Electrosurgery

Electrosurgery

[ih-lek-troh-sur-juh-ree]
Electrosurgery is the application of a high-frequency electric current to human (or other animal) tissue as a means to remove lesions, staunch bleeding, or cut tissue. Electrosurgery can be used 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.

In electrosurgical procedures, the tissue is burned by an electrical current. Although electrosurgical devices may be used for the cauterization of tissue in some applications (as for example during hemorrhoid surgery), electrosurgery is usually used to refer to a quite different method than that used by many dedicated electrocautery devices. The latter uses heat conduction from a hot probe heated by a direct current (much in the manner of a soldering iron), whereas electrosurgery uses alternating current to directly heat the tissue itself (diathermy), while the probe tip remains relatively cool.

Electrosurgery is commonly used for such dermatological procedures as removal of skin tags, removal/destruction of benign skin tumors, and the removal of warts. It is now often preferred by dermatologists over laser surgery and cryosurgery for several procedures.

Electrosurgery is performed using a device called an Electrosurgical Generator, sometimes referred to as an RF Knife.

History

The development of the first commercial electrosurgical device is credited to Dr. William T. Bovie, who developed the device during the period of 1914 to 1927 while employed at Harvard University The first use of an electrosurgical generator in an operating room occurred on October 1, 1926. The surgery was performed by Dr. Harvey Williams Cushing.

Electricity generates heat

Electrosurgery works in the same manner as the heating coil in an electric toaster or hairdryer. Applying a voltage causes current flow which in turn causes the material (the heating coil in the case of the toaster, or the tissue in the case of electrosurgery) to rise in temperature.

To perform electrosurgery, a voltage source is applied across the tissue, which causes an electrical current to flow. The voltage source and tissue form a simple electrical circuit, with the tissue acting as a resistor. The resistance of the tissue determines the current flow:

I=frac{V}{R}
This relationship is known as Ohm's law. Current flowing through a resistor causes the generation of heat. This heat is generated in the tissue itself, and the heat is the destructive power that causes the tissue damage. In other words, the resistance of the tissue converts the electrical energy of the voltage source into heat (thermal energy) which causes the tissue temperature to rise:
mbox{Heat produced } = mbox{ Electrical energy expended}.
The electrical power (energy per time) expended and can be calculated using:
P=I cdot V=I^2 cdot R=frac{V^2}{R}
where P represents the electrical power, typically measured in watts. This result also gives the heat production rate (heat produced per unit time).

Why doesn't the electrode heat up? The answer to this question is that the resistance of the metal electrode and metal wire is so much smaller than that of the tissue that very little power is expended inside the metal conductors. The same principle explains why a toaster gets exceedingly hot, but (thankfully) the power cord and the wires in the wall do not heat appreciably.

The voltage source used in electrosurgery is a specialized electronic instrument. It is sometimes referred to as an electrosurgical generator.

Temperature rise and current density

The change in temperature that an object experiences when heated is inversely proportional to its heat capacity; the heat needed is proportional to the mass of the object. Considering two objects of the same material but of different sizes, a larger amount of heat is required to increase the temperature of the larger of the two objects by (for example) one degree. Moreover, when heat is added to a small region of an object, the temperature of that localized region will rise much higher than when the heat is evenly dispersed over the entire object.

Current density is a measure of the concentration of electrical current. A higher current density results in a higher concentration of heat generation. By this result and those of the previous paragraph, the local temperature in a piece of tissue will rise in proportion to the current density in that region.

Frequency of the electricity

A steady electrical current is referred to as DC current or 0 Hz. A varying current is referred to as AC current and consists of waves of one or more frequency (greater than 0 Hz).

The human nervous system is very sensitive to low-frequency (0 Hz to about 1000 Hz) electricity. Application of low-frequency electricity stimulates the nervous system. At even low currents low-frequency electricity causes electric shock which may involve acute pain, muscle spasms, and/or cardiac arrest. The sensitivity of the nervous system to electricity decreases with increasing frequency. At frequencies above 100 kHz, electricity does not stimulate the nervous system.

