This is followed by vasodilation, which causes an increase in the amount of blood in the capillary and a rise in mean intercapillary pressure. At this stage, the capillary walls are only slightly more permeable than normal and the fluid leaking into the tissue is mainly water and electrolytes, rather than protein or cells. These changes in pressure and capillary wall permeability combine to produce edema. Depending upon the severity of the inflammation, as well as its duration, various scavenger cells, such as neutrophils, lymphocytes, and monocytes, attack the bacteria or other types of debris present at the injury site. Heat, says Zablotsky, can assist in the healing process, as well as help to eliminate pain.
Heat can modulate pain through two methods, vascular changes and neurological changes.
The application of heat, or other stimulation, to an injured part might sufficiently overload the pain transmission with a preponderance of sensory input from thermoreceptors. It has also been noted that vascular changes reduce pain by accelerating the natural heating process. There is also an increase in flow due to the blood which moves in to cool the heated area. As the flow of blood increases, the concentration of nutrients in the area increases and the potentially toxic waste products in the injured tissue are removed at an accelerated rate. The heating of tissues through the use of electromagnetic devices has been used since the early part of this century, says Zablotsky.