Crest Factor and Electrosurgical Generators

Electrosurgery is the passage of radio frequency current through tissue to create a desired surgical effect, such as coagulation or cutting. There are several characteristics of electrosurgical generators that can influence the surgical effect in each mode. In Cut mode, the Power Efficiency Rating (PER), indicates the ability of the generator to provide the selected power over a wide range of tissue types or impedances. In Coag mode, a different parameter, the crest factor is used.

The crest factor of an electrosurgical generator is an important indicator of the generator’s ability to provide tissue hemostasis with little or no cutting. When evaluating the ability of an electrosurgical generator to deliver the selected power into a wide range of tissue types, the PER should be used. The PER is an excellent gauge of performance for the Cut/Blend modes of the generator. For the Coag modes, a different parameter must be used due to the nature of the high-voltage coagulation waveform and the important clinical effect of hemostasis.

An effective coagulation waveform provides a high degree of hemostasis, with minimal cutting, and can be evaluated by the waveform’s crest factor. The technical definition of crest factor is the ratio of two different voltage measurements: a waveform’s peak voltage (Vpeak) divided by its rms voltage1 (Vrms). These voltages are measured at the generator’s rated load resistance and at the maximum power setting for each mode:

The formal equation is:
Waveforms with low crest factors can be effective in cutting tissue, but have little or no coagulation effect. These include cut and some blend waveforms, which typically have crest factors between 1.4 and 3.5. Waveforms with crest factors between 3.5 and 5.0 have a medium coagulation effect, with low cutting ability, and include desiccate and some aggressive blend modes. Waveforms with crest factors greater than 5.0 are the traditional sparking coagulation modes, such as fulgurate and spray, and are used because of their superior hemostatic effect.

For procedures requiring a great deal of hemostasis, such as liver, surgeons often prefer a higher crest factor (7+) for broad, superficial coagulation. For general surgery procedures, such as bowel resections, a medium crest factor of 5.0 is adequate. For procedures where cutting with some hemostasis is required, such as breast reductions, a crest factor of 2.5 provides excellent cutting performance while providing adequate hemostasis.

c02b.gif (5759 bytes)
* Measurements should be taken at
rated load. T = Repetition of the periodic
waveform in seconds

Crest factor is determined by three parameters: pulse amplitude, pulse repetition rate and pulse width. By modifying one or more of these parameters while keeping the others constant, the crest factor can be raised. For example, an increase in pulse amplitude raises the crest factor if the pulse width and repetition rate remain the same. However, if the pulse voltage goes too high, accessory insulation may be stressed or an increase in high-frequency (HF) leakage may occur. Decreasing the pulse repetition rate also raises the crest factor, although a large decrease results in audible buzzing at the surgical site. Finally, a decrease in pulse width raises the crest factor; however, a pulse width that is too short increases the stress on the generator’s output circuitry. Only a careful balance of these components produces an effective and reliable coagulation waveform.

Careful attention to the desired clinical effect and expected use for each Coag mode, coupled with extensive customer feedback, has enabled Valleylab to optimize these components for the Coag modes offered in Valleylab electrosurgical generators. Valleylab routinely provides crest factor information for all of its electrosurgical generators.

When contemplating the purchase of a new electrosurgical generator, users should review the crest factors of the coagulation modes offered by manufacturers and compare them with those currently used in the procedures performed in their facility. Use of the crest factor measurement assists with comparisons between different coagulation modes within a single generator; it also helps when comparing similar modes from different manufacturers. When deciding on the most beneficial generator for your facility, request crest factor information from the electrosurgical generator manufacturers and have this information included in your bid packages.

Charles D. Allen, BSBME, MBA, is product manager, electrosurgical hardware, at Valleylab (Boulder, Colo.).

1. The term rms voltage means that the root mean square method is used to measure the effective value of the voltage over one complete cycle.