The word diathermy means through heating. Surgical diathermy is a technique that allows bleeding from small blood vessels to be arrested without the need for mechanical clips or sutures and enables tissues to be cut without the use of a knife, with sealing of small blood vessels at the same time. It is carried out by the application of heat to small areas of tissue in a highly controlled way. In microsurgery the heat may be applied with a needle which has been preheated in a flame (for example in ophthalmic surgery, or by applying a piece of resistance wire through which an electric current is passed (as in cautery of the nasal septum). However the heat is usually generated by passing a radiofrequency electric current through the tissues themselves.

Animal tissue is a conductor of electricity but has a considerable resistivity. Resistivity is the intrinsic property of a material to resist the passage of electricity: the higher the resistivity, the poorer a conductor and vice versa. When an electric current is passed through a poor conductor energy is lost in the form of heat and the temperature of the material increases. The amount of heat produced depends upon the current (amperes) and the resistance of the tissue (ohms). The heating power developed may be expressed as Equation 2

(where W = power (W), R = resistance (&Ohgr;), and I = current). One watt of power is converted into 1 J/s heat. Surgical diathermy uses 30 to 400 watts of power, depending upon the degree of heating required.

Under normal circumstances, passing the required amount of current through the body, either as direct current or at conventional mains frequency (50 Hz in the United Kingdom and Europe, 60 Hz in North America) would cause severe muscle spasm, intense pain, and fatal cardiac arrhythmias. However as frequency is increased, more current may be passed through the body with fewer of these effects, allowing the use of higher currents and, therefore, higher temperatures (Fig. 1) 44. For this reason, surgical diathermy uses radiofrequency currents in the range 0.5 to 1.5 MHz.

Application of radiofrequency current across living tissue would cause heating of the whole volume. It is therefore necessary to limit the amount of tissue heated to avoid damaging the surrounding mass. This goal is achieved ensuring a high current density in the volume of tissue that requires diathermy and a very low current density in all other tissues. The same heating effect will occur along all of the current pathway but the temperature will only rise significantly where the current density is high. Where there is a low current density the rise in temperature will be totally dissipated by the large volume of surrounding tissues. An understanding of the concept of current density is vital to the safe use of radiofrequency surgical diathermy.

There are two ways of delivering a high current density to a localized area: monopolar and bipolar diathermy. With the monopolar technique the current is passed through a large volume of tissue (Fig. 2) 45 from an ‘indifferent’ or ‘plate’ electrode of comparatively large surface area which is in good electrical contact with a large area of the body. The current then passes through an active electrode of very small contact surface, which is under the control of the surgeon. A very low current density is therefore passed through most of the body, but at the point of contact between the active electrode and the tissues the current density is very high, and therefore has a large heating effect.

Bipolar surgical diathermy involves the current being passed between two point electrodes, in the form of insulated forceps limbs placed immediately adjacent to the tissue to be heated (Fig. 3) 46. A very high current density, and hence a high heating effect, is produced over a very small volume of tissue, with virtually no heat generated elsewhere in the body. Bipolar diathermy can only be used with relatively low currents and is only suitable for the coagulation of small blood vessels. Its greatest application is in microsurgery, especially of the hand, and in neurosurgery.

Radiofrequency surgical diathermy generators are basically continuous wave radio transmitters without antennae. The earliest types consisted of a spark-gap and a tuned circuit supplied with electricity. Production of an arc across the gap caused the circuit to oscillate at its resonant frequency for as long as current was supplied to it. With the advent of vacuum valves (radio tubes) the quality of the diathermy current improved and it became more controllable. Such valves are expensive and large, requiring power supplies for their filaments as well as high tension for the output current, and are less reliable than the semiconductors which have superseded them.

Modern diathermy generators employ high-power, high-frequency transistors which provide an output equivalent to that available from valve generators. Although the main part of the generator is a high-powered radiofrequency oscillator, semiconductor electronics and microprocessors are used to control the oscillator, monitor its performance, and to monitor safety, especially in regard to the connection of the indifferent electrode.

The waveforms of the radiofrequency currents used in surgical diathermy have been chosen empirically. Tissue to be cut has to be heated very rapidly, but only in close proximity to the tip of the electrode. Cutting is achieved by striking an arc between the tip and the tissue, thus charring it: a continuous sinuosoidal waveform of high power is the most effective. Coagulation requires less heat applied over a slightly greater volume of tissue: this is best achieved by repetitive short bursts of a few cycles of the current. There are some applications, such as transurethral prostatectomy, where a combination of these two waveforms may be blended to achieve more widespread coagulation during cutting.

Many manufacturers produce machines which generate complex waveforms for special situations but their advantages are hard to prove.

Radiofrequency diathermy is most commonly used to coagulate small blood vessels, using appropriate forceps, and to cut tissues, using a fine pointed electrode. Both of these procedures may be carried out endoscopically using appropriate insulated electrodes. The most frequent applications for such techniques are the treatment or removal of bladder tumours and hypertrophic prostate tissue. This requires distension of the bladder with a non-conductive fluid, such as a glycine solution, and the use of a blended current.

Skin damage is probably the most common surgical problem associated with radiofrequency diathermy. It is caused either by coagulation of blood vessels close to the skin, or by inadvertent contact between a skin edge and a conducting instrument while treating deeper tissues.

Knowledge of the principle of current density obviates the disaster of inadvertent coagulation of pedicles. The classical cause of this problem is the use of monopolar diathermy on the testicle whilst it is supported in the surgeon's hand. The current density is then very high in the spermatic cord, which has a small cross-sectional area and is the sole current pathway between the rest of the body and the testis. Whenever monopolar diathermy is used in this situation, the testicle should remain in contact with the main bulk of the body with, if possible a saline-soaked swab to improve conduction. This disaster can be totally avoided by the use of bipolar diathermy.

Increased current density is also the cause of skin burns caused by poor contact between the indifferent ‘plate’ electrode and the skin. This, and also faults in the indifferent electrode lead, may cause skin burns at other sites as the current attempts to find other pathways to complete the circuit. The generator should have self-diagnosis systems for lead faults, the indifferent electrode must be carefully applied, and the patient's skin must be protected from contact with any other conductive material which may form an alternative current pathway.

There is an enormous variety of internal and external electronic cardiac pacemakers and one must assume that they may all be inhibited or even damaged by radiofrequency diathermy, unless the manufacturer's data states otherwise. If diathermy is vital then it is necessary to ensure that the pacemaker and its lead are not included in the maximum current pathway: bipolar diathermy is the safer method. There is also an increased risk of arrhythmias and even intracardiac burns due to leakage of current through leads attached to external pacemakers.

Explosion and fire are still risks during diathermy, despite the almost total disappearance of flammable anaesthetic agents. If flammable anaesthetic agents are still in use diathermy should ideally be avoided entirely, although it may be used if it is kept away from the zone of risk.

Factors associated with a high risk of fire include the use of inflammable skin preparation solutions, especially when there is a likelihood that some of the solution has flowed into skin folds or the perineal area and not been allowed to evaporate. Several such fires have required extensive plastic reconstruction of the groin and perineal area. The atmosphere around a patient, especially under the surgical drapes, during general anaesthesia has an increased concentration of oxygen. Because nitrous oxide supports combustion as well as, if not better than, oxygen, special care must be taken when diathermy is used in the oral cavity, where the concentration of oxygen/nitrous oxide is always elevated. Methane in obstructed bowel may detonate if struck by a diathermy spark, especially if mannitol has been used as a bowel evacuant.