Common Electrode Configurations For Ground-return-pad Devices
There are several commonly used electrode configurations or circuit topologies:
In bipolar configuration the voltage is applied to the patient using a pair of similarly-sized electrodes. For example, special forceps, with one tine connected to one pole of the AC generator and the other tine connected to the other pole of the generator. When a piece of tissue is held by the forceps, a high frequency electric current flows from one to the other forceps tine, heating the intervening tissue.
In monopolar configuration the patient is attached to the return electrode, a relatively large metal plate or a flexible metalized plastic pad which is connected to the return electrode of the AC source. The surgeon uses a pointed electrode to make contact with the tissue. The electric current flows from the active electrode, through the body to the return electrode, and then back to the electrosurgical generator. Since electric current spreads from the pointed electrode as it enters the body the current density is rapidly (quadratically) decreasing with distance from the electrode. Since the rate of heating is proportional to the square of current density, the heating occurs in a very localized region, only near the probe tip. On an extremity such as a finger, there is limited cross-sectional area for the return current to spread across, which might result in higher current density and some heating throughout the volume of the extremity.
There is also a common intermediate configuration, when both electrodes are located on the same probe, but the return electrode is much larger than the active one. Since current density is higher in front of the smaller electrode, the heating and associated tissue effects take place only (or primarily) in front of the active electrode, and exact position of the return electrode on tissue is not critical. Sometimes such configuration is called sesquipolar, even though the origin of this term in Latin (sesqui) means a ratio of 1.5.
Read more about this topic: Electrosurgery
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