Pathophysiology
Clinical meaning
Pressure injuries result from sustained mechanical loading on soft tissue overlying bony prominences, causing localized ischemia, cellular death, and tissue necrosis. When external pressure exceeds capillary closing pressure (approximately 32 mmHg in healthy tissue, though this threshold is lower in critically ill, malnourished, or edematous patients), blood flow to the compressed tissue ceases. Ischemia lasting longer than 2 hours initiates an irreversible cascade: ATP depletion causes failure of the Na-K-ATPase pump, intracellular sodium and water accumulation leads to cellular swelling, lysosomal membrane breakdown releases proteolytic enzymes that digest cellular contents, and calcium influx activates destructive phospholipases and endonucleases. Reperfusion injury compounds the damage when pressure is relieved โ reactive oxygen species generated during reoxygenation cause additional endothelial and parenchymal cell injury. Shear forces (tangential mechanical stress from the body sliding against a surface, such as when the head of the bed is elevated beyond 30 degrees) cause deep tissue deformation by stretching and angulating perforating blood vessels between the skin and deeper fascial layers, producing ischemia in subcutaneous tissue while the skin surface may appear intact โ this is the...