyfrecator
A hyfrecator is a low-powered medical apparatus used in electrosurgery on conscious patients, usually in an office setting. It is used to destroy tissue directly, and to stop bleeding during minor surgery. It works by emitting low-power high-frequency A.C. electrical pulses, via an electrode...
. This term began in 1940 as a Birtcher Corporation brandname Hyfrecator® for "High Frequency Eradicator", but now serves generically to describe a general class of single-electrode, non-isolated (earth-referenced) low-powered electrosurgical machines intended mainly for office use. An accidental additional return path through an earth-ground provides a danger of a burn at a site far away from the probe electrode, and for this reason single-electrode devices are used only on conscious patients who would be aware of such complications, and only on carefully insulated tables.
In such a setting, hyfrecators are not used to cut tissue, but to destroy relatively small lesions, and also to stop bleeding in surgical incisions made by blade instruments under local anesthesia.
Electrosurgical modalities
In cutting mode electrode touches the tissue, and sufficiently high power density is applied to vaporize its water content. Since water vapor is not conductive under normal cirumstances, electric current cannot flow through the vapor layer. Energy delivery beyond the vaporization threshold can continue if sufficiently high voltage is applied (> +/-200 V) to ionize vapor and convert it into a conductive plasma. Vapor and fragments of the overheated tissue are ejected, forming a crater . Electrode surfaces intended to be used for cutting often feature a finer wire or wire loop, as opposed to a more flat blade with a rounded surface.Coagulation is performed using waveforms with lower average power, generating heat insufficient for explosive vaporization, but producing a thermal coagulum instead.
Electrosurgical desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
occurs when the electrode touches the tissue open to air, and the amount of generated heat is lower than that required for cutting. The tissue surface and some of the tissue more deep to the probe dries out and forms a coagulum (a dry patch of dead tissue). This technique may be used for treating nodules under the skin where minimal damage to the skin surface is desired.
In fulguration
Fulguration
Fulguration, also called electrofulguration, is a procedure to destroy tissue using a high-frequency electric current applied with a needlelike electrode.- External links :...
mode, the electrode is held away from the tissue, so that when the air gap between the electrode and the tissue is ionized, an electric arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
discharge develops. In this approach the burning to the tissue is more superficial, because the current is spread over the tissue area larger than the tip of electrode. Under these conditions, superficial skin charring or carbonization is seen over a wider area than when operating in contact with the probe, and this technique is therefore used for very superficial or protrusive lesions such as skin tags. Ionization of an air gap requires voltage in the kV range.
Besides the thermal effects in tissue, electric field can produce pores in the cellular membranes - a phenomenon called electroporation
Electroporation
Electroporation, or electropermeabilization, is a significant increase in the electrical conductivity and permeability of the cell plasma membrane caused by an externally applied electrical field...
. This effect may affect cells beyond the range of thermal damage.
Wet field electrosurgery
There are wet and dry field electrosurgical devices. Wet field devices operate in a saline solution, or in an open wound. Heating is as a result of an alternating current that passes between two electrodes. Heating is usually greatest where the current density is highest. Therefore it is usually the smallest or sharpest electrode that generates the most heat.Cut/Coag Most wet field electrosurgical systems operate in two modes: "Cut" causes a small area of tissue to be vaporized, and "Coag" causes the tissue to "dry" (in the sense of bleeding being stopped). "Dried" tissues are killed (and will later slough or be replaced by fibrotic tissue) but they are temporarily physically intact after electrosurgical application. The depth of tissue death is typically a few millimeters near the contact of the electrode.
Cut If the voltage level is high enough, the heat generated can generates a vapour pocket. The vapour pocket typically reaches temperatures of approximately 400 degrees Celsius, which vaporizes and explodes a small section of soft tissue, resulting in an incision.
Coag When the system is operating in "coag mode" the voltage output is usually lower than in cut mode and less power is delivered. This therefore generates less heat and a vapour pocket is not generated. Tissue remains grossly intact, but cells are destroyed at the point of contact, and smaller vessels are destroyed and sealed, stopping capillary and small-arterial bleeding.
Electrosurgical waveforms
Different waveformWaveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
s can be used for different electrosurgical procedures. For cutting, a continuous single frequency sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
is often employed. Rapid tissue heating leads to explosive vaporization of interstitial fluid
Interstitial fluid
Interstitial fluid is a solution that bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and transcellular fluid...
. If the voltage is sufficiently high (> 400 V peak-to-peak) the vapor sheath is ionized, forming conductive plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
. Electric current continues to flow from the metal electrode through the ionized gas into the tissue. Rapid overheating of tissue results in its vaporization, fragmentation and ejection of fragments, allowing for tissue cutting. In applications of a continuous wave the heat diffusion typically leads to formation of a significant thermal damage zone at the edges of the lesion. Open circuit voltage in electrosurgical waveforms is typically in the range of 300–10,000 V peak-to-peak.
