Acute Respiratory Distress Syndrome (ARDS): Mechanism

Overview

Triggers and Initiating Insults

Three Sequential Pathologic Phases

Phase 1 — Exudative (Days 0–7)

1. Initial insult activates Toll-like receptors (TLRs) on alveolar type I (AT1) epithelial cells and resident alveolar macrophages.
2. Macrophages secrete pro-inflammatory cytokines — TNF-α, IL-1β, IL-6, IL-8 — that recruit circulating neutrophils via chemotaxis.
3. Neutrophil sequestration: Activated neutrophils become mechanically "stiff" (actin cytoskeleton remodeling) and are physically trapped in narrow pulmonary capillaries (capillary diameter < neutrophil diameter). This produces the transient leukopenia that often precedes ARDS.
4. Sequestered neutrophils then transmigrate across the alveolar-capillary membrane, releasing a cascade of cytotoxic mediators:
   • Reactive oxygen species (ROS) — oxidative damage to membranes
   • Proteases — neutrophil elastase (NE) degrades epithelial/endothelial cadherins (adherens junctions), destroys surfactant protein A, and degrades growth factors
   • Neutrophil extracellular traps (NETs) — web-like DNA–histone structures that trap pathogens but also cause endothelial damage and thrombus formation; NET-associated IL-6 and TNF amplify inflammation
5. Monocytes migrate into the lung and cause epithelial apoptosis via IFN-β–dependent TRAIL release (activating death receptors on epithelial cells).
6. Platelets form aggregates with PMNs and monocytes, amplifying NET formation and inflammatory signaling.
7. Endothelial and epithelial barrier failure: Loss of both the microvascular endothelium (necessary and sufficient for edema formation) and the alveolar epithelium leads to flooding of protein-rich fluid and RBCs into the airspaces.

• Hyaline membranes (composed of cellular debris, fibrin, and denatured surfactant)
• Alveolar edema (protein-rich fluid)
• Interstitial and alveolar neutrophil infiltration
• Type I pneumocyte necrosis
• Congestion and interstitial edema

• Massive right-to-left intrapulmonary shunt (fluid-filled/atelectatic alveoli perfused but not ventilated) → refractory hypoxemia
• Decreased respiratory system compliance (stiff lungs)
• Increased dead-space ventilation (vascular compression, intravascular fibrin deposition)
• Pulmonary hypertension (hypoxic vasoconstriction + fibrin microthrombi + vascular compression by PEEP)

Phase 2 — Proliferative (Days 7–21)

• Neutrophil-predominant infiltrate shifts to lymphocyte-predominant
• Type II pneumocytes proliferate along denuded basement membranes, synthesize new surfactant, and transdifferentiate into type I pneumocytes (reepithelialisation)
• Hyaline membranes are reorganized; early fibrotic changes appear (collagen deposition)
• Interstitial inflammation and early architectural disruption develop
• N-terminal procollagen peptide III (a collagen synthesis marker) is detectable in BAL fluid as early as 24 hours after injury onset, indicating fibroproliferation begins simultaneously with inflammation, not after it

Phase 3 — Fibrotic (>21 days, subset of patients)

• Alveolar-duct and interstitial fibrosis replace inflammatory exudate
• Emphysema-like changes with large bullae from architectural destruction
• Intimal fibroproliferation in pulmonary microcirculation → progressive vascular occlusion → pulmonary hypertension
• Marked reduction in lung compliance; increased dead space; risk of pneumothorax
• Lung biopsy evidence of fibrosis in any phase correlates with increased mortality

The Cytokine/Mediator Network

Coagulation Dysregulation

• Tissue factor activation and fibrin deposition in pulmonary microvasculature
• Platelet-neutrophil complexes amplify both coagulation and NET release
• Microthrombi contribute to pulmonary hypertension and dead-space ventilation
• Fibrin components in the alveolar space further impair surfactant function

Impaired Alveolar Fluid Clearance

• Hypoxemia and hypercapnia impair sodium transport, reducing fluid clearance
• Loss of type I/II pneumocytes removes the cellular machinery for active clearance
• Edema therefore accumulates and persists, worsening hypoxemia further in a positive-feedback loop

Why DAD Is Found in Only ~50% of ARDS Autopsies

Summary Schematic

Triggering insult (sepsis, pneumonia, aspiration, trauma, etc.)
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Activation of TLRs on macrophages and AT1 cells
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Release of TNF-α, IL-1β, IL-6, IL-8
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Neutrophil recruitment → capillary sequestration → transmigration
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PMN degranulation: ROS + elastase + NETs + cytokines
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Endothelial + epithelial barrier breakdown
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Protein-rich exudate floods alveoli + surfactant dysfunction
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Hyaline membrane formation + microatelectasis + shunt
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Refractory hypoxemia + ↓ compliance + ↑ dead space + pulm. HTN
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(If unresolved) → Type II pneumocyte proliferation → fibroproliferation
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(Subset) → Fibrotic phase: pulmonary fibrosis + bullae + cor pulmonale

• Murray & Nadel's Textbook of Respiratory Medicine, Chapter 134 (Etiology and Pathogenesis of ARDS)
• Harrison's Principles of Internal Medicine, 22nd Ed., Chapter 312 (Clinical Course and Pathophysiology)
• Robbins & Kumar Basic Pathology (Acute Lung Injury and ARDS)
• Goldman-Cecil Medicine, Chapter 90–91 (ARDS and Mechanical Ventilation)