7.1 Pressure Injury Pathophysiology & NPIAP Staging

7.1 Pressure Injury Pathophysiology & NPIAP Staging

Pressure injuries represent one of the most critical challenges in clinical wound management, serving as a primary indicator of patient care quality, systemic vulnerability, and nursing practice across acute, long-term, and home care settings. The Certified Wound Specialist (CWS) examination rigorously evaluates a clinician's understanding of the cellular mechanisms driving pressure-induced tissue damage, the physical forces responsible for skin breakdown, and the standardized 2016 National Pressure Injury Advisory Panel (NPIAP) staging system. Mastery of these concepts is essential for accurate clinical documentation, risk mitigation, and appropriate therapeutic intervention.

---

Etiology & Pathophysiology of Pressure Injuries

A pressure injury is localized damage to the skin and underlying soft tissue, usually over a bony prominence or related to a medical or other device. The injury occurs as a result of intense and/or prolonged pressure or pressure in combination with shear. Soft tissue tolerance for pressure and shear is further influenced by microclimate, nutrition, perfusion, co-morbidities, and tissue condition.

1. External Pressure & Ischemic Hypoxia

The foundational mechanism of pressure injury development is mechanical compression of soft tissues between an internal anatomical bony prominence and an external surface (such as a mattress, chair cushion, or medical device).

• Capillary Closure Pressure: Historically, physiological research established that average capillary perfusion pressure ranges between 16 and 32 mmHg (with 32 mmHg accepted as the standard arteriolar capillary pressure). When external interface pressure exceeds capillary closure pressure, microvascular collapse occurs.
• Ischemic Cascade: Vessel compression occludes local blood flow, halting the delivery of oxygen and essential nutrients to surrounding tissues while preventing the clearance of metabolic waste products (such as lactic acid and carbon dioxide).
• Tissue Hypoxia & Necrosis: Sustained ischemia triggers an anaerobic metabolic shift, intracellular acidosis, depletion of adenosine triphosphate (ATP), cellular swelling, cell membrane rupture, and eventual ischemic tissue necrosis.

2. Ischemia-Reperfusion Injury

When external pressure is relieved, blood flow returns to previously ischemic tissues. Although reperfusion is vital for tissue recovery, the sudden restoration of oxygenated blood initiates a secondary inflammatory cascade known as reperfusion injury.

• Reoxygenation activates xanthine oxidase and NADPH oxidase, producing high concentrations of reactive oxygen species (ROS) (free radicals such as superoxide and hydrogen peroxide).
• ROS induce lipid peroxidation of cell membranes, microvascular endothelial injury, and leukocyte adhesion, leading to local capillary thrombosis, increased microvascular permeability, and secondary tissue destruction that can exceed the initial ischemic damage.

3. Direct Mechanical Cell Deformation

Modern biomechanical research demonstrates that pressure injuries do not result solely from vascular ischemia. Direct, high-magnitude mechanical strain causes rapid cell deformation.

• Uniphasic mechanical distortion directly stretches cell membranes, disrupting the cytoskeleton, opening mechanosensitive ion channels, and altering cell membrane permeability within minutes.
• This direct mechanical damage causes rapid cell death before vascular ischemic necrosis fully develops, particularly in deep muscle layers subjected to high shear deformation over bony prominences.

4. Shear Forces vs. Friction

Distinguishing between shear and friction is a fundamental requirement for the CWS exam.

• Shear Dynamics: Shear occurs when a patient's skeleton shifts downward under gravity while the outer epidermal layer remains anchored to bed linens by friction. For example, elevating the head of the bed (HOB) above 30 degrees causes the sacral bone to slide inferiorly while the sacral skin remains fixed. This creates a twisting force that stretches and occludes the deep perforating arterioles supplying the muscle and subcutaneous fat, resulting in deep-seated necrosis that may initially present under intact skin.

---

The NPIAP 2016 Staging System

In 2016, the National Pressure Injury Advisory Panel updated its staging definitions, replacing the term "pressure ulcer" with "pressure injury" to accurately reflect both intact and ulcerated skin lesions. Staging describes the extent of anatomical tissue loss.

