Mechanism of Acute Respiratory Distress Syndrome (ARDS)

1. Triggers and Classification of Injury

• Murray & Nadel's Textbook of Respiratory Medicine, Table 134.2

2. Core Pathophysiology: Increased Alveolar-Capillary Permeability

• Protein-rich, exudative fluid floods the alveolar spaces
• Alveolar fluid clearance is impaired (loss of epithelial pumping function)
• Respiratory system compliance falls dramatically
• Right-to-left shunting develops, causing profound hypoxemia
• Dead space ventilation increases, driving up minute ventilation
• Pulmonary hypertension develops via hypoxic vasoconstriction, intravascular fibrin deposition, and compression from positive-pressure ventilation

3. Pathological Stages (Diffuse Alveolar Damage - DAD)

Phase 1 - Exudative (Days 1-7)

• Hyaline membranes form in alveoli (composed of cellular debris, proteins, surfactant components)
• Protein-rich fluid fills airspaces
• Widespread epithelial disruption
• Massive neutrophil infiltration of interstitium and airspaces
• Elevated N-terminal procollagen peptide III in BAL fluid detectable within 24 hours (early signal of fibrogenesis)

Phase 2 - Proliferative (Days 7-21)

• Hyaline membranes are reorganized
• Fibrosis begins to appear
• Obliteration of pulmonary capillaries
• Deposition of interstitial and alveolar collagen
• Decrease in neutrophil number and extent of edema

Phase 3 - Fibrotic (>2 weeks, subset of patients)

• Frank pulmonary fibrosis in patients with persistent ARDS

Figure: CT demonstration of ARDS phases. (A) Baseline. (B) Exudative phase - bilateral ground-glass opacities. (C) Proliferative phase - architectural distortion. (D) Fibrotic phase - linear and reticular abnormalities. - Murray & Nadel's, Fig. 134.2

4. The Central Role of Neutrophils

Figure 134.3: Activated PMNs exit the bloodstream, transmigrate across the alveolar-capillary membrane, and release cytokines, proteases, ROS, and NETs. - Murray & Nadel's

Mechanism of Neutrophil-Mediated Injury

1. Reactive oxygen species (ROS) - oxidative damage to epithelial and endothelial cells
2. Proteolytic enzymes - especially neutrophil elastase (NE) and matrix metalloproteinases (MMPs). NE degrades surfactant protein A, ECM components, and disrupts interepithelial tight junctions
3. Cytokines/chemokines - TNF-alpha, IL-1beta amplify the inflammatory cascade and recruit more neutrophils
4. Neutrophil Extracellular Traps (NETs) - web-like structures of DNA, histones, myeloperoxidase, NE, and cathepsin G. In sepsis, large-scale NET release causes endothelial damage and thrombosis. Animal models show that DNase treatment (which degrades NETs) attenuates lung injury and lowers IL-6/TNF levels. BAL fluid from ARDS patients shows NET concentration inversely correlated with degree of ARDS.

Note: ARDS can occur in profoundly neutropenic patients, suggesting alveolar macrophages may serve as an alternative driver of injury in some cases.

5. Surfactant Dysfunction

• The proportion of large (active) to small (inactive) surfactant aggregates is diminished
• Surfactant production by type II pneumocytes falls
• Plasma proteins that leak into alveoli interfere with surfactant function
• Neutrophil elastase directly degrades surfactant protein A (confirmed in BAL fluid from ARDS patients)
• Phospholipase A2 (relevant in pancreatitis-associated ARDS) enzymatically degrades surfactant and promotes alveolar collapse and increased vascular permeability

6. Inflammation and Coagulation Crosstalk

• TNF-alpha increases thrombin and fibrin formation, stimulates tissue factor expression on endothelial cells, and inhibits fibrinolysis
• Fibrin fragments are chemotactic for neutrophils - a self-amplifying loop
• Intravascular fibrin deposition contributes to pulmonary hypertension
• Activated protein C (endogenous anticoagulant) has anti-inflammatory effects including IL-6 downregulation and neutrophil activation attenuation
• Other key mediators include IL-8 (potent neutrophil chemoattractant), platelet-activating factor, adhesion molecules, and reactive oxygen species

7. Cytokine-Driven "Cytokine Storm" Amplification

• TNF-alpha and IL-1beta are produced by macrophages and activated neutrophils early in the response
• These stimulate further neutrophil recruitment, endothelial activation, and coagulation
• IL-8 acts as the principal chemokine drawing more neutrophils to the lung
• In pancreatitis-associated ARDS specifically: TNF-alpha and IL-8 augment neutrophil recruitment, while phospholipase A2 and elastase from pancreatic tissue directly damage the lung; reactive oxygen species and platelet-activating factor contribute further

8. Functional Consequences

Summary Diagram of Mechanism

Precipitating Insult (Sepsis / Pneumonia / Aspiration / Trauma)
             ↓
Systemic / Local Inflammatory Activation
             ↓
Cytokines (TNF-α, IL-1β, IL-8) + Complement + Coagulation activation
             ↓
Neutrophil sequestration in pulmonary capillaries
             ↓
Neutrophil transmigration → ROS + Elastase + NETs + Cytokines
             ↓
Alveolar-capillary barrier disruption (endothelium + epithelium)
             ↓
Protein-rich exudate floods alveoli + Surfactant inactivation
             ↓
Diffuse Alveolar Damage (DAD) = Hyaline membranes + Atelectasis
             ↓
Hypoxemia (shunt) + Decreased compliance + Pulmonary hypertension
             ↓
Exudative → Proliferative → Fibrotic phases

• Murray & Nadel's Textbook of Respiratory Medicine, Chapter 134 (ETIOLOGY AND PATHOGENESIS OF ARDS, THE ALVEOLAR-CAPILLARY MEMBRANE, NEUTROPHILS AND OTHER INFLAMMATORY MEDIATORS, INFLAMMATION AND COAGULATION)
• Goldman-Cecil Medicine, Chapter 91 (Mechanical Ventilation in ARDS)