Electrosurgical Waveforms
Different waveforms can be used for different electrosurgical procedures. For cutting, a continuous single frequency sine wave is often employed. Rapid tissue heating leads to explosive vaporization of interstitial fluid. If the voltage is sufficiently high (> 400 V peak-to-peak) the vapor sheath is ionized, forming conductive plasma. Electric current continues to flow from the metal electrode through the ionized gas into the tissue. Rapid overheating of tissue results in its vaporization, fragmentation and ejection of fragments, allowing for tissue cutting. In applications of a continuous wave the heat diffusion typically leads to formation of a significant thermal damage zone at the edges of the lesion. Open circuit voltage in electrosurgical waveforms is typically in the range of 300–10,000 V peak-to-peak.
Higher precision can be achieved with pulsed waveforms. Using bursts of several tens of microseconds in duration the tissue can be cut, while the size of the heat diffusion zone does not exceed the cellular scale. Heat accumulation during repetitive application of bursts can also be avoided if sufficient delay is provided between the bursts, allowing the tissue to cool down. 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 an amplitude of a periodic waveform is called modulation.
For coagulation, the average power is typically reduced below the threshold of cutting. Typically, sine wave is turned on and off in a rapid succession. The overall effect is a slower heating process, which causes tissue to coagulate. In simple coagulation/cutting mode machines, the lower duty cycle typical of coagulation mode is usually heard by the ear as a lower frequency and a rougher tone than the higher frequency tone typical of cutting mode with the same equipment.
Many modern electrosurgical generators provide sophisticated waveforms with power adjusted in real time, based on changes of the tissue impedance.
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