Acute Respiratory Distress Syndrome (ARDS): Mechanism

Definition & Berlin Criteria

• Bilateral pulmonary infiltrates on CXR/CT
• PaO₂/FiO₂ (P/F) ratio <300 mmHg (mild), <200 (moderate), <100 (severe)
• Known clinical insult within 7 days
• Not fully explained by cardiac failure or fluid overload

Causes (Triggers)

Phases of ARDS

Phase 1: Exudative Phase (Days 0–7)

1. Initial Insult → Pattern Recognition & Innate Immune Activation

• Pathogens, DAMPs (damage-associated molecular patterns), or toxins are recognized by Toll-like receptors (TLRs) on alveolar type I (ATI) epithelial cells and resident alveolar macrophages.
• Macrophages secrete pro-inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-8/CXCL8), initiating a cytokine cascade.

2. Neutrophil Recruitment & Activation (Central Mediators)

• IL-8 and other chemokines recruit circulating PMNs from the bloodstream into the alveolar interstitium and airspaces.
• Activated neutrophils migrate across the endothelium and epithelium, releasing:
  • Proteases (elastase, matrix metalloproteinases) — digest basement membrane and tight junction proteins
  • Reactive oxygen species (ROS) — oxidant-mediated membrane and organelle injury
  • Neutrophil extracellular traps (NETs) — chromatin-protease complexes that amplify injury

3. Endothelial & Epithelial Barrier Disruption

• Type I pneumocytes (gas exchange cells, ~95% of alveolar surface area) are highly vulnerable to injury and lack regenerative capacity.
• Disruption of tight junctions and ENaC (epithelial sodium channels) impairs active fluid clearance from alveoli, compounding edema.

4. Protein-Rich Alveolar Edema

• Plasma proteins (including fibrinogen) flood the airspace.
• Fibrinogen is converted to fibrin, which precipitates on the denuded alveolar walls along with necrotic cell debris → forming hyaline membranes (the histologic hallmark of DAD).
• Surfactant dysfunction: Plasma proteins inactivate surfactant; type II pneumocyte injury reduces surfactant production. Loss of surfactant increases alveolar surface tension → alveolar collapse and atelectasis.

5. Additional Injury Pathways

• Monocytes migrate into the lung and cause epithelial apoptosis via IFN-β–dependent TRAIL (TNF-related apoptosis-inducing ligand) release.
• Platelets form aggregates with PMNs (PMN-platelet aggregates), amplifying NET formation and oxidant injury.
• Red blood cells that extravasate release cell-free hemoglobin, causing further oxidant-mediated damage.
• Microvascular thrombi form due to local coagulation activation, contributing to increased pulmonary dead space and hypercapnia.

Physiologic Consequences of the Exudative Phase

• Hypoxemia: Edema-filled alveoli cannot participate in gas exchange → intrapulmonary shunting (perfusion without ventilation); refractory to supplemental O₂ alone.
• Decreased compliance: Fluid and collapse stiffen the lung ("baby lung" concept — only a small proportion of alveoli remain aerated and recruitable).
• Increased dead space: Microvascular obstruction impairs CO₂ excretion → hypercapnia.
• Bilateral infiltrates on CXR/CT, predominantly in dependent zones due to gravitational distribution of edema.

Phase 2: Proliferative Phase (Days 7–21)

• Neutrophil infiltrate is replaced by lymphocyte-predominant inflammation.
• Type II pneumocytes proliferate along denuded basement membranes — they synthesize new surfactant and differentiate into type I cells to restore the epithelial barrier.
• Alveolar exudates begin to organize; macrophages clear fibrin and debris.
• Many patients improve during this phase.

Phase 3: Fibrotic Phase (>Day 21, subset of patients)

• Alveolar edema and inflammatory exudates convert to alveolar-duct and interstitial fibrosis.
• Disrupted acinar architecture leads to emphysema-like changes and large bullae.
• Intimal fibroproliferation in pulmonary microvessels → progressive vascular occlusion → pulmonary hypertension.
• Results in reduced compliance, increased dead space, elevated pneumothorax risk, and oxygen dependence.

Summary Cascade

Trigger (sepsis, trauma, aspiration...)
        ↓
TLR activation on ATI cells & macrophages
        ↓
Cytokine storm (TNF-α, IL-1β, IL-6, IL-8)
        ↓
PMN recruitment & activation into alveoli
        ↓
Proteases + ROS + NETs released
        ↓
Endothelial & epithelial barrier disruption
  ├── Protein-rich edema floods alveoli
  ├── Fibrin deposition → hyaline membranes
  ├── Surfactant inactivation → alveolar collapse
  └── Impaired Na/fluid clearance (ENaC/ATPase)
        ↓
Hypoxemia (shunt) + Reduced compliance + Dead space
        ↓
Respiratory failure (ARDS)

Key Points

• Harrison's Principles of Internal Medicine, 22e, Chapter 312
• Robbins & Kumar Basic Pathology, Chapter 11
• Goldman-Cecil Medicine, Chapter 79 / Chapter 91
• Sabiston Textbook of Surgery, Chapter 11