Porth's Essentials of Pathophysiology, 4e

1171

Disorders of Skin Integrity and Function

C h a p t e r 4 6

Mechanisms of Development Many factors contribute to the development of pressure ulcers, such as length of stay in the hospital, vasopres- sure infusion, spinal cord injury, age, and body mass index. 54 Pressure, shearing forces, friction, and moisture contribute to the incidence of pressure ulcers. External pressures that exceed capillary pressure interrupt blood flow in the capillary beds. When the pressure between a bony prominence and a support surface exceeds the normal capillary filling pressure, capillary flow essen- tially is obstructed. If this pressure is applied constantly for 2 hours, oxygen deprivation coupled with an accu- mulation of metabolic end products leads to irreversible tissue damage. While pressure magnitude and duration are important in the creation of a pressure ulcer, no spe- cific amount of pressure necessary to compress capil- laries and interrupt blood flow has been determined. 55 Tolerance to pressure loads differs according to tissue, location, and metabolism. 55 Persons with impaired cir- culation require less pressure to interrupt circulation. The same amount of pressure causes more damage when it is distributed over a small area than over a larger area. Whether a person is sitting or lying down, the weight of the body is borne by tissues covering the bony promi- nences. Most pressure ulcers are located on the lower part of the body, such as the sacrum, the coccygeal area, the ischial tuberosities, and the greater trochanters. Pressure over a bony area is transmitted from the surface to the underlying dense bone, compressing all of the interven- ing tissue. As a result, the greatest pressure occurs at the surface of the bone and dissipates outward in a cone- like manner toward the surface of the skin (Fig. 46-19). Thus, extensive underlying tissue damage can be present when a small superficial skin lesion is first noticed. Altering the distribution of pressure from one skin area to another prevents tissue injury. Pressure ulcers most commonly occur in persons with conditions such as spinal cord injury in which normal sensation and the ability to move to redistribute body weight are impaired. Normally, persons unconsciously shift their weight to redistribute pressure on the skin and underlying tis- sues. For example, during the night, people turn in their sleep, preventing ischemic injury of tissues that overlie the bony prominences that support the weight of the body; the same is true for sitting for any length of time.

A split-thickness skin graft can be sent through a skin mesher that cuts tiny slits into the skin, allowing it to expand up to nine times its original size. These grafts are used frequently because they can cover large surface areas and there is less autorejection. Full-thickness skin grafts include the entire thickness of the dermal layer. They are used primarily for reconstructive surgery or for small deep areas. The donor site of a full-thickness skin graft requires a split-thickness skin graft to help it heal. Two-layered synthetic skin grafts, such as Apligraf or Integra , are composed of a layer of silicone, mim- icking the properties of the epidermis, and a layer or matrix of fibers. 52 Skin cells attach to the fibers, enabling dermal skin growth. Once the dermal skin has regenerated, the silicone layer is removed and a thin epidermal skin graft is applied, thus requiring less skin grafting overall. Other treatment measures include positioning, splint- ing, and physical therapy to prevent contractures and maintain muscle tone. Because the normal body response to disuse is flexion, the contractures that occur with a burn are disfiguring and cause loss of limb or appendage use. Once the wounds have healed sufficiently, elastic pressure garments, sometimes for the full body, often are used to prevent hypertrophic scarring. Pressure Ulcers Pressure ulcers are ischemic lesions of the skin and underlying structures caused by unrelieved pressure that impairs the flow of blood and lymph. Pressure ulcers often are referred to as decubitus ulcers or bed- sores. The word decubitus comes from the Latin term meaning “lying down.” However, a pressure ulcer may result from pressure exerted when seated or lying down. Pressure ulcers are most likely to develop over a bony prominence, but they may occur on any part of the body that is subjected to external pressure, friction, or shear- ing forces. Approximately 2.2 million people in the United States develop pressure ulcers annually. 53 Several subpopula- tions are at particular risk, including persons with quad- riplegia, elderly persons with restricted activity and hip fractures, and persons in the critical care setting.

Pressure

Epidermis Dermis

Skin

Subcutaneous fat

Deep fascia

Muscle

FIGURE 46-19. Pressure over a bony prominence compresses all intervening soft tissue, with a resulting wide, three-dimensional pressure gradient that causes various degrees of tissue damage. (From Shea JD. Pressure sores: classification and management. Clin Orthop Relat Res. 1975;112:90.)

Periosteum Bone