Superficial heat treatments are very commonly used for aches and pains by the public although rather less so by physiotherapists nowadays. These types of treatment do not heat deep into the body so cannot affect muscles directly as the subcutaneous fat layer below the skin functions as a heat insulator. Superficial heating also causes increased blood flow through the skin of the heated area, which carries off some of the heat being applied. The methods of heating are varied and are generally described as heat conduction, heat convection and heat conversion.
Conductive heating is accomplished by the transfer of heat from one object to another without any movement involved, with direct contact between the source of heat and the tissues to be heated. Hot packs, hot water baths, electric heat pads, paraffin or wax baths and hot compresses all fall into this category. Convective heating is less commonly used and works by movement of the heat medium which is a fluid or the air. Whirlpool baths and hot air devices come into this category. Conversion heating involves the conversion of one type of energy to another such as light energy from a heat lamp being converted into heat at the skin surface.
Various parameters of the heat application determine how the bodily tissues react to the temperature changes. This depends on the size of the region under treatment, how quickly the temperature rises, how long the heat is applied for and the level of temperature achieved in the tissues. Hot packs, sometimes called hydrocollator packs, uses the conductive heating effect and can be effective but there is a risk of burns as the tissues can be overheated. Hot packs do not mould very effectively to uneven body parts such as the foot or ankle, making predictable heating unlikely. Reduction in the local blood supply can occur with pressure of the pack on the leg or vice versa, giving increased heating.
Refractory ankle swelling after fractures or repeated sprains of the ankle and joint inflammatory problems can be treated by contrast bathing where a 25 degree centigrade difference is established between a cold and a hot bath. Moving from one to the other causes great changes in the circulation and typically initial treatment is ten minutes in the hotter container then one in the colder one, then changing to four minutes in the hotter with one following in the colder. Repeated cycles can be performed for up to thirty minutes a few times a day. Pool therapy or hydrotherapy uses this therapeutic technique also, by convective heating.
Hydrotherapy involves immersion of the body in a large pool of warm water and the heat and support from the water helps pain and weakness and allows increased joint ranges particularly in arthritic conditions. A temperature above 34 degrees centigrade is used and this heat can have side effects such as changes in blood pressure as the heat and water pressure affects the body.
A technique of conversion heating is to use radiant energy of which the commonest form is infra-red radiation, whereby high energy light particles enter the skin and are converted into heat. Heat lamps, which can be purchased on the high street, are an example, and have some visible spectrum of red light also to indicate they are on. These can be used for minor aches and pains with muscle spasms and for osteoarthritic joints or backache.
This technique does not heat the tissues deeply but can soothe the local area due to warmth sedating the local sensory nerves and by allowing heat input to affect central processing. The distance of the light source to the skin is critical to avoid the risk of burns, as halving the distance will increase the intensity of the radiation by four times
Careful assessment and clinical judgement are needed by the physiotherapist before prescribing heat treatments. Since superficial heating may have relatively small effects, the clinical reason for doing so should be clear. Contraindications should be considered such as loss of normal sensation in the area rendering the patient unable to sense whether skin overheating is occurring. As heat can increase the tissue metabolism in the treated areas, areas of inflammation or infection might not be appropriate for this method.
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