SUPERFICIAL HEAT
In contrast to deep heating modalities, superficial heating modalities usually do not heat deep tissues, including muscles, because the subcutaneous layer of fat beneath the skin surface acts as a thermal insulator and inhibits heat transfer.
Additionally, increased cutaneous blood flow from superficial heating causes a cooling reaction as it removes the heat that is applied externally.
In general, the transfer of heat (whether the purpose is heating or cooling) often is classified into 3 general types of heat transfer: conduction, convection, and conversion.
They are characterized as follows:
Conductive heating - This is defined as heat transfer from one point to another without noticeable movement in the conducting medium. Typically, direct contact takes place between the heat source and the target tissues. Superficial heat is usually conductive heat (eg, hot water baths, hot packs, electric heating pads, warm compresses).
Convective heating - This form of heating is produced by the movement of the transferring heating medium, usually air or a fluid. Methods for providing convective superficial heat include Fluidotherapy, whirlpool, moist air baths, and hot air baths.
Conversion heating - This involves the conversion of one energy form (eg, light, sound) into another (heat). Superficial heat is produced by heat lamps or radiant light bakers, with heat being transferred when the conveying medium (light energy) is converted to heat energy at the skin surface.
Superficial Heat Modalities categorized by primary heat transfer:
v Conduction - Hot pack, paraffin bath
v Convection - Fluidotherapy, hydrotherapy, moist air
v Conversion - Radiant heat
Factors which determine extent of Physiologic Response to Heat:
level of the tissue temperature (usually 40-45 º C)
Duration of the tissue temperature increase
Rate of increase in the tissue temperature
Size of the area being treated
CONDUCTIVE HEATING
Conductive heating is usually a simple modality that can be taught to the patient for independent home use. The disadvantages of this modality include potential burns, difficulty with its application to regions exhibiting abnormalities (eg, the foot), and a skin-drying effect (with the exception of paraffin and water media).
Another complication that may occur is impairment of local vascular supply, due to the weight of the modality on the limb or the weight of the limb on the modality. This phenomenon also may be caused by an uneven distribution of pressure on the anatomical region that the medium contacts.
Hot packs or hydrocollator packs contain silicate gel in a cotton bag. These packs are placed in a hot water tank, which is thermostatically controlled at 71.1-79.4°C. The silicate gel absorbs a large quantity of water and has a high heat capacity.
Hot packs are applied over layers of towels for 20-30 minutes. Most of the heat transfer from the hot pack to the patient is by conduction. Increasing the towel thickness reduces the heat flow and produces an intentional slowing in the temperature rise.
Acceleration of heat transfer occurs if the hot pack leaks into the towel. The patient never should lie on the hot pack, as the body weight could squeeze hot water out of the pack into the towel and potentially cause a burn. The maximum skin temperature is obtained after 8 minutes, followed by a reduction in temperature due to increased blood flow.
Repeated application of hot packs may prolong the period of temperature elevation but does not alter the temperature distribution.
Forms of Hot Packs:
§ Kenny packs (vigorous, short-term stimulation/heating),
§ Rubber hot water bottles, and
§ Electric heating pads.
Heat transfer is enhanced if the pad is moist or if the heated part is wrapped in moist material.
Complications that can arise from using an electric heating pad include shock hazard if the device is not insulated properly and burns if the patient falls asleep lying on the pad. Heat output increases over time until equilibrium is achieved. Keep in mind that heat may be sufficiently analgesic to produce burns.
Indications for the application of hot packs may include painful muscle spasms, abdominal muscle cramping, menstrual cramps, and superficial thrombophlebitis.
Hospitalized patients receive circulated warm-fluid hot packs to minimize the potential for burns due to prolonged superficial heating application, a particular concern in sensory impaired or mentally challenged patients.
Chemical packs often are available in containers that, when properly manipulated, allow previously separated ingredients to mix, thereby producing an exothermic chemical reaction that causes heat production.
A Paraffin Bath is another form of conductive heating. Paraffin baths are particularly useful for contractures due to rheumatoid arthritis, burns, and progressive systemic sclerosis (scleroderma).
Paraffin usually is applied to the hands, arms, and feet. Paraffin wax is melted and mixed with liquid paraffin. For therapeutic use, the paraffin bath is maintained at the melting point of 51.7-54.4°C in an insulated, thermostatically controlled container.
The 2 application methods include (1) The Dip Method and (2) Immersion.
THE DIP METHOD
The hand or foot is placed in the liquid paraffin bath and withdrawn when a thin layer of warm, solid paraffin forms, becomes adherent, and covers the skin. The dipping procedure is repeated until a thick paraffin glove is formed.
The heat can be retained by wrapping with towels for 20 minutes; then, the cool, solid paraffin glove is peeled away and the paraffin is recycled. The dip method is a mild heat application because only a limited amount of heat is available for transfer to the skin.
IMMERSION
Alternatively, the body part is immersed in the paraffin bath for 20-30 minutes. The immersion method transfers heat not only from the solid paraffin block but also from the liquid paraffin bath itself. The heat transfer rate from the liquid paraffin bath to the skin is slowed as the solid paraffin glove provides a poor thermal conductor.
