The National Pressure Ulcer Advisory Panel (NPUAP) defines a pressure ulcer as a “localized injury to the skin and/or underlying tissue, usually over a bony prominence as a result of pressure or pressure combined with shear and/or friction.”

Kosiak¹ showed that 70 mm Hg of pressure applied continuously over 2 hours produced moderate histologic changes in rat muscle. Dinsdale² showed that shear can significantly reduce the amount of pressure necessary to disrupt blood flow.

Secondary factors include

Bony prominences are particularly at risk; muscle is more sensitive to breakdown from pressure than skin. Ulcers are commonly staged according to NPUAP guidelines as follows:

Stage I – Intact epidermis with nonblanchable erythema not resolved within 30 minutes. Warmth, edema, induration, or discoloration may be indicators of stage I ulcers in patients with darker skin.

Stage II – Partial-thickness epidermal or dermal skin loss. These may appear as blisters with erythema.