Deep Heat Technique

Updated: Dec 13, 2022
  • Author: Milton J Klein, DO, MBA; Chief Editor: Dean H Hommer, MD  more...
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Shortwave Diathermy

Shortwave diathermy involves the therapeutic application of high-radiofrequency (usually 27.12 MHz [λ = 11.06 m]) electrical currents. Hyperemia, sedation, and analgesia are the basic physiologic effects. [12, 13]  The reduction in muscle spasm resulting from muscle relaxation is caused by an increased vascular supply to the treated area. A transverse technique is applied to treat a larger anatomic area, with the primary concentration at the midpoint between electrodes.

Proper application and tuning are required for this modality. The patient's electrical impedance becomes part of the impedance of the patient's own circuit. The patient's circuit must be set to resonance, so the patient's circuit frequency is equal to that of the machine. The patient should feel only a comfortable heat.

For therapeutic benefit, the tissue temperature should be elevated to between 40º and 45°C. Continuous supervision and observation of the patient are required. The treatment time is usually 20-30 minutes. At clinically relevant energies, shortwave diathermy can increase subcutaneous fat temperature by 15°C and muscle temperature by 4-6°C at a depth of 4-5 cm. Patients should be placed on a wooden table or chair when shortwave diathermy is applied.

One means of applying shortwave diathermy is through the condenser method. In this, the treatment site is placed between two electrodes functioning as capacitor plates. Monitoring of patient movement is required, because movement can affect the amplitude of the heat concentration being applied. Another technique, the inductive coil method, involves coil applicators that selectively heat superficial musculature (unless these applicators are used on joints with minimal overlying soft tissue, resulting in selective heating of the joint).

Inductively coupled units use induced eddy currents to heat tissue, especially tissue, such as muscle, with high water content. Units joined to provide aggregate capacity use electrical fields to heat tissue with low water content, such as fat. Self-adjusting resonators minimize the positioning effect.

Felt or plastic spacers should be used with the condenser method. When the condenser or inductive coil method is applied, a towel should be used to absorb perspiration, thereby avoiding localized heat concentration. The patient must be instructed to remain motionless. The output of the machine should be adjusted to a desired level so that movement does not change the impedance circuit and increase current flow (which would mean a greater risk of a dose increase and resultant burns). The shortwave diathermy unit should be tuned to low power as per patient tolerance, and the meter readings should be properly documented. Heating localization depends on the coupling of radio waves to the patient.


Microwave Diathermy

Microwave diathermy, which employs electromagnetic radiation delivered at frequencies substantially higher than those seen with shortwave diathermy, is another deep heat modality that selectively heats tissues with high water concentration. [14, 15]  Hyperemia, sedation, and analgesia are the physiologic effects, similar to those of shortwave diathermy. [12, 16]  Secondary local vascular dilatation results in increased local metabolism.

The two frequencies designated for microwave diathermy are 915 MHz and 2456 MHz. The lower frequency is preferred and more commonly used because it provides selective heat deep into muscle and because less energy is converted to heat in the subcutaneous fat. [15]

Because the frequencies are higher than those used in shortwave diathermy and the wavelengths are the same size as the applicator, microwave diathermy can be focused more easily than shortwave diathermy can. Direct contact applicators with full aperture skin contact are optimal for improved coupling and reduction of stray radiation.

A microwave director is used to aim the microwaves at the area of treatment, allowing observation of the treatment site. Heat can be reduced by increasing the distance between the microwave director and the treatment site. As with shortwave diathermy, microwave diathermy can result in hot spots and burns; these can occur secondary to localized perspiration associated with selective heating of the treatment zone. The microwave diathermy equipment should be adjusted to provide comfortable heating, with treatment time in the range of 20-30 minutes.


Ultrasound Diathermy

Ultrasound diathermy is a deep heating modality that uses high-frequency acoustic vibrations. The frequencies employed are above the human audible spectrum (ie, higher than 17 kHz), typically in the range of 0.8-1.0 MHz. Energy is generated via the piezoelectric effect; electrical energy is then applied to a crystal, causing the crystal to vibrate at a high frequency and thereby produce ultrasound. Ultrasound is delivered by continuous or pulsed waves (the goal being to produce nonthermal effects, such as streaming and cavitation) and provides a high heating intensity. [17, 18]

Ultrasound energy is absorbed and transformed into heat energy as it propagates through tissue. The therapeutic dose is computed by the power output (total W) and the size of the ultrasound head/trasnducer. The usual initial dose is 1 W/cm2 and is adjusted to patient tolerance, as well as to the treatment goals. The practitioner must select the wave form (continuous or pulsed), intensity, and duration.

The patient should experience a comfortable heating or no sensation at all. The treatment time is 5-10 minutes, taking into account the patient's tolerance and comfort. After the skin is cleansed, a coupling agent, such as an ultrasound gel, is required to provide effective conduction between the head/transducer and the skin surface. To avoid hot spots, the device head must be continuously moved over the treatment site.

Ultrasound diathermy produces the following biologic effects:

  • Temporary analgesia [12, 19, 9]
  • Increased peripheral blood flow
  • Increased vascularity with associated hyperemia/inflammatory response [12]
  • Increased cell membrane permeability
  • Peripheral nerve conduction changes (reversible conduction block with high-intensity ultrasound exposure)
  • Relief of muscle spasms [9]

The following factors influence the propagation of ultrasound in biologic tissue:

  • Transmission
  • Absorption
  • Refraction
  • Reflection

As mentioned above, the nonthermal effects of ultrasound diathermy include cavitation, which disrupts chemical and cellular bonds, thereby assisting with the treatment of fibrous tissue, scar tissue, and joint capsule adhesions. A therapeutic application of ultrasound known as phonophoresis is used to diffuse a topical medication (eg, a steroid, analgesic, or anesthetic in a gel) into the subcutaneous tissue. [19]