Higher precision can be achieved with pulsed waveforms . Using bursts of several tens of microseconds in duration the tissue can be cut, while the size of the heat diffusion zone does not exceed the cellular scale. Heat accumulation during repetitive application of bursts can also be avoided if sufficient delay is provided between the bursts, allowing the tissue to cool down .
The proportion of ON time to OFF time can be varied to allow control of the heating rate. A related parameter, duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
, is defined as the ratio of the ON time to the period (the time of a single ON-OFF cycle). In the terminology of electrical engineering
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, this process of altering an amplitude of a periodic waveform is called modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
.
For coagulation, the average power is typically reduced below the threshold of cutting. Typically, sine wave is turned on and off in a rapid succession. The overall effect is a slower heating process, which causes tissue to coagulate. In simple coagulation/cutting mode machines, the lower duty cycle typical of coagulation mode is usually heard by the ear as a lower frequency and a rougher tone than the higher frequency tone typical of cutting mode with the same equipment.
Many modern electrosurgical generators provide sophisticated waveforms with power adjusted in real time, based on changes of the tissue impedance.
Prevention of unintended burns in patients
For high power surgical uses during anesthesia the monopolar modality relies on a good electrical contact between a large area of the body (typically at least the entire back of the patient) and the return electrode or pad (also known as dispersive pad or patient plate). Severe burns (3rd degree) can occur if contact with the return electrode is insufficient, or when a patient comes into contact with metal objects serving as an unintended (capacitative) leakage path to Earth-ground.To prevent unintended burns, the skin is cleaned and a conductive gel is used to enhance contact with the return electrode. Proper electrical grounding practices must be followed in the electrical wiring of the building. It is also recommended to use a modern electrosurgical unit that includes a return electrode monitoring system that continuously tests for reliable and safe patient contact. These systems interrogate the impedance of a split or dual-pad return electrode and will alarm out, disabling further generator output in case of fault. Prior generators relied on single pad return electrodes and thus had no means of verifying safe patient connection. Return electrodes should always have full contact with the skin and be placed on the same side of the body and close to the body part where the procedure is occurring.
If there is any metal in the body of the patient, the return electrode is placed on the opposite side of the body from the metal and be placed between the metal and the operation site. This prevents current from passing selectively through metal on the way to the return electrode. For example, for a patient who has had a right sided hip replacement who is scheduled for surgery, the return electrode is placed on the left side of the body on the lateral side of the lower abdomen, which places the return electrode between the location of the metal and the surgical site and on the opposite side from the metal. If there is metal on both sides of the body, the return electrode is placed between the metal and the procedure site when possible. Common return electrode locations include lateral portions of the outer thighs, abdomen, back, or shoulder blades.
The use of the bipolar option does not require the placement of a return electrode because the current only passes between tines of the forceps or other bipolar output device.
Electrosurgery should only be performed by a physician who has received specific training in this field and who is familiar with the techniques used to prevent burns.
See also
- CryosurgeryCryosurgeryCryosurgery is the application of extreme cold to destroy abnormal or diseased tissue. The term comes from the Greek words cryo and surgery meaning "hand work" or "handiwork"....
- Laser surgeryLaser surgeryLaser surgery is surgery using a laser to cut tissue instead of a scalpel. Examples include the use of a laser scalpel in otherwise conventional surgery, and soft tissue laser surgery, in which the laser beam vaporizes soft tissue with high water content...
- Electrocautery
- Dielectric heatingDielectric heatingDielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...
- microwave minimaze procedure
- Harmonic scalpelHarmonic scalpelThe Harmonic scalpel is a cutting instrument used during surgical procedures to simultaneously cut and coagulate tissue.The Harmonic brand is manufactured and distributed by Ethicon Endo-Surgery a subsidiary of Johnson & Johnson....
External links
- A Simple Guide to the Hyfrecator 2000, Richard J Motley, Schuco International Ltd. a primer for low-powered outpatient dermatological devices, such as the Hyfrecator 2000 device.
- Electrosurgery for the Skin, Barry L. Hainer M.D., Richard B. Usatine, M.D., American Family Physician (Journal of the American Academy of Family Physicians), 2002 Oct 1;66(7):1259-66.
- Electrosurgical Generator Testing Online Journal of he Biomedical Engineering Association of Ireland (BEAI), May 1997.
- Update on Electrosurgery, Judith Lee, Contributing Editor, Outpatient Surgery Magazine, February, 2002.