DIATHERMY EQUIPMENT

Typical Number in Hospital: 20

Cost Bands: 4

References: 3,4

Diathermy means internal heating where the heat is generated by a direct conversion to heat of some other form of energy passing through the tissues. In the case of surgical diathermy as used in the operating theatre for cutting and coagulating tissue an electric current passes through the body and heat is generated by resistive loss. In short-wave or microwave diathermy as used in the physiotherapy department, heating is caused by the dielectric loss of the tissues when electromagnetic waves pass through (the same effect as used in microwave ovens). Diathermy can also be produced by the absorption of ultrasonic waves, and this effect is also employed in the physiotherapy department.

In the case of surgical diathermy, high-frequency electric current is made to pass through the body between two contact electrodes. High frequencies (0.5 to 3.0 MHz) are used because direct current, or low-frequency alternating current (e.g. the 50 Hz or 60Hz of the mains supply) would cause muscle spasm. Bipolar diathermy (less often used) causes the current to pass between the two arms of a special forceps so that tissue held between the tips is heated causing the death of the tissue and the cessation of bleeding. In the more conventional monopolar diathermy one electrode is large (e.g. 100 cm2) and attached to the skin whilst the other is a pointed or ball-tipped probe held in contact with the tissue to be cut or coagulated. This causes intense heating at the point of contact between the probe and the tissue since the current density is very high. Little or no heating is produced at the large electrode because the current density is low. The large and small electrodes are known as the indifferent (or dispersive), and active electrodes respectively. The current generators for surgical diathermy may be spark gap types (very old), valve (old) or transistor. Since the current generated is in the radio-frequency part of the electromagnetic spectrum they can cause widespread interference with other electronic equipment in or near the operating theatre (e.g. EEG equipment).

Diathermy equipment used in the physiotherapy department usually involves the production of an electrostatic field in the body from two large (e.g. 10 cm diameter) electrodes placed each side of the body and operating at 27 MHz. Other types direct microwave radiation at the skin to produce a more superficial heating. Both types cause mild internal heating from which the therapeutic effect is derived.

Ultrasound is absorbed by living tissues to a varying degree depending on the frequency employed (the higher the frequency the greater the proportion of energy absorbed), and the type of tissue. They are used with frequencies in the range 1-3 MHz and total power of 1-3 W to cause heating of tissue (particularly joint capsules) where the heating is said to speed repair processes and relieve inflammation. See also entries on Surgical diathermy, Short-wave and Microwave diathermy, and Ultrasonic diathermy.