This modality represents a vigorous heat application, causing a significant increase in skin tissue temperature, up to 46°C, with a marked temperature decrease in the subcutaneous tissue. Water at the same temperature applied by the same method would be intolerable because of the high specific heat and thermal conductivity.
CONVECTIVE HEATING
Fluidotherapy is a form of convective heating that uses a bed of round, uniform, finely divided solids, such as glass beads, into which thermostatically controlled warm air is blown to generate a warm, semifluid mixture.
Part of the patient's hand, foot, or limb can be immersed for superficial heating. This technique applies dry heat, and the temperature is equivalent to the hot air that is blown into the bed of beads. The usual treatment temperature range is 45.6-47.8 º C.
Uses of Fluidotherapy may include pain relief in arthritic conditions of small joints, joint mobilization following trauma/mobility, and analgesia / sedation in young patients undergoing exercise programs with painful and contracted joints due to sickle cell anemia.
Hydrotherapy can include total immersion in a large hot tub or Hubbard tank. Whirlpool baths can be used for partial immersion of the upper or lower extremities. Because hydrotherapy also may be used in treating infected draining wounds, the equipment must be sterilized between uses.
The water is agitated, and the size of the tank determines the capacity (the entire body or just the upper or lower extremities). For total body immersion in water, the temperature should not exceed 40.6 º C. Partial immersion of a limb should have a maximum temperature of 46.1 º C. The treatment time is limited to 20-30 minutes each session.
As a precaution during total immersion of a patient in water temperatures over 37.8 º C, his/her oral temperature should be observed to prevent a rise of body core temperature. With total body immersion, heat loss occurs primarily through the head and neck; therefore, the heat regulatory mechanism is impaired significantly.
Total body immersion has a relaxing effect and may predispose the patient to hypotension due to peripheral blood pooling (secondary to vasodilatation of all 4 limbs).
Another convection modality is the moist air cabinet. Air saturated with water vapor at a controlled temperature is blown over the patient, causing superficial heating over a large area.
The temperature distribution in this modality provides heating of skin and superficial tissue. This modality is used most commonly for back muscle spasms and polyarticular arthritic conditions. The recommended temperature maximum is 40.6 º C.
Contrast baths provide a method of therapeutic hyperemia for management of rheumatoid arthritis or sympathetically mediated pain (eg, rheumatoid arthritis of distal joints, hands, feet; prolonged ankle swelling after an ankle sprain/strain in refractory joint effusions).
A differential of approximately 25°C exists between the hot and cold water. The hot water is at a temperature of 40.6-43.3°C. The cold water temperature is maintained at 15-20°C. The greatest hyperemia response is produced by a 10-minute hot water immersion followed by cold water for 1 minute.
The cycle continues with hot water immersion for 4 minutes and cold water for 1 minute; this 4:1 cycle is repeated for a total of 30 minutes at each physical therapy appointment or during each home-based self-treatment session.
Other approaches to convective heating include water-based exercise and spa therapy (balneotherapy).
CONVERSION HEATING
Radiant heat therapy is a type of conversion heating; high-energy photons penetrate the tissues, and this energy is converted into heat. Because photons of longer wavelengths process less energy, penetration is more superficial; shorter wavelengths have a greater therapeutic benefit.
The therapeutic, radiant heat – producing temperature rise in tissues ranges from the spectrum of far infrared to visible yellow. Longer wavelengths of light, from green to ultraviolet, produce photochemical reactions that do not significantly raise tissue temperature.
Most other commercially available radiant light sources produce infrared with some visible light. These lamps contain heating elements of Carborundum (silicon carbide), special quartz tubes, or metal alloys. The higher-energy photons are produced by shorter-wavelength radiant heat, resulting in a greater penetration of superficial tissue.
INDICATIONS:
§ Muscle spasms from underlying joint/skeletal conditions,
§ Rheumatic joints in which direct heating of the joint is contraindicated, and
§ The treatment of superficial skin breakdown in the intertriginous areas.
A treatment time of 20-30 minutes is recommended, with the maximum effect occurring at a minimum of 20 minutes. The radiant energy source is positioned 15-24 inches (38.1-61 cm) from the treatment site.
The intensity is controlled by the light source, distance, type/quality of reflector, and air movement. With heat lamps, guidance concerning treatment time is given by the patient's subjective feeling of warmth.
Infrared radiation can be used for reflex vasodilatation where vasospasm is present. Because of the photochemical effect of lower-energy, ultraviolet photon, this radiant energy is used most effectively for the treatment of psoriasis and other dermatologic conditions.
CONTRAINDICATIONS TO SUPERFICIAL HEAT APPLICATION
Neither paraffin baths nor Fluidotherapy should be used in open wounds that are either clean or infected.
Hydrotherapy is contraindicated in patients immediately following surgery, because a healing wound should be kept dry.
Special precautions should be used for therapy provided in a Hubbard tank for patients with either a tracheostomy or an ostomy.
Radiant heat should not be used in patients who have the following conditions:
Photosensitivity
Acute inflammation or hemorrhage
Bleeding disorder
Decreased sensation
Because total immersion in a Hubbard tank elevates core body temperature, patients with the following conditions, which generate temperature sensitivity, should avoid this heating modality:
Multiple sclerosis
Adrenal suppression or failure
Systemic lupus Erythematosus
Pregnancy