As discussed in Chapter 4, when a voltage is applied across a conductor, a current will flow. If the voltage is applied across the body via suitable electrodes the body becomes part of the circuit and a current will also flow, the magnitude depending on the properties of the tissues in its path, particularly the resistance. This current can cause heating or other physiological effects, depending on the frequency of the driving voltage. The effects of the domestic mains current flowing through the body was discussed in Chapter 6, but different effects occur as the frequency of the voltage is increased. As the frequency goes up, the heating increases but the tissue stimulation decreases and, at frequencies above 100 kHz (i.e. radio frequencies), the effect is entirely heating. This heating effect in the body by electric current is called diathermy, but the location, concentration and how this heat is used is dependant on the electrode design and the current concentration or current density at any point in the circuit. For a certain applied voltage, the average current throughout the circuit will be the same. The current density is the current per unit area, and so if the material in which the current passes is smaller, the heating effect increases. The resistance of the material is proportional to its size, so as the material becomes smaller then its resistance gets larger. The heating power is the product of the current squared and the resistance (power = I2 × R). Surgical diathermy (or electrosurgery) is where either one or both of the electrodes are very small, and it is used to cut and coagulate tissue. The smaller electrode can be made into a pointed surgical tool and localised heating will occur at the tip of the instrument. The smaller and more pointed the instrument is, the greater the current density will be at the tip. This electrode is classified as the active or live one. The current densities around this electrode can be as much as 10 A cm−2, and the total heating power typically around 200 W.
Keywords: coagulation diathermy, current density, cutting diathermy, desiccation diathermy, diathermy, fulguration, interference, diathermy as source, monopolar electrodes, diathermy, passive electrode, diathermy
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