Pressure Ulcer - Etiology

Etiology

Pressure ulcers are accepted to be caused by three different tissue forces: Pressure, or the compression of tissues and/or destruction of muscle cells. In most cases, this compression is caused by the force of bone against a surface, as when a patient remains in a single decubitus position for a lengthy period. After an extended amount of time with decreased tissue perfusion, ischemia occurs and can lead to tissue necrosis if left untreated. Pressure can also be exerted by external devices, such as medical devices, braces, wheelchairs, etc. Shear, or a force created when the skin of a patient stays in one place as the deep fascia and skeletal muscle slide down with gravity. This can also cause the pinching off of blood vessels which may lead to ischemia and tissue necrosis. Friction is related to shear but is considered less important in causing pressure ulcesr Microclimate, is temperature and moisture of the skin in contact with the surface of the bed or wheelchair. Moisture on the skin causes the skin to loose the dry outer layer and reduces the tolerance of the skin for pressure and shear. Aggravating the situation may be other conditions such as excess moisture from incontinence, perspiration or exudate. Over time, this excess moisture may cause the bonds between epithelial cells to weaken thus resulting in the maceration of the epidermis. Temperature is also a very important factor. The cutaneous metabolic demand rises by 13% for every 1°C rise in cutaneous temperature. When supply can't meet demand, ischemia therefore occurs. There are currently two major theories about the development of pressure ulcers. The first and most accepted is the deep tissue injury theory which claims that the ulcers begin at the deepest level, around the bone, and move outward until they reach the epidermis. The second, less popular theory is the top-to-bottom model which says that skin first begins to deteriorate at the surface and then proceeds inward.