Electrosurgical Theory

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).

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.¹ Therefore, electrosurgical devices use frequencies in the range of 350 to 500 kHz. This range is referred to as the medium RF electromagnetic spectrum (Figure 5.1).

Waveforms

There are three basic types of waveforms generated in electrosurgery: cutting waveform, coagulation waveform, and blended waveforms (Figure 5.2). Cutting waveforms are continuous waveforms, and coagulation and blend are intermittent waveforms.

The continuous cutting waveform uses less peak voltage at a similar power setting to the intermittent waveform, resulting in less lateral thermal tissue damage. With pure cut, the amplitude of the continuous waveform is the same. This induces a localized effect on the tissues, creating high tissue temperatures (>100°C), vaporization of the interstitial fluid, and tissue separation with little hemostasis. Heat is absorbed by water released from the cells, which minimizes thermal damage and provides minimal coagulation. Power settings for the cut waveform are often between 50 and 80 W².

The intermittent waveform of the coagulation waveform produces a high current density delivered in pulses. This waveform has higher voltage than the continuous waveforms and delivers the electrical charge deeper into the tissues. The pauses between the pulses lead to decreased tissue heating, resulting in coagulation rather than cutting. Coagulation is achieved with a power setting between 30 and 50 W.² Coagulation can either be performed using desiccation or fulguration. Whereas fulguration uses a noncontact technique to control diffuse hemorrhage, desiccation is a contact technique used for local bleeding. Fulguration uses an intermittent waveform, which allows for proteins to melt and recongeal, forming a coagulum. Some ESUs also contain a spray mode, which is useful for oozing capillary beds. Spray mode does not penetrate as deeply into tissues and can be used in more delicate tissues. Desiccation or a contact technique leads to less heat production than fulguration. A coagulum is formed by tissues drying out and proteins melting.

A blended waveform results in simultaneous coagulation and cutting. Whereas blend 1 is more effective at cutting with minimal hemostasis, blend 3 results in better hemostasis and decreased cutting (see Figure 5.2). Many surgeons prefer to use the blend waveform because it provides a trade-off between thermal tissue damage and hemostasis.

Despite its name, the cutting waveform can be used for coagulation, and in some scenarios, it is recommended over the coagulation waveform. An example is when applying electrosurgery to the hemostat or forceps in what is referred to as coaptive coagulation. In this scenario, cutting energy is recommended because this produces deeper hemostasis and less thermal spread compared with coagulation energy. Coagulation results in rapid increase in impedance from char at the electrode. As impedance increases, more power is needed to penetrate deeper tissues. Instead, cutting energy heats up the tissue more quickly, minimizing char accumulation and allowing energy to penetrate deeper into the target tissues. With coaptive coagulation, the flattened vessel wall becomes fused as the current is applied to the instrument. Heat denatures the outer vessel wall and dehydrates the vessel, halting blood flow.

Monopolar Electrosurgery

The monopolar system is the most commonly used electrosurgical device. It consists of a generator; an electrosurgical pencil (active electrode); and a return electrode, which is the grounding pad (passive electrode). The generator directs a current through the electrosurgical pencil, which contains the active electrode. The current then passes to the tissue through the patient’s body to the grounding pad and back to the generator (Figure 5.3). The tissue effect is determined by the power setting, waveform, technique (contact, noncontact, and time), and electrode configuration. The passive electrode should maintain a wide contact area to minimize the chance of concentrating the current and patient burning. Previous rigid metal plates were associated with increased burns because of rigidity, which often led to decreased contact with the plate to the patient. Newer electrosurgery generators also come with return electrode monitoring (REM) in which a generator will cease operation if the return signal is outside programmed limits.