Shortwave or Microwave Diathermy: A Fire Hazard?
FAQ [Health Devices Jun 1983;12(8):197]
The cover material of a couch was burned
during shortwave diathermy treatment in which the cables were positioned close
to the couch. When we duplicated the effect in our biomedical engineering
laboratory, a hole was burned through the material in about three minutes. No
flame was observed in either case. Does this represent a fire hazard and does it
indicate defective equipment?
In shortwave and microwave diathermy
treatment, tissue heating occurs as electromagnetic energy, induced by or
radiated from the diathermy head, is absorbed in the electrically conductive
tissue and converted to heat. In most shortwave diathermy units, a significant
radio-frequency field exists around the cables that carry electrical energy from
the generator to the applicator head. (Microwave diathermy units usually have
well-shielded leads between the generator and head, or the generator and
applicator head are integral, and there is normally little radiation except from
the applicator.) There is little or no heating of the air that surrounds the
cables since it absorbs little energy from the radio-frequency fields.
However, heating can occur if a conductive or partially
conductive material is located within the field produced by the cables or
applicator. The Canadian Bureau of Medical Devices has drawn attention to the
fact that certain plastics and synthetics (e.g., nylon, polyvinyl chloride, and
polyethylene terephthalate), which are usually regarded as good insulators, can
also be heated significantly by shortwave and microwave diathermy
units.(1,2) Some fabric blends that contain synthetics can also be heated. In contrast, two other synthetics widely used in medical practice, silicone and polytetrafluoroethylene (Teflon), are relatively unaffected in radio-frequency fields.
The amount of heating depends on a number of factors, including the output setting of the generator and the degree to which the radiated field (energy) is concentrated in a small area. A very concentrated field exists when a cable is located near a grounded or conductive object or when the cables are positioned close to each other.
This heating has been known to cause smoking and melting of some materials, but we know of only one report (verbal and undocumented) in the last ten years of a fire caused by radiated energy from a shortwave diathermy unit used for physical therapy applications. We attribute this low incidence of fire to general recognition of the need to keep diathermy cables properly spaced and positioned and to the likelihood that many of the materials close to the cables were nonconductive.
(Diathermy is also being investigated and used for cancer treatment. While this discussion is primarily limited to physical therapy applications, similar concerns may also apply in cancer treatment applications. We are also aware of one report of a fire during use of a specially designed microwave diathermy unit for cancer treatment.)
We do not consider shortwave and microwave diathermy units a significant fire risk, nor do we think it likely that a defective unit was responsible for the incident reported at the start of this article. However, the observations of the Canadian BMD (including laboratory tests that indicate that synthetics can be ignited by diathermy units) suggest a heightened vigilance regarding materials that, otherwise, might have been considered nonconducting and innocuous.
The following recommendations will minimize the risk of equipment damage and injury while further reducing any fire risk:.
keep cables spaced apart. Route cables as designed for the unit, using all
spacing insulators and cable supports provided. Don't put anything else
between the cables and never cross cables.
- Keep cables at least several inches away from any
objects or material, especially metal or grounded objects. Be wary that some
synthetics and plastics, though assumed to be nonconductive, may be heated
- Do not lean on or hold the cables while the
generator is activated. In addition to the strong heating effect, a
deteriorated cable could break down and expose the user to high voltages.
- Keep all line cords away from the diathermy unit
cables. Do not store or coil line cords where they can come close to the
cables on an operating diathermy unit.
- Before increasing generator output in response to a
report of inadequate patient heating, verify that cables are properly
routed, evenly spaced, and away from any metal or grounded objects. The
heating effect may be "stolen" from the patient at a spot where
the cables are close to a metal object or each other. Increasing the
generator output under these circumstances will cause increased heating at
- Use a nonconductive treatment table and a mattress
or couch without metal parts (e.g., decorative buttons, springs) near the
patient, applicators, or cables. Do not use conductive mattresses or
This article does not discuss the contraindications for diathermy treatment or precautions to avoid patient burns. In general, keep cables away from the patient. Some authors also caution against the use of diathermy around the eyes; this caution takes on greater significance in patients with implanted intraocular lenses or plastic contact lenses. Some additional discussion and precautions are included in our evaluation of shortwave diathermy units (see Health Devices, Vol. 8, p. 175).
- Health and Welfare Canada. Health Protection Branch. Guidelines for
microwave diathermy devices. Information Letter, No. 585;Sep 19, 1980.
- Fire hazard from shortwave and microwave diathermy units.
Alert-Medical Devices 1979, No. 24;Aug
- Diathermy Units, Microwave [11-245]
- Diathermy Units, Shortwave [11-246]