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Electricity is the movement or flow of electrically charged particles from one electrode to another. Electrosurgery instruments apply an electrical current to tissue, enabling cutting, coagulating, desiccating, or fulgurating by generating heat. There are three properties of electricity that affect the rise in temperature of the tissue: Voltage, current, and resistance, or impedance. The interaction of these three properties is explained by Ohm's law, which describes the flow of electricity along a circuit:

Whereas current is a measure of electron movement through tissue in a given time, voltage is the driving force that moves the electrons against the tissue resistance or impedance within the circuit. Tissue resistance or impedance is a function of both the composition of the tissues and blood supply. As voltage drives electrons through the circuit against impedance, heat is generated. This tissue resistance or impedance produces heat rather than the active electrode. Therefore, tissues with greater impedance will result in the generation of more heat. Tissue impedance constantly changes as an electrical current is applied and the tissues become desiccated. The degree of heat leads to varying tissue effects (Table 5.1).

Table 5.1 Tissue effect in relation to temperature.

Source: Modified from Dubiel et al. [1].

Temperature (°C)	Tissue effect
250	Tissue carbonized from dehydration
100	Cell wall rupture
90	Tissue desiccation
70	Protein denaturing
50	Enzymatic activity inactivated
40	Inflammation and edema

Another important concept in understanding electrosurgery is the concept of power. Power is a measure of work per unit time. It is a function of voltage and current and is measured in watts. Power tells you the rate at which the energy works. Power rises exponentially with increases in voltage and decreases inversely with increases in resistance or impedance. However, voltage is the main determinant of tissue effect and is a function of the waveform that the generator delivers (see waveform section below).

An ESU is composed of four basic components: A generator, an active electrode, the patient, and the return electrode. The ESU uses low‐frequency alternating current (AC) from a wall outlet and converts it to a higher voltage radiofrequency (RF) output. The current can be used to induce diathermy but also stimulates muscle and nerve cells. Stimulation of muscle and nerve cells can lead to pain, muscle spasm, and even cardiac arrest. The sensitivity of nerves and muscles cells to electrical stimulation decreases and the excitability threshold increases with increasing frequency, meaning that nerve and muscle cell stimulation is refractory to electrical stimulation above 100‐kHz [2]. Therefore, electrosurgical devices use frequencies in the range of 350–500 kHz. This range is referred to as the medium RF electromagnetic spectrum (Figure 5.1).