Mechanism of Acute Respiratory Distress Syndrome (ARDS)

1. Precipitating Insults

2. The Alveolar-Capillary Membrane — The Central Target

• Endothelial injury: Loss of pulmonary vascular endothelial barrier integrity is both necessary and sufficient for ARDS development. Endothelial cells become damaged via direct insult, cytokine-mediated apoptosis, and neutrophil-induced injury, causing tight junction disruption and massive increases in vascular permeability.
• Epithelial injury: Type I pneumocyte death (via necrosis, apoptosis, coagulation, and mechanical stretch) disrupts the epithelial barrier and impairs alveolar fluid clearance. Type II cell injury reduces surfactant production.

3. The Neutrophil — Central Cellular Mediator

1. Sequestration: The average pulmonary capillary is narrower than a neutrophil. Activated neutrophils stiffen (due to actin cytoskeleton remodeling) and cannot deform adequately, becoming physically trapped in capillary segments. This produces a transient leukopenia — one of the earliest clinical signs.
2. Transmigration: Sequestered neutrophils migrate across the endothelium into the interstitium and alveolar space, aided by chemokines (e.g., IL-8) released by macrophages and epithelial cells. Early migration can occur even without classic adhesion molecules (L-selectin, β2-integrins).
3. Tissue destruction: Once in the alveolar space, activated PMNs release:
   • Reactive oxygen species (ROS) — oxidative damage to membranes
   • Proteolytic enzymes (elastase, matrix metalloproteinases) — degrade extracellular matrix and interepithelial junctions, and directly degrade surfactant protein A
   • Cytokines: TNF-α, IL-1β — amplify the inflammatory response and recruit additional cells
   • Neutrophil extracellular traps (NETs) — chromatin-based structures that trap pathogens but also promote tissue injury and coagulopathy
4. Neutrophil–platelet interaction: Mutual activation of neutrophils and platelets amplifies both inflammation and microvascular thrombosis.

4. Cytokines, Macrophages, and the Inflammatory Cascade

• Recognize pathogen-associated (PAMPs) and damage-associated (DAMPs) molecular patterns via toll-like receptors (TLRs)
• Release TNF-α, IL-1β, IL-6, IL-8, and platelet-activating factor (PAF)
• Activate the p38 MAP kinase pathway, further amplifying TNF-α production and macrophage inflammatory protein-2 (a neutrophil chemokine)

5. Surfactant Dysfunction

• Decreased production of surfactant (especially dipalmitoylphosphatidylcholine)
• Shift from large (active) to small (inactive) surfactant aggregates
• Plasma protein leak into alveoli, which inhibits surfactant function
• Elastase-mediated degradation of surfactant protein A

6. Angiopoietin Axis and Vascular Permeability

• Angiopoietin-1 (Ang1) stabilizes endothelial tight junctions via the Tie2 receptor.
• Angiopoietin-2 (Ang2) is released by injured endothelial cells as a competitive antagonist, disrupting barrier integrity. Elevated plasma Ang2 is a biomarker of severity in sepsis and ARDS. Genetic variants in Ang2 are associated with increased ARDS risk.

7. Failure of Alveolar Fluid Clearance

• Hypoxia suppresses ENaC expression and Na⁺/K⁺-ATPase activity
• Nitric oxide (from inflammatory activation) impairs β-adrenergic-mediated fluid clearance upregulation

8. Phases of ARDS

9. Ventilator-Induced Lung Injury (VILI) — A Perpetuating Mechanism

• Volutrauma/Barotrauma: Overdistension of aerated alveoli at high tidal volumes → epithelial and endothelial tears
• Atelectrauma: Repetitive opening and collapse of unstable alveoli at low end-expiratory volumes
• Biotrauma: Mechanical stretch triggers cytokine release (IL-6, IL-8, TNF-α) from the lung into the systemic circulation → distal organ dysfunction

Summary Diagram of Mechanism

Precipitant (sepsis, aspiration, trauma)
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Macrophage activation → Cytokine storm (TNF-α, IL-1β, IL-8)
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Neutrophil sequestration in pulmonary capillaries
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PMN transmigration + activation
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ROS + Elastase + NETs → Alveolar-capillary barrier destruction
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Protein-rich edema floods alveoli
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Surfactant dysfunction + Type I/II pneumocyte death
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Alveolar collapse (atelectasis) + impaired fluid clearance
          ↓
Severe V/Q mismatch → Refractory hypoxemia (ARDS)
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Mechanical ventilation → VILI → Perpetuates injury