Acute Respiratory Distress Syndrome (ARDS): Mechanism

Definition and Triggers

• Bilateral infiltrates on chest radiograph/CT not fully explained by effusions, collapse, or nodules
• PaO2/FiO2 ratio <300 mmHg (mild <300, moderate <200, severe <100)
• Onset within 7 days of a known clinical insult
• Respiratory failure not fully explained by heart failure or fluid overload

Core Pathophysiological Concept

Three Sequential Phases

Phase 1: Exudative Phase (Days 1-7)

1. Alveolar-capillary barrier disruption - Damage to both the pulmonary microvascular endothelium and alveolar epithelium increases barrier permeability. Loss of the epithelium disrupts barrier integrity and also prevents alveolar fluid clearance. Loss of endothelial barrier integrity is both necessary and sufficient for the development of ARDS.
2. Neutrophil sequestration - One of the earliest manifestations of ARDS (even before hypoxemia) is transient leukopenia due to neutrophil sequestration in the lung microvasculature. Because pulmonary capillaries are narrower than neutrophils, they must deform to pass through. Activated neutrophils become "stiff" via actin cytoskeletal changes and cannot negotiate these narrow segments, causing them to lodge in capillaries. They then migrate into the interstitium and alveoli, even without the usual adhesion molecules.
3. Neutrophil-mediated tissue injury - Sequestered neutrophils release a battery of cytotoxic compounds:
   • Reactive oxygen species (ROS) - oxidative damage to membranes
   • Neutrophil elastase (NE) - degrades epithelial and endothelial cadherins (adherens junction proteins), predisposing to alveolar flooding; also degrades surfactant protein A
   • Metalloproteinases - further extracellular matrix degradation
   • Cytokines: TNF-α, IL-1β, IL-8 - amplify inflammatory recruitment
   • Neutrophil extracellular traps (NETs) - chromatin-based webs that trap but also damage surrounding tissue
4. Cytokine signaling cascade - Activated macrophages release TNF-α, IL-1β, IL-6, and IL-8. IL-8 is a potent neutrophil chemoattractant. p38 MAP kinase (activated by LPS) stimulates TNF-α and macrophage inflammatory protein-2, further driving neutrophil chemotaxis. This creates a positive-feedback inflammatory loop.
5. Alveolar flooding - Protein-rich fluid, cellular debris, and fibrin pour into alveolar spaces, forming hyaline membranes - a hallmark finding on histology (termed diffuse alveolar damage, or DAD).
6. Surfactant dysfunction - The proportion of large (active) to small (inactive) surfactant aggregates is diminished by decreased production and increased conversion. Leaked plasma proteins interfere with surfactant function. Neutrophil elastase degrades surfactant protein A. The net result is loss of alveolar surface tension reduction, promoting collapse.
7. Profound hypoxemia - Alveolar flooding causes ventilation-perfusion (V/Q) mismatch and right-to-left intrapulmonary shunting. Flooded or collapsed alveoli are perfused but not ventilated.
8. Decreased compliance - Fluid-filled and collapsed alveoli make the lung stiff; the functional residual capacity drops.
9. Pulmonary hypertension - Caused by hypoxic vasoconstriction, intravascular fibrin deposition, and compression of blood vessels by positive-pressure ventilation.

Phase 2: Proliferative Phase (Days 7-21)

• Hyaline membranes begin to reorganize
• Type II pneumocyte hyperplasia - type II cells proliferate to replace destroyed type I pneumocytes and attempt to restore epithelial integrity
• Alveolar septal thickening develops
• Neutrophil numbers decrease; macrophages and lymphocytes increase
• Fibroblast proliferation begins in the interstitium and alveolar spaces
• In situ thrombi may appear in small pulmonary arteries
• Some resolution of pulmonary edema begins

Phase 3: Fibrotic Phase (>2 weeks, in a subset)

• Occurs in patients with persistent or severe ARDS
• Alveolar septal thickening from organizing fibrosis
• Obliteration of pulmonary capillaries
• Deposition of interstitial and alveolar collagen
• BAL fluid from ARDS patients stimulates cultured fibroblasts to proliferate in vitro - suggesting active profibrotic signaling

Alveolar Fluid Clearance - Why It Fails

• Apoptosis and necrosis of the alveolar epithelium
• Defects in transcellular ion transport induced by proinflammatory cytokines, oxidants, and hypoxia
• Clotting of extravasated plasma proteins within airspaces slows protein removal (protein clearance is 1-2%/hour vs. fluid clearance at 10-20%/hour, leading to progressive alveolar protein accumulation)

Ventilator-Induced Lung Injury (VILI) and Biotrauma

• Further endothelial and epithelial injury
• Enhanced neutrophil margination
• Increased release of proinflammatory cytokines from alveolar macrophages and epithelium (biotrauma)
• These mediators can spill into the systemic circulation, contributing to multi-organ dysfunction syndrome (MODS)

Pathological Correlate: Diffuse Alveolar Damage (DAD)

• Intra-alveolar hyaline membrane formation
• Interstitial and intra-alveolar edema
• Acute inflammation (neutrophil infiltration)
• Epithelial cell necrosis (temporal uniformity - suggesting a single acute insult)
• Later: type II pneumocyte hyperplasia, fibroblast proliferation, collagen deposition

Summary Cascade

Inciting insult (sepsis, pneumonia, trauma, etc.)
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Activation of alveolar macrophages + systemic inflammatory response
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Release of TNF-α, IL-1β, IL-6, IL-8, complement, platelet-activating factor
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Neutrophil sequestration in pulmonary capillaries
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Neutrophil transmigration → release of ROS, elastase, proteases, NETs
       ↓
Damage to alveolar epithelium + microvascular endothelium
       ↓
Increased alveolar-capillary permeability
       ↓
Protein-rich exudate floods alveoli → hyaline membrane formation
       ↓
Surfactant dysfunction → alveolar collapse
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V/Q mismatch + intrapulmonary shunt → refractory hypoxemia
       ↓
Decreased lung compliance + pulmonary hypertension
       ↓
[Ongoing] → proliferative → fibrotic phase

• Murray & Nadel's Textbook of Respiratory Medicine (Chapter 134: ARDS - Etiology, Pathogenesis, Pathology)
• Goldman-Cecil Medicine (Chapter 90/91: ARDS, Mechanical Ventilation)
• Fishman's Pulmonary Diseases and Disorders (Chapter 140: Resolution of Acute Lung Injury)
• Sabiston Textbook of Surgery (Chapter: Critical Care / ARDS)