Two main types of current are provided, one for cutting and one for coagulation. The optimal cutting current is a continuous wave output from the electrosurgical unit. For smooth cutting a continuous arc is required between the active probe and the patient. Upon application of a high power continuous wave arc, the tissue cells volatize resulting in a smooth cutting action as the probe is moved along the surface of the tissue. To introduce hemostasis, the cutting current wave form is pulsed. The lower the duty cycle, the greater will be the amount of hemostasis and the less the cutting effect. Duty cycle is defined as the ratio of pulse on time to duration of the total pulse period times 100%. For effective coagulation a current with a duty cycle of approximately 20% to less than 5% is required. The longer off-time with a low duty cycle allows the tissue to cool off, so as to avoid volatization of cells, but enough power must be applied to sear off exposed blood vessels.
Both electrodes are available in various configurations to be selected by the surgeon according to the intended use. The active probe selected by the surgeon can range in size from a pair of forceps or a knife blade to a fine needle. The contact area of the probe and the type of tissue encountered are factors determining the amount of power necessary to effectively cut or coagulate the blood vessels contigous to the operating situs.
Electrosurgical units have previously used either spark gap or vacuum tube methods to achieve radio frequency levels of several hundred watts. For many years the generator used for producing a coagulation current was a spark gap type of generator. A spark gap oscillator can generate large peak powers at a low duty cycle while maintaining about 120 watts of average power. Spark gap methods, however, generate while noise whereas spectrum purity is desirable with electrosurgical units, particularly since electronic equipment is becoming more prevalent in hospitals. Vacuum tube units are capable of generating a power output of several hundred watts in the megahertz range, but, they generally also operate at low efficiency and have low reliability compared to presently available solid state circuitry. With the advent of solid state units it has been found that presently available transistors cannot generate the large amounts of peak power required under some conditions. Hence, so the duty cycle had to be increased to allow for adequate power, but, the larger duty cycle introduced a cutting effect in the coagulation mode. To minimize the cutting effect in the coagulation mode, a low duty cycle is required.
The amount of power required varies depending upon whether the active probe is arcing or in physical contact with the tissue and is also dependent upon the effective current density at the operating site, as determined by the contact area of the probe. All electrosurgical units on the market today employ amplitude control to vary the amount of coagulation power. Since a low duty cycle results in less cutting effect it would, therefore, be desirable to vary the duty cycle of electrosurgical units in response to load conditions as opposed to varying the amplitude control.