Stage 1 Pressure Injury: Non-Blanchable Erythema of Intact Skin

• Definition: Intact skin with a localized area of non-blanchable erythema, which may appear differently in darkly pigmented skin.
• Clinical Characteristics: Presence of blanchable erythema or changes in sensation, temperature (warmth or coolness), or tissue firmness (induration) may precede visual color changes. Color changes do not include purple or maroon discoloration (which indicates DTPI).
• Dark Skin Tone Assessment: In deeply pigmented skin, erythema may be difficult to detect visually. Clinicians must rely on palpation for changes in skin temperature (hyperthermia or hypothermia), localized tissue edema, induration (stiffness), and patient reports of localized pain or hyperesthesia compared to adjacent intact tissue.

Stage 2 Pressure Injury: Partial-Thickness Loss with Exposed Dermis

• Definition: Partial-thickness loss of skin with exposed dermis. The wound bed is viable, pink or red, moist, and may also present as an intact or ruptured serum-filled blister.
• Clinical Characteristics: Adipose (fat) and deeper tissues are not visible. Granulation tissue, slough, and eschar are absent.
• Diagnostic Exclusions: Stage 2 should not be used to describe moisture-associated skin damage (MASD), incontinence-associated dermatitis (IAD), intertriginous dermatitis (ITD), skin tears, medical adhesive-related skin injuries (MARSI), or burn wounds.

Stage 3 Pressure Injury: Full-Thickness Loss of Skin

• Definition: Full-thickness loss of skin, in which adipose (fat) is visible in the ulcer and granulation tissue and epibole (rolled wound edges) are often present.
• Clinical Characteristics: Slough and/or eschar may be visible but do not obscure the depth of tissue loss. Undermining and tunneling may occur.
• Anatomical Variation: Depth of tissue damage varies by anatomical location. Areas of significant adiposity (e.g., buttocks, sacrum) can develop deep Stage 3 wounds. Conversely, areas lacking subcutaneous tissue (e.g., bridge of the nose, ear, occiput, malleolus) may present as shallow Stage 3 ulcers. Fascia, muscle, tendon, ligament, cartilage, and bone are not exposed.

Stage 4 Pressure Injury: Full-Thickness Skin and Tissue Loss

• Definition: Full-thickness skin and tissue loss with exposed or directly palpable fascia, muscle, tendon, ligament, cartilage, or bone in the ulcer.
• Clinical Characteristics: Slough and/or eschar may be visible. Epibole, undermining, and tunneling frequently occur. Depth varies by anatomical location.
• Clinical Implication: Direct exposure of bone or tendon dramatically increases the risk of osteomyelitis and deep space infections. If bone is palpable with a sterile metallic probe (Positive Probe-to-Bone test), advanced diagnostic evaluation for osteomyelitis is indicated.

Unstageable Pressure Injury: Obscured Full-Thickness Skin and Tissue Loss

• Definition: Full-thickness skin and tissue loss in which the extent of tissue damage within the ulcer cannot be confirmed because it is obscured by slough or eschar.
• Clinical Characteristics: If slough or eschar is removed, a Stage 3 or Stage 4 pressure injury will be revealed.
• Debridement Exception: Stable, dry, adherent eschar on the heels or an ischemic limb should not be softened or removed. In these ischemic anatomical regions, stable eschar serves as the body's natural, sterile protective cover. Debriding stable eschar in an arterial or dry heel wound introduces bacteria and converts a closed ischemic wound into an open, high-risk infected ulcer.

Deep Tissue Pressure Injury (DTPI)

• Definition: Persistent non-blanchable deep red, maroon, or purple discoloration, or epidermal separation revealing a dark wound bed or blood-filled blister.
• Etiology: Results from intense and/or prolonged pressure and shear forces at the bone-muscle interface. The damage initiates in deep muscle and subcutaneous tissue over bony prominences.
• Clinical Progression: The lesion may evolve rapidly to reveal the true extent of tissue injury (rapidly progressing to a Stage 3 or Stage 4 injury despite optimal care), or it may resolve without tissue loss if prompt offloading and microvascular perfusion are restored.

Mucosal Membrane Pressure Injury

• Definition: Pressure injury found on mucous membranes with a history of a medical device in use at the location (e.g., endotracheal tubes, nasogastric tubes, tracheostomy ties, Foley catheters, fecal management systems).
• Staging Rule: Mucosal membrane pressure injuries cannot be staged using the NPIAP staging system. Because mucosal tissues lack a keratinized stratified squamous epidermis and typical dermal/subcutaneous architecture, anatomical depth cannot be evaluated using cutaneous staging criteria. They must be documented simply as "Mucosal Membrane Pressure Injuries" with device etiology specified.