Deep Heat

Deep heat is produced when energy is converted into heat as it passes through body tissues. ^[1]  Energy sources include the following:

• High-frequency currents (shortwave diathermy)
• Very-high-frequency electromagnetic radiation (microwave diathermy)
• High-frequency sound (ultrasound diathermy)

The temperature distribution in the tissues heated by any of these modalities results from the pattern of relative heating, which is the amount of energy converted to heat at any given location. The practitioner should choose a heating modality that produces the highest temperature at the site of concern without exceeding the temperature tolerance at the affected site or in tissues above or below that site. The temperature rise depends on the properties of the tissue, including the specific heat, thermal conductivity, and the length of time for which the heating modality is applied.

The temperature rise and distribution of heat that are associated with these modalities are superimposed on the physiologic temperature distribution in the tissues prior to diathermy application. Usually, the temperature is low at the skin surface and higher at the core.

The physiologic effects of temperature occur at the site of the application and in distant tissue. The local effects are caused by the elevated temperature response of cellular function by direct and reflex action. Locally, there is a rise in blood flow with associated capillary dilatation and increased capillary permeability. Initial tissue metabolism increases, and there may be changes in the pain threshold. Distant changes from the heated target location include reflex vasodilatation and a reduction in muscle spasm (as a result of skeletal muscle relaxation).

Vigorous heating produces the highest temperature at the site where the therapeutic result is desired. The tissue undergoes a rapid temperature rise, with the temperature coming close to the tolerance level. Vigorous heating is used for chronic conditions that require deep structures, such as large joints, to be heated. When acute inflammatory processes are occurring, deep heating requires extreme care, because it can obscure inflammation.

Local tissue temperature is maintained during mild heating, the primary effect being the production of a higher temperature at a site distant from the heating modality's application. Reflex vasodilatation occurs when the rise in temperature is slow for short periods, such as during a subacute process. With the proper application, superficial and deep heating methods can accomplish mild heating.

Shortwave diathermy

The best method for large-area deep heating is shortwave diathermy. This modality is useful for various indications.

The following problems can be treated with shortwave diathermy, depending on the individual condition of each patient and the desired treatment goals ^{[2, 3, 4, 5]} :

• Localized musculoskeletal pain
• Inflammation (joint or tissue) ^[6]
• Pain/spasm
• Sprains/strains
• Tendonitis
• Tenosynovitis
• Bursitis
• Rheumatoid arthritis ^[6]
• Periostitis
• Capsulitis

Microwave diathermy

Because microwave diathermy selectively heats muscles and deep heat improves the flexibility of collagen tissues, muscle contractures can be treated with this modality (in combination with a physical therapy stretching program). ^[7]

Microwave diathermy can also be used to reduce secondary muscle spasm under a trigger point.

In addition, this modality can effectively treat the superficial joints of the hands, feet, and wrist because of the thin soft-tissue layer overlying these joints.

Ultrasound diathermy

Therapeutic use of ultrasound is ideal for providing deep heat to large joints. For example, it is effective in treating the shoulder or hip, because a standing wave is produced as a result of the curved reflection of the glenoid or the acetabulum; this effect concentrates heat energy at the articular surfaces of the joint. ^[8]

In combination with a physical therapy program utilizing range of motion (ROM) and stretching activities, the localized, intra-articular heating produced by ultrasound diathermy greatly facilitates the mobilization of joint adhesions or capsular restrictions caused by tightness or scarring. This modality is also used to treat osteoarthritis, tendinitis, and bursitis. ^[9] Ultrasound-based deep heating has not been found to be effective for the preventive management of posteccentric exercise, delayed-onset muscle soreness. ^[10]

Most of the indications for ultrasound diathermy are similar to those for other deep heating modalities. Additional indications include the following:

• Joint contracture
• Joint adhesions
• Calcific bursitis
• Hematoma resolution
• Neuromas

Other conditions that may be treated with ultrasound, though with limited therapeutic benefit, include the following:

• Fibrosis
• Phantom limb pain
• Myofascial pain
• Reflex vasodilatation
• Ulcer debridement

Shortwave diathermy

Shortwave diathermy has the following precautions or contraindications:

• Malignancy
• Sensory loss
• Tuberculosis
• Metallic implants or foreign bodies
• Pregnancy
• Application over moist dressings
• Ischemic areas or arteriosclerosis
• Thromboangiitis obliterans
• Phlebitis
• Cardiac pacemakers
• Contact lenses
• Metal-containing intrauterine contraceptive devices
• Metal in contact with skin (eg, watches, belt buckles, jewelry)
• Use over epiphyseal areas of developing bones
• Active menses
• In addition, extreme care must be used with pediatric or geriatric patients.

The literature is not clear on the amount of heating that occurs in the case of metal surgical clips; in addition, the effect of shortwave diathermy on actual bony growth plates is not known with certainty. The most common complication of shortwave diathermy is the development of burns, which may be caused by a number of factors, including the following:

• Faulty equipment
• Improper technique
• Inadequate patient supervision
• Inappropriate positioning of the patient

Microwave diathermy

The previously mentioned contraindications for shortwave diathermy also apply to microwave diathermy.

Additional precautions include synovitis with joint effusion, systemic/local infection, and use over bony prominences.

Ultrasound diathermy

Precautions to be noted in the use of ultrasound diathermy are the same as those for most other deep heating modalities. Although the literature for physical therapy and for physical medicine has differing opinions, therapeutic ultrasound can be used over metal implants with caution and with constant motion of the device head. 

Additional contraindications include conditions in which the application of deep heat would require direct exposure of the eye, pregnant uterus, spine, laminectomy sites, brain, heart, or known ischemic areas, which can result in detrimental cavitation and heating of those tissues. ^[11]