Mechanism of Acute Respiratory Distress Syndrome (ARDS)

1. Inciting Triggers

2. Three Sequential Pathological Phases

Phase 1 — Exudative Phase (Days 1–7)

• Injury to alveolar capillary endothelial cells and type I pneumocytes breaks down the normally tight alveolar-capillary barrier
• Protein-rich edema floods the interstitium and alveolar spaces
• Condensed plasma proteins aggregate with cellular debris and dysfunctional surfactant to form hyaline membrane whorls lining the denuded alveolar walls
• Widespread neutrophil infiltration of the interstitium and airspaces occurs
• Pulmonary vascular injury leads to microthrombi and vascular obliteration

Phase 2 — Proliferative Phase (Days 7–21)

• Hyaline membranes are reorganized
• Type II pneumocytes proliferate in an attempt to restore the epithelial barrier
• Fibrosis begins to appear; obliteration of pulmonary capillaries and deposition of interstitial/alveolar collagen are observed
• Neutrophil numbers decrease and pulmonary edema begins to resolve

Phase 3 — Fibrotic Phase (>2–3 weeks)

• A subset of patients develop pulmonary fibrosis
• Notably, fibroproliferation may begin simultaneously with early inflammatory injury — elevated N-terminal procollagen peptide III (a marker of collagen synthesis) can be detected in BAL fluid within 24 hours of ARDS onset

CT demonstration of the phases of ARDS. — Murray & Nadel's Textbook of Respiratory Medicine

3. Cellular and Molecular Mechanisms

3a. Alveolar-Capillary Barrier Breakdown

1. Microvascular endothelium — loss of endothelial barrier integrity is both necessary and sufficient for ARDS development
2. Alveolar epithelium — damage to type I pneumocytes (which cover ~95% of the alveolar surface) is the key precipitating event

3b. Neutrophil-Mediated Injury

• Activated neutrophils are sequestered within the alveolar and interstitial spaces in massive numbers
• They degranulate, releasing:
  • Proteases (elastase, matrix metalloproteinases) — degrade the extracellular matrix and tight junctions
  • Reactive oxygen species (ROS) — cause oxidative damage to cell membranes
  • Myeloperoxidase (MPO) — amplifies oxidative injury
  • Neutrophil extracellular traps (NETs) — directly injure endothelial and epithelial cells
• Neutrophil elastase also degrades surfactant protein A, impairing host defense and surface tension regulation
• Platelet-neutrophil complexes (formed via P-selectin / PSGL-1 binding) amplify this injury cascade, triggering necroptosis and pyroptosis

3c. Cytokine Storm / Inflammatory Amplification

• Alveolar macrophages activated via TLR/PRR pathways polarize to the M1 (pro-inflammatory) phenotype
• They release pro-inflammatory cytokines: TNF-α, IL-1β, IL-6, IL-8 (CXCL8), IL-17
• These cytokines recruit and activate more neutrophils and promote endothelial cell activation
• IL-8 in particular is a potent neutrophil chemoattractant responsible for sequestration within the lung
• The Th17/T-reg imbalance (↑Th17, ↓T-reg) further skews the milieu toward inflammation
• Anti-inflammatory mediators (IL-10, TGF-β) are relatively suppressed

3d. Surfactant Dysfunction

• Plasma proteins leaking into the alveolar space inhibit surfactant function
• The proportion of large (active) to small (inactive) surfactant aggregates is diminished due to decreased production and increased conversion
• Neutrophil elastase degrades surfactant protein A
• Net result: loss of surface tension reduction → microatelectasis and alveolar collapse
• Unlike neonatal RDS (where surfactant deficiency is the primary event), in adult ARDS the surfactant abnormality is secondary to barrier disruption — explaining why surfactant replacement trials in adults have failed

3e. Coagulation–Inflammation Interplay

• Activation of the coagulation cascade within the injured lung leads to intravascular fibrin deposition and microthrombi in pulmonary capillaries
• Fibrin also accumulates in alveolar spaces, contributing to hyaline membrane formation
• This causes vascular obliteration and contributes to pulmonary hypertension
• Simultaneous suppression of fibrinolysis (via elevated PAI-1) prevents clot resolution

3f. Impaired Alveolar Fluid Clearance (Na⁺/Water Transport)

• Active fluid reabsorption from the alveolar space is driven by Na⁺-K⁺-ATPase on type II pneumocytes and epithelial Na⁺ channels (ENaC)
• Injury to type II pneumocytes disrupts this active transport, preventing clearance of alveolar edema
• This is compounded by the fact that type II pneumocytes are also the source of surfactant synthesis

3g. Angiopoietin Dysregulation

• Angiopoietin-2 (Ang2) is elevated in sepsis and ARDS and acts as a competitive antagonist to Ang1, disrupting endothelial barrier integrity
• Ang2 promotes pulmonary vascular leak; exogenous Ang2 in animal models induces pulmonary leak reversed by Ang1
• Genetic variants in Ang2 are associated with higher plasma levels and increased ARDS risk
• The Ang2/Tie2 receptor axis is an emerging therapeutic target — Murray & Nadel's Textbook of Respiratory Medicine

4. Physiological Consequences

5. Ventilator-Induced Lung Injury (VILI) — A Secondary Mechanism

• Volutrauma: Large tidal volumes overdistend "normal" lung units (the heterogeneous ARDS lung funnels volume preferentially to compliant regions)
• Barotrauma: Excessive airway pressure → pneumothorax, pneumomediastinum
• Atelectrauma: Repetitive opening/closing of collapsed terminal units generates high shear stresses at the interface of aerated and collapsed tissue
• Biotrauma: Mechanical stretch activates local inflammatory cells, releasing cytokines and ROS that spill into the systemic circulation, contributing to multi-organ failure

Summary Flow

Inciting insult (sepsis, pneumonia, aspiration, trauma…)
        ↓
Activation of alveolar macrophages → Cytokine storm (TNF-α, IL-1β, IL-6, IL-8)
        ↓
Neutrophil sequestration in lung → ROS, proteases, NETs, platelet-neutrophil complexes
        ↓
Injury to type I pneumocytes + microvascular endothelium
        ↓
↑ Alveolar-capillary permeability
        ↓
Protein-rich edema floods alveoli + Coagulation activation → Fibrin/hyaline membranes
        ↓
Surfactant dysfunction → Microatelectasis
        ↓
Impaired Na⁺ transport (type II pneumocyte injury) → Failed fluid clearance
        ↓
ARDS: Hypoxemia + ↓ Compliance + Pulmonary Hypertension + ↑ Dead Space
        ↓
        ±
Ventilator-Induced Lung Injury (perpetuates the cycle)
        ↓
Resolution (proliferative → fibrotic) OR Death

• Murray & Nadel's Textbook of Respiratory Medicine, Chapter 134 (Etiology and Pathogenesis of ARDS)
• Harrison's Principles of Internal Medicine, 22nd ed., Chapter 312
• Goldman-Cecil Medicine, Chapter 90/91
• Morgan & Mikhail's Clinical Anesthesiology, 7th ed.