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Localized heating of tissue without muscle twitch or spasm can be effected by passing a large alternating current of high frequency through parts of the body. This is surgical diathermy or electrosurgery. If the power is sufficient, the temperature reached causes coagulation of the blood or even disintegration of the tissue due to boiling.
This technique is employed in a controlled manner during surgery to seal small blood vessels by coagulation, and to cut through layers of tissue. Cutting and coagulation is achieved by applying electric current to the tissues via small hand-held probes (electrodes). The current flows out of the body (usually) through a very large electrode placed on the skin at some remote site (e.g. on the thigh). At this electrode (the indifferent electrode) the current density is very low, so little heating occurs. However, at the hand-held electrode the density is very high due to its small contact area, and so great heat is developed.
Diathermy currents may be generated by valve or transistor circuits and may include a range of protection and monitoring systems. The frequencies used are typically in the range 0.5 to 3 MHz and so it is difficult to achieve true earth-free operation due to the capacitance between the patient and earth, and the capacitance of the leads of monitoring equipment. Most versions have power output up to a few hundred watts, with slightly higher power necessary for urology applications where the surgery often takes place under water (in the bladder). Low power versions exist, sometimes called hyfrecators, for excision of warts, polyps, and skin flaps.
The system described above is monopolar, whereas bipolar systems are available in which the heating currents only flow between the two tips of special forceps. These are useful for fine work and when the patient has a pacemaker which might malfunction in the presence of the very large circulating high- frequency current.
Content and Design Copyright 2000 Dr. Malcolm C Brown. See Title Page for more details