To avoid electric shock, electrosurgical equipment operates in the frequency range of 200 kHz to 5 MHz. This region of the Electronics:Frequency Spectrum corresponds roughly to that of the Medium Frequency (MF) band where AM radio stations can be found. However, electrosurgery does NOT use propagating radio waves; electrosurgery uses an electrical circuit that comprises the voltage source and the tissue that it is applied to, as explained above.

Common Electrosurgical modalities: Monopolar and Bipolar

There are two commonly used electrosurgical modalities or circuit topologies: monopolar and bipolar. The bipolar modality is used less often, but is easier to explain. Voltage is applied to the patient using a special forceps, with one tine connected to one pole of the A.C. voltage source and the other tine connected to the other pole of the voltage source. When a piece of tissue is held by the forceps, a high frequency electrical current flows from one to the other forceps tine, through the intervening tissue. The direction of this current alternates at high frequencies, but heating takes place no matter which direction the current flows. In this manner, the intervening tissue is heated.

In the monopolar modality the patient lies on top of the return electrode, a relatively large metal plate or a relatively large flexible metalized plastic pad which is connected to the other electrode of the A.C. current source. The surgeon uses a single, pointed, probe to make contact with the tissue. The electrical current flows from the probe tip, through the body and then to the return electrode, from which it flows back to the electrosurgical generator. It might seem that the monopolar modality would cause heating of the entire body cavity. However, the heating is actually very precisely confined to the tissue that is near the probe tip. This results from the fact that the current rapidly spreads out laterally as it enters the body, causing a dramatic decrease in the current density. Because the current density is much greater near the tip than it is in the interior of the body, or at the large surface return electrode, the heating occurs in a very localized region, only near the probe tip.

On an extremity such as a finger or penis, however, there is limited crosssectional area for the return current to spread across, resulting in high current density and heating throughout the volume of the extremity. For this reason monopolar electrocautery must not be used for circumcision.

Other Electrosurgical modalities: fulgeration with spark gap

For relatively low-powered monopolar electrosurgery performed on conscious outpatients (such as in a dermatologist's office), the spark gap or fulguration modality may be used.

At low-power, this technique requires no return electrode or patient-contact-plate at all . This requires that the patient be insulated from alternatve paths to ground, and to be conscious. Absence of a ground at the machine (a return plate or wire) is possible, because at the very high frequencies and low currents generated by low powered electrosurgical devices, the capacitance between the patients body and the machine's ground potential is large enough to allow the resulting displacement current to act as a return path. At low power, if a ground path spark develops between the patient and something touching a better "ground" than air, it will be small and may cause relatively minor shocks or burns at the ground point, but these are not a problem in a low-powered setting with conscious patients because they are immediately noticed. For high-power or surgical anesthesia settings, however, a ground pad is always necessary to insure that all such stray ground currents enter the machine safely through a large-skin-surface contact, and dedicated wire.

In fulgeration mode, with or without a ground pad used, the electrode is held away from the skin, so that a spark gap develops, and the burning to the skin is more superficial, because it is spread out at entry to the body. Under these conditions, superficial skin charring or carbonization is seen over a wider area than when the probe point it touched to the skin, and this wider area of superficial charring, useful for destruction of structures such as skin tags, is synonymous with fulgeration.

Prevention of unintended burns in anesthetized 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. If such a contact is not made, severe burns (3rd degree) can occur in unintended areas on the patients skin and beneath the skin.

To prevent unintended burns, the skin should be clean and dry and a conductive jelly should be used to enhance contact. Proper electrical grounding practices must be followed in the electrical wiring of the building. It is also recommended to use a newer electrosurgical unit that includes alarms for ground circuit interruption.

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.

Electrosurgical waveforms

Different waveforms can be used for different electrosurgical procedures. For cutting, a continuous single frequency sine wave is generated. This produces rapid heating. At the cellular level, rapid heating causes tissue cells to boil and burst. At a larger scale, the ruptured cells create a fine tear in the tissue, creating a clean incision.

For coagulation, the sine wave is turned on and off in rapid succession. The overall effect is a slower heating process, which causes cells to coagulate. 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 a sinewave is called modulation. More specifically, it is referred to as a continuous wave (CW) modulation or on-off keying (OOK).

Notes

See also

Manufacturers of Electrosurgical Equipment

In alphabetical order.

External links

In alphabetical order

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