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WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
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ITG SUBJECT: DIATHERMY
Introduction: Recent investigations involving medical devices have brought increased attention to medical diathermy equipment. This ITG has been written to give those who are unfamiliar with high frequency electrical therapy equipment a fundamental knowledge of diathermy theory.
Diathermy is the controlled production of "deep heating" beneath the skin in the subcutaneous tissues, deep muscles and joints for therapeutic purposes. There are basically two types of diathermy devices on the market today: radio or high frequency and microwave. Ultrasonic or ultrasound therapy is also a form of diathermy, and is sometimes combined with electrical stimulation. Radio frequency (r.f.) diathermy is assigned an operating frequency of 27.12MH Z (short wave) by the Federal Communications Commission. Older radio frequency units were assigned an operating frequency of 13.56MH Z. Microwave diathermy is assigned 915MH Z and 2450MH Z as operating frequencies (these are also Microwave oven frequencies).
The present informal position of the Food and Drug Administration is that a diathermy device should be capable of producing heat in tissue from a minimum of 104 F to a maximum of 114 F at a depth of two inches in not more than 20 minutes. When diathermy equipment is utilized, the power output is maintained below the pain threshhold of the patient.
There are basically two methods of applying high or radio frequency diathermy - Dielectric and Inductive.
- Dielectric - When dielectric coupled diathermy is used, a rapidly alternating voltage differential is created between two electrodes producing a rapidly alternating electric field between the electrodes. The electrodes are placed either one on each side or both on the same side of the part of the body to be treated so that the electric field penetrates the tissues of the concerned area of the body. Because of the electrical charges within the tissue molecules, the tissue molecules will try to align themselves with the rapidly changing electric field. This rapid movement, or alternation, of the molecules, causing friction or collisions with other molecules, produces heat in the tissues. The electric field strength is determined by the degree of difference in potential between the electrodes set by the unit power control. Since frequency is not varied, the average power output determines the intensity of heating. The electrodes are usually small metal plates mounted in cushion like enclosures, but may be made of a flexible material such as wire mesh so that they may be contoured to fit a particular portion of the body.
- Inductive - In Inductive coupled r.f. diathermy, high frequency current is generated through a coil to produce a rapidly reversing magnetic field. The coil is normally wound inside an applicator attached to the diathermy unit by an adjustable arm. The applicator is made in various forms for ease of application to the concerned area and is positioned directly over or next to the area to be treated. The rapidly reversing magnetic field induces circulating currents and electric fields into the body tissues, producing heat in the tissues. Induction coupling is generally employed at the lower r.f. diathermy region. Intensity of heating is again determined by average power output.
It is the opinion of FDA and the consensus of experts that pulsing the output of r.f. diathermy (as opposed to continuous wave) produces no extra beneficial therapeutic effects. Any physiological responses produced by pulsed r.f. diathermy are attributable to heat produced by the average power output.
Microwave diathermy produces heat in the body tissues through application of microwave energy to the concerned area. The microwave energy stimulates tissue molecules, as r.f. diathermy does, converting electrical energy to heat. Microwave energy is beamed from a director or applicator to the treatment area with intensity of heating controlled by the average power output of the microwave source and the spacing of the applicator from the concerned area. Microwave diathermy applicator size and shape is normally determined by the desired directivity and amount of tissue area to be treated.
When ultrasonic diathermy is used, a coupling oil or cream is applied to the concerned area. The ultrasonic applicator is then brought in contact with the cream which acts as a coupling medium for the ultrasonic energy between the transducer and the body. For an irregular surface such as a hand or foot, the limb and the applicator are immersed in water which readily transmits the energy to the tissues. Ultrasonic diathermy must operate at a frequency within the 800 to 1,000KH Z range. Power output must be plane wave (unfocused) and not exceed 3 watts per square centimeter. This does not apply to the experimental focussed ultrasound surgery devices or new ultrasound diagnostic devices.
Diathermy electromagnetic energy can propagate by radiation or other coupling through air, tissue or current conductors to effect the operation of sensitive electronic equipment. Diathermy electromagnetic fields may induce currents that cause excessive heating of metal devices in the body, e.g., bone pins, implanted electrodes, dental fillings and metal sutures, producing burns in the adjacent tissues \1\. A patient under treatment with r.f. diathermy is in a strong electrical field and is a conductor at some voltage above electrical ground. If this patient touches a bare metal part of the diathermy cabinet or any other electrically grounded part, he can receive a shock or burn \1\.
\1\ High - Frequency Electrical Equipment in Hospitals; 1970, NFPA No. 76CM, Part III, Section 31, National Fire Protection Association, Boston, Massachusetts