Electrosurgery in the Uterus

During RF electrosurgery, high frequency electromagnetic energy is converted in the cell first to kinetic energy, then to thermal energy. Rapid intracellular heating to 100ᴼ C results in boiling of the intracellular water, that comprises a liquid to gas conversion that results in rapid volumetric expansion and cellular rupture – a process called “vaporization”. If the elevation in intracellular temperature remains below100ᴼ C but above 60ᴼ C, two processes occur; one is the loss intracellular water is by dehydration (desiccation); and the other a breakdown in the protein molecular bonds, which when they reform create a homogenous coagulum that is collectively a process called coagulation. The type of tissue effect achieved is determined by variety of factors that include those that relate to RF electrical properties as well those related to the interface between the electrode and tissue, including the tissue exposure time, the size and shape of the electrode, and the relationship of the electrode to the target tissue. 

RF electrosurgery requires the creation of a contiguous circuit allowing the passage of electrons. The elements of a circuit that include the two electrodes, the electrosurgical generator or unit (ESU) the wires that connect them, and, of course some part of the patient. All RF electrosurgery requires two electrodes, at least one of them designed to create a tissue effect, a circumstance that makes all RF electrosurgery “bipolar”. The difference between bipolar and monopolar instruments is the location and purpose of the second electrode. 

There are three fundamental and interacting properties of electricity that necessary to understanding RF electrosurgery. These include current (I), voltage (V) and impedance or resistance (R). Current is the measure of the movement of electrons past a given point in the circuit in a defined period of time and is measured in amperes. The concept of voltage is one of pressure, in this instance the difference in electromotive pressure between the two electrodes, and which reflects the pressure with which the electrons are pushed through the circuit. It is measured in volts. Resistance, measured in ohms, reflects the difficulty that the circuit, or a part of the circuit presents to the passage of electrons. Technically, the term resistance is reserved for constant polarity circuits (also called direct current circuits, or DC) while for alternating polarity circuits the term impedance is the appropriate term. In any electrical circuit these electrical properties are related by Ohm's Law: I = V/R.  Power reflects work per unit time, is calculated as the product of current and voltage and is measured in watts (W = V x I). If Ohm's Law is used to create a substitution for current [W = V x (V/R)], then wattage can be expressed in another way (W = V2/R).

The ESU converts the 60 cycle per second (60 Hertz or Hz) waveform from a standard AC output, into a radiofrequency current that typically ranges from 300 to 500 Kilohertz (KHz). If such a RF output is viewed on the screen of an oscilloscope, it is displayed as a waveform that is usually symmetrical above and below "0" volts, a circumstance that reflects the alternating polarity of the circuit. The peak voltage of the generated waveform is depicted by the distance from the baseline to the apex of the wave.

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Electrosurgical tissue effects depend upon a number of factors including the ESU power setting and waveform, target tissue impedance, the type of distending media and the shape and size of the electrode and its proximity to tissue. A concept that is critical to the understanding of electrosurgical tissue effects is current or power density, the total wattage striking the tissue per unit area. When the ESU output is held constant, power density is determined by the shape and size of the electrode and by its relationship to the tissue - contact vs. non-contact. An electrode with a small surface area such as a needle or a wire loop, when held in proximity to tissue, will concentrate currant so the energy is focused on a small area that facilitates rapid elevation of cellular temperature and the creation of a narrow zone of vaporization. If the electrode is changed to a wider, larger design such as a ball or barrel the power density is diluted somewhat to that the temperature is enough only to dehydrate and coagulate the tissue. The opposite power density extreme the dispersive electrode, with a surface area so large that it dissipates the current thereby preventing significant elevation in tissue temperature.

During hysteroscopic surgery, tissue cutting is best achieved with a continuous or near continuous low voltage output, using a pointed or or loop shaped electrode held adjacent to tissue. Provided the voltage is adequate, and the electrode is near to the tissue, vaporization occurs, which, in fluid media manifests with simultaneous transaction and the formation of gas bubbles from the intracellular liquid gas conversion, that become free in the media.  The depth of thermal injury in the remaining tissue depends on a number of factors including the ESU power output and waveform, the electrode size and shape and the speed and skill of the surgeon. While the minimum depth of thermal injury has been described as low as a few microns, more typical reported endomyometrial injury studies suggest about 0.2 to 1.0 mm. 

Bulk vaporization describes the use rather large surface area to remove large tissue masses with electrosurgical vaporization. The technique requires a high-power output and its function can be facilitated by a design that includes grooves or spikes that can concentrate current. To achieve the required power in monopolar systems, the ESU is typically set at three to four times the power required for loop or needle electrodes. When using this technique, surgeons should be reminded to safe a tissue specimen for histopathological examination.

As previously discussed, tissue coagulation comprises the combination of cellular and tissue dehydration and coagulation, the latter a process whereby tissue proteins are denatured and reconstituted into a homogenous coagulum. Coagulation is achieved by placing an electrode with a relatively large surface area on the target tissue, followed by activation of the ESU. Preferably, low voltage outputs are preferred as the modulated, high voltage outputs called “coagulation” in North America tend to cause only a superficial effect. As previously stated, all bipolar systems are designed to be used with low voltage waveforms. Provided the selection of an appropriate electrode, any waveform may be used to create tissue coagulation. However, the cutting and blended outputs for monopolar instruments are preferred to those labeled "coagulation" because they are at least equally efficacious and they are not associated with the risks of current diversion. High voltage outputs damage electrodes more quickly and are associated with an increased risk for capacitative coupling, to be discussed later. 

The process of fulguration generally applies to electrosurgery in gaseous media. The dynamics are vastly different in fluid media. Fulguration is is also known as spray coagulation or black coagulation, being a technique that superficially coagulates and carbonizes tissue using the repeated high voltage electrosurgical arcs, that, when applied to desiccated tissue, induce the process of resistive heating that can quickly elevate the temperature to 200C or more resulting in organic breakdown and release of the carbon atoms that provide the black appearance.

Direct coupling can be associated with monopolar hysteroscopic instruments. Any metallic object including speculae and cervical tenaculi also can, following contact with the external sheath, serve to conduct current to locations in the vagina and vulva. Care should be taken to avoid contact of these instruments with the resectoscope.