Shortwave diathermy is a high-frequency electrical current that produces deep tissue heating. Similar to therapeutic ultrasound, shortwave diathermy is delivered in pulsed or continuous output modes to produce thermal and nonthermal effects. Also refer to the basic physiological responses to heat described in Chapter 5.
• Shortwave diathermy (SWD) uses oscillating highfrequency non-ionizing electromagnetic energy (similar to broadcast radio waves) to produce deep heat within the tissues. The electromagnetic energy lacks the wavelength duration needed to depolarize motor or sensory nerves. The Federal Communications Commission (FCC) has reserved the frequencies of 13.56, 27.12, and 40.68 MHz for medical use, with the 27.12-MHz (wavelength of 11 m) frequency being the most common.124 Another type of radiofrequency deep heating modality, microwave diathermy (MWD), is not used clinically in the United States (Box 9-1).
Box 9-1. MICROWAVE DIATHERMY
Microwave diathermy (MWD) is a deep-heating modality that uses a magnetron to produce high-frequency electromagnetic energy that is converted into heat within the body. The FCC has reserved 915 Hz and 2450 Hz for the medical use of microwave diathermy. Although microwave diathermy is similar to SWD, there are differences between the two.
Electrical fields are predominant with microwave diathermy, in contrast to the magnetic fields that predominate in SWD. Heating occurs through a dipole response created within the cell membrane. The rotation of these molecules causes friction, resulting in heat production. Because of the spreading of the radio waves and absorption of the energy, superficial tissues tend to be heated more than deeper tissues. Although microwave diathermy produces biophysical effects similar to those of SWD, the treatment is more superficial because the microwave radiation cannot penetrate the fat layer to the same extent as shortwave radiation. Because the energy is collected by the adipose tissue, the effects occur at about one-third the depth of SWD effects, but the energy is reflected at tissue interfaces, creating standing waves that can result in unsafe increases in tissue temperatures.
The indications and contraindications for the use of microwave diathermy are similar to those for SWD. However, there can be no metal within the treatment field (4 feet from the pads, drums, or coils). This includes not only metal on the patient but implanted metal (e.g., plates, screws, intrauterine devices •) as well.
Microwave diathermy is not commercially available in the United States, partially because the energy tends to be reflected and scattered into the surrounding environment and has been associated with an unacceptably high incidence of miscarriages among female therapists who regularly operate these units.125
The thermal effects of SWD are similar to those described in Chapter 5, but occur deeper in the tissues. Shortwave diathermy can also be applied in a nonthermal mode. The thermal and nonthermal effects are similar to therapeutic ultrasound, but SWD does not reflect from bone and is less likely to create hotspots (Table 9-1).124...