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ElectrosurgeryElectrosurgery 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 fulguratetissue. Its benefits include the ability to make precise cuts with limited blood loss. In electrosurgerical procedures, the tissue is burned by an electrical current. Electrosurgery is different from electrocautery, in that the latter uses heat conductionfrom a hot probe whereas electrosurgery directly heats the tissue itself while the probe tip remains cold. 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 also used to cauterizeduring hemorrhoidsurgery. It is now often preferred by dermatologists over laser surgery and cryosurgery for several procedures.
Electricity Generates HeatElectrosurgery works in the same manner as the heating coil in an electric toasteror 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 voltagesource is applied across the tissue, which causes an electrical currentto 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:
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 energyof the voltage source into heat (thermal energy) which causes the tissue temperature to rise:
The electrical power(energy per time) expended and can be caculated using:
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 DensityThe change in temperaturethat an object experiences when heated is inversely proportional to its heat capacity, which is proportional to the massof 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 densityis 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 ElectricityA steady electrical current is refered to as DCcurrent or 0 Hz. A varying current is referred to as ACcurrent and consists of wavesof one or more frequency(greater than 0 Hz). The human nervous systemis very sensitive to low-frequency (0 Hz to about 1000 Hz) electricity, due to the fact that the nervous system is in itself a complex web of electrical circuits. Application of low-frequency electricity stimulates the nervous system. At even low currents low-frequency electricity causes electric shockwhich 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 frequency spectrumcorresponds 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 circuitthat is comprised of the voltage source and the tissue that it is applied to, as explained above. Electrosurgical ModalitiesThere are two commonly used electrosurgical modalities or circuit topologies: monopolar and bipolar. The biploar 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 the positive side of the voltage source and the other tine connected to the negative side of the voltage source. When a piece of tissue is grabbed by the forceps, an electrical current flows from the + to the - tine, through the intervening tissue. In this manner, the intervening tissue is heated. In the monopolar modality the patient lies on top of the return electrode, a large metal plate. The return electrode is maintained at ground potential. The surgeon uses a single, pointed, probeto 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. To the reader, it may 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 enter the body, causing a dramatic decrease in the current density. Because the current density is much much greater near the tip than it is in the interior of the body, the heating occurs in a very localized region near the probe tip. Prevention of Unintended BurnsThe 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 WaveformsDifferent waveformscan be used for different electrosurgical procedures. For cutting, a continuous single frequency sine waveis 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). See also
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