EFFECT OF COLD ON SKIN
The rate of skin cooling is faster than the rate of re-warming, implying that a shorter period of cold appli-cation suffices to cool the skin. The depth of cold pen-etration depends on the duration and the area of application. Areas of the body containing more adi-pose tissue take a longer time to change temperature. If deeper structures are to be cooled, the duration of application is increased. When cold, in the form of wa-ter, is applied locally, it results in peripheral vasocon-striction and pallor. The vasoconstriction, in turn, re-sults in a decrease in skin temperature and reduction of edema, muscle spasm, and further hemorrhage.
Analgesic effects begin when skin temperature is lowered to approximately 13.6°C (56.5°F). Analgesia is produced by the reduction in nerve conduction ve-locity by cold. Systemic reactions, such as increase in heart rate, respiratory rate, blood pressure, and shiv- ering, may be produced. Soon after cold application has ended, peripheral vasodilatation may occur, with redness of skin, feeling of warmth, slowing of pulse and respiratory rates, and relaxation. This reaction may last for 20–30 minutes.
For therapeutic purposes, both types of reactions may be desirable and cold and hot applications may be alternated.