Acute Respiratory Distress Syndrome (ARDS): Mechanism

Precipitating Triggers

Pathophysiology: Three Sequential Phases

Phase 1: Exudative Phase (Days 0-7)

• Bacteria, viruses, or systemic mediators activate Toll-like receptors (TLRs) on alveolar type I (ATI) epithelial cells and resident alveolar macrophages
• Macrophages release proinflammatory cytokines: IL-1, IL-6, IL-8 (CXCL8), and TNF-α
• IL-8 is a potent neutrophil chemoattractant, driving massive pulmonary neutrophil recruitment

• Neutrophils (PMNs) migrate from pulmonary capillaries across the endothelium and epithelium into the alveolar space - this is the central step in injury
• Activated neutrophils release:
  • Proteases (elastase, collagenase, MMP-8/9) - degrade basement membrane
  • Reactive oxygen species (ROS) - cause oxidative injury to lipid membranes
  • Neutrophil extracellular traps (NETs) - contribute to microvascular occlusion
  • Platelet-activating factor - amplifies inflammatory signaling

• The barrier has two layers: the capillary endothelium and the alveolar epithelium (type I and II pneumocytes)
• Protease and ROS attack disrupts tight junctions and destroys type I pneumocytes (thin, gas-exchange cells covering ~95% of alveolar surface)
• Result: markedly increased permeability allows protein-rich exudative fluid to flood the interstitium and alveolar spaces (unlike cardiogenic edema which is low-protein hydrostatic)

• Circulating monocytes migrate into the lung and differentiate into macrophages
• They release IFN-β, which triggers monocyte-derived secretion of TRAIL (TNF-related apoptosis-inducing ligand)
• TRAIL activates death receptors on alveolar epithelial cells, causing apoptosis and further barrier breakdown
• PMN-platelet aggregates form and worsen NET formation

• Disrupted tight junctions allow RBCs into the alveolar space
• Lysed RBCs release cell-free hemoglobin, which generates additional oxidative stress via Fenton chemistry

• Type II pneumocytes are also injured, reducing surfactant production
• Phospholipase A2 (elevated in pancreatitis-related ARDS) enzymatically degrades existing surfactant
• Loss of surfactant raises surface tension → alveolar collapse → atelectasis

• Condensed plasma proteins, cellular debris, and dysfunctional surfactant components aggregate in the alveolar space to form characteristic hyaline membranes (named for their glassy histologic appearance)

• Vascular injury leads to microthrombosis (fueled by NETs and platelet aggregates) and fibrocellular proliferation in vessel walls
• Results in: dead-space ventilation, pulmonary vascular resistance, and pulmonary hypertension
• Mechanisms of pulmonary hypertension: hypoxic vasoconstriction + intravascular fibrin deposition + compression by positive-pressure ventilation

• Normally, alveolar fluid is cleared by ENaC (epithelial sodium channel) and Na+/K+-ATPase on type II pneumocytes
• Hypoxemia and hypercapnia directly impair sodium transport, so edema clearance is reduced, worsening the cycle

• Alveolar edema (gravity-dependent, heterogeneous distribution) → ↓ functional residual capacity
• Collapsed dependent zones → intrapulmonary right-to-left shunt → refractory hypoxemia (hallmark)
• Dead space ventilation in non-collapsed regions → hypercapnia
• Reduced lung compliance (stiff lung) → increased work of breathing → respiratory fatigue and failure

Phase 2: Proliferative Phase (Days 7-21)

• The shift from neutrophil-predominant to lymphocyte-predominant inflammation signals the beginning of repair
• Type II pneumocytes (cuboidal, surfactant-producing) proliferate to cover denuded alveolar surfaces and differentiate into type I cells
• Hyaline membranes are reorganized and early fibrosis appears
• Fibroblast activation and collagen deposition begin (procollagen peptide III detectable in BAL fluid as early as 24 hours after onset - suggesting fibroproliferation begins simultaneously with injury)
• Many patients recover during this phase; some develop progressive lung injury

Phase 3: Fibrotic Phase (>Day 21)

• Occurs in a subset of patients with persistent ARDS (>2 weeks)
• Obliteration of pulmonary capillaries, extensive interstitial and alveolar collagen deposition, and bullae formation
• Loss of normal lung architecture
• Associated with prolonged mechanical ventilation dependence, increased risk of pneumothorax, and high mortality

Pathologic Hallmark: Diffuse Alveolar Damage (DAD)

• Alveolar edema
• Type I pneumocyte necrosis
• Neutrophil accumulation in interstitium and alveoli
• Hyaline membrane formation
• Interstitial and alveolar collagen (later stages)

Summary of Key Mediators

What Makes It Non-Cardiogenic

• Normal or low pulmonary capillary wedge pressure (<18 mmHg)
• High-protein exudative edema (reflecting barrier permeability, not hydrostatic pressure)
• Absence of cardiomegaly, pleural effusions, or pulmonary vascular redistribution on CXR (though these can coexist)

• Harrison's Principles of Internal Medicine 22e, Chapter 312 - "Clinical course and pathophysiology"
• Murray & Nadel's Textbook of Respiratory Medicine - "Etiology and Pathogenesis of ARDS / Pathology / Alveolar-Capillary Membrane"
• Robbins & Kumar Basic Pathology - "Acute Lung Injury and Acute Respiratory Distress Syndrome"
• Sabiston Textbook of Surgery - "ARDS" (Berlin definition, management framework)