Mechanism of Acute Respiratory Distress Syndrome (ARDS)

1. Definition & Diagnostic Context

• Mild: PaO₂/FiO₂ 200–300
• Moderate: 100–200
• Severe: <100

2. Inciting Stimuli

3. Pathologic Phases

Phase 1 — Exudative (Days 1–7)

• Protein-rich, exudative fluid floods the alveolar spaces
• Hyaline membranes form — composed of cellular debris, proteins, and surfactant remnants
• Widespread epithelial disruption
• Massive neutrophil infiltration of interstitium and airspaces
• Loss of surfactant → alveolar collapse and decreased compliance

Phase 2 — Proliferative (Days 7–21)

• Hyaline membranes are reorganized
• Early fibrosis appears
• Type II pneumocyte proliferation attempts epithelial repair
• Decreased neutrophil infiltration
• Reduction in pulmonary edema

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

• Established pulmonary fibrosis
• Obliteration of pulmonary capillaries
• Deposition of interstitial and alveolar collagen
• Notably, evidence of fibroproliferation (elevated N-terminal procollagen peptide III in BAL fluid) may begin as early as 24 hours after injury onset

4. Core Mechanism: Alveolar-Capillary Barrier Disruption

Endothelial Injury

• Normally, the pulmonary microvascular endothelium presents a tight barrier maintained by cell-cell junctions (adherens junctions, tight junctions)
• In ARDS, inflammatory mediators (TNF-α, IL-1β, IL-8) damage these junctions
• Destruction of endothelial cadherins (key adherens junction proteins) by neutrophil elastase leads to paracellular leak
• Result: protein-rich plasma fluid escapes into the interstitium and alveolar spaces

Epithelial Injury

• Type I pneumocytes (covering ~95% of alveolar surface) are particularly vulnerable
• Their loss impairs the normal fluid-clearing function of the epithelial barrier
• Type II pneumocytes are the primary survivors and must regenerate type I cells
• Injury to type II cells impairs surfactant production, worsening alveolar instability

5. The Neutrophil as Central Effector

1. Sequestration: Neutrophils are retained in pulmonary capillaries and recruited into alveolar spaces following exposure to chemoattractants (IL-8/CXCL8, C5a, leukotriene B4)
2. Reactive oxygen species (ROS): Oxidative burst generates superoxide, hydrogen peroxide, and hypochlorous acid — directly damaging epithelial and endothelial cells
3. Proteolytic enzymes:
   • Neutrophil elastase (NE): Degrades growth factors, cytokines, and epithelial/endothelial cadherins — predisposing to alveolar flooding
   • Matrix metalloproteinases (MMPs): Destroy basement membrane components
4. Neutrophil Extracellular Traps (NETs): Web-like structures of DNA, histones, myeloperoxidase, NE, and cathepsin G — released to trap pathogens. In ARDS, NETs cause endothelial damage and promote thrombus formation. BAL fluid from ARDS patients shows NET concentrations inversely correlated with PaO₂/FiO₂
5. Platelet-neutrophil complexes: Via P-selectin/PSGL-1 interactions, platelets and neutrophils form complexes that amplify ROS and NET release, driving cell injury through necroptosis and pyroptosis

6. Cytokine Storm & Macrophage Activation

• Alveolar macrophages (M1 phenotype) activated via TLR/PRR pathways release proinflammatory cytokines: TNF-α, IL-1β, IL-6, IL-8
• This cytokine storm recruits and hyperactivates neutrophils, creating a feedforward loop
• T-cell dysregulation: Th17 responses increase (↑IL-17), while T-regulatory cells decrease, reducing anti-inflammatory mediators (IL-10, TGF-β)
• Endothelial cell activation by TNF-α and proinflammatory cytokines amplifies permeability

7. Surfactant Dysfunction

• Normal pulmonary surfactant (primarily phosphatidylcholine + surfactant proteins SP-A, SP-B, SP-C, SP-D) reduces alveolar surface tension and prevents collapse
• In ARDS, phospholipase A₂ (released during pancreatitis or by inflammatory cells) degrades surfactant
• Plasma proteins leaking into alveoli inhibit surfactant function
• Loss of surfactant → progressive alveolar collapse → reduced functional residual capacity (FRC) → intrapulmonary shunting and refractory hypoxemia
• There is a strong correlation between serum phospholipase A₂ levels and respiratory insufficiency in pancreatitis-associated ARDS — Murray & Nadel's

8. Coagulation-Inflammation Cross-Talk

• TNF-α → stimulates tissue factor expression on endothelial cells → activates extrinsic coagulation cascade → thrombin and fibrin formation
• TNF-α also inhibits fibrinolysis
• Fibrin fragments act as chemoattractants for neutrophils, amplifying inflammation
• Intravascular fibrin deposition causes microthrombi → pulmonary hypertension
• Angiopoietin-2 (released from activated endothelium) destabilizes the vessel wall and promotes permeability; angiopoietin-1 is protective — levels of Ang-2 are elevated in ARDS and correlate with severity

9. Sodium & Water Transport Failure

• Hypoxia and cytokine-mediated injury impair these transport mechanisms
• Failure of active sodium transport allows edema fluid to persist in airspaces even after capillary leak has been controlled
• Recovery of alveolar fluid clearance capacity is a key determinant of outcome

10. Ventilator-Induced Lung Injury (VILI)

• Overdistend the aerated lung (volutrauma/barotrauma)
• Promote cyclic atelectrauma in recruitable lung units
• Trigger biotrauma: mechanical stretch activates inflammatory signaling (NF-κB pathway), releasing additional cytokines that can spread injury systemically → multi-organ failure

11. Pulmonary Hypertension in ARDS

• Hypoxic vasoconstriction (HPV): alveolar hypoxia constricts local vasculature
• Intravascular fibrin microthrombi obstruct vessels
• Compression by positive-pressure ventilation
• Mediator-driven vasoconstriction (thromboxane A₂, endothelin-1)

Summary Flowchart

Inciting Insult (sepsis, pneumonia, trauma, aspiration)
         ↓
Alveolar macrophage + pattern recognition receptor activation
         ↓
Cytokine storm (TNF-α, IL-1β, IL-6, IL-8)
         ↓
Neutrophil sequestration & activation in pulmonary capillaries
         ↓
ROS, NE, NETs, proteases released
         ↓
Endothelial & epithelial barrier disruption (cadherin degradation)
         ↓
Protein-rich exudate floods alveoli
+ Surfactant degradation / dysfunction
+ Coagulation activation → microthrombi
+ Sodium transport failure
         ↓
Diffuse Alveolar Damage (DAD)
- Hyaline membrane formation
- Alveolar collapse (↓FRC)
- Right-to-left shunting
- ↓ Compliance, refractory hypoxemia
         ↓
ARDS (± progression to fibrosis, pulmonary hypertension, multi-organ failure)