External factors that lead to the development of pressure ulcers include pressure, shearing forces, friction, and moisture.
Pressure applied to the skin in excess of the arteriolar pressure (32 mm Hg) prevents the delivery of oxygen and nutrients to tissues, resulting in the accumulation of metabolic waste products. Pressures are greatest over bony prominences where weight-bearing points come in contact with external surfaces. A patient lying on a standard hospital mattress may generate pressures of 150 mmHg; sitting produces pressures as high as 300 mmHg over the ischial tuberosities. Pressure in excess of 70 mmHg for two hours results in irreversible tissue damage in animal models.
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