5. Surgical Diathermy

Diathermy is used widely in surgical practice, both for coagulation and for cutting, and relies on the heat generated as an electric current passes through a resistance that is concentrated in the probe itself

Heat generation is proportional to the power that is developed, typically 50–400 W. Heat is proportional to I2 (current)/A (area).

high-frequency sine waveform is used for cutting, typically 0.5 MHz

damped waveform is used for coagulation, typically 1.0–1.5 MHz.

High frequency is necessary because muscle is very sensitive to DC and to AC at low frequencies. Mains frequency is low, at 50 Hz, which is a frequency that is particularly efficient at precipitating VF. Very high-frequency current has minimal tissue penetration and passes across the myocardium without ill effect.

two connections to the patient; the neutral (or indifferent) patient plate, and the active coagulation or cutting electrode Current passes through both, but the current density at the active electrode is very high and generates high temperatures. current density is dispersed over a wide area and heating does not occur patient plate, and hence the patient, is kept at earth potential, which reduces the risks of capacitor linkage incorporate isolating capacitors to minimize the problem. An alternative is to use an earth-free or floating circuit

The current is localized to the instrument; it passes only from one blade of the forceps to the other. Bipolar diathermy uses low power, and this limits its efficacy in the coagulation of all but small vessels. The circuit is not earthed.

• thermal injury a - Electrode b - metal contact site c - prothesis

at the site of the indifferent electrode (the diathermy plate) which must be in close and even contact with a large area of skin, ideally an area that is well perfused and so which will dissipate heat. Adhesive and conductive gels are useful. If the area of contact is small, the current density increases to the point at which a burn is probable may occur if the plate is detached or malpositioned. The diathermy current may flow to earth through any point at which the patient is touching metal (such as the operating table, lithotomy poles or ECG electrodes) and cause a burn. The plate should not be placed over an area where there is a metal prosthesis in place (usually the hip). Metal has a low resistance in comparison with tissue, and so the current will flow preferentially through the prosthesis, generating a potentially dangerous current density.

Alcoholic skin preparation solutions have ignited after diathermy activation completed via a route that does not include the indifferent electrode; this may also result in a burn interfere with monitoring devices. This problem can be minimized by the use of electrical filters Diathermy may lead to ischaemia and infarction of structures supplied by fine endarteries. Classic examples include the penis (hence unipolar diathermy must be avoided in circumcision) and the testis, which has a vulnerable vascular pedicle

The indifferent electrode should be sited as far distant as possible from the pacemaker, and bipolar diathermy should be used wherever possible. If the use of unipolar diathermy is unavoidable, it should be deployed in short bursts. Cutting diathermy causes more of a problem than coagulation. Implantable cardiac defibrillators (ICDs) are increasingly common. In emergency surgery these can be protected from the effects of surgical diathermy by the application of a magnet which will deactivate the ICD but not the pacing function.