Technical Aspects and Safety of Capacitive Coupling

Electrosurgery is the passage of radio frequency current through tissue to create a desired surgical effect, such as coagulation or cutting. The use of radio frequency (RF) energy (above 100 kHz) prevents neuromuscular stimulation while still providing excellent tissue heating. Unfortunately, clinicians often do not fully understand the potentially negative consequences of using radio frequency energy. These include RF leakage current and high levels of capacitively coupled current that can cause alternate site burns. In order to prevent capacitive coupling, it is important to understand how capacitors are formed in the patient circuit, how capacitively coupled current occurs and what preventative measures can be taken to reduce the likelihood of patient injury.

What is a capacitor?
A capacitor is an electric component that is capable of storing an electrical charge. It is formed by two conductive plates that are separated by an insulating material called a dielectric. Unlike a resistor that dissipates energy, capacitors store energy and return it to the circuit in which they are connected. The amount of energy a capacitor can store is measured in Farads. The formula for calculating capacitance is:

K = the dielectric constant of the insulator, Eo = a constant called the permittivity of free space and is equal to 8.55 x 10-12 Farads/meter, A = the area of the plates in square meters, and D = the distance between the plates in meters.

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