Acute Respiratory Distress Syndrome (ARDS): Mechanism

1. The Berlin Definition (Diagnostic Framework)

• Bilateral opacities on chest X-ray or CT not fully explained by effusions, collapse, or nodules
• PaO2/FiO2 ratio <300 mmHg (mild: <300; moderate: <200; severe: <100)
• Onset within 7 days of a known clinical insult
• Respiratory failure not fully explained by cardiac failure or fluid overload
• Sabiston Textbook of Surgery, p. 940

2. Inciting Causes

• Murray & Nadel's Textbook of Respiratory Medicine

3. The Three Pathological Phases

Phase 1 - Exudative Phase (Days 1-7)

• Hyaline membranes form from cellular debris, proteins, and denatured surfactant components
• Protein-rich fluid fills alveolar spaces due to barrier breakdown
• Widespread epithelial and endothelial disruption
• Massive neutrophil infiltration of the interstitium and airspaces
• This is termed Diffuse Alveolar Damage (DAD)

Phase 2 - Proliferative Phase (Days 7-21)

• Hyaline membranes are reorganized
• Early fibrosis appears
• Obliteration of pulmonary capillaries
• Deposition of interstitial and alveolar collagen
• Neutrophil numbers decrease

Phase 3 - Fibrotic Phase (>2 weeks)

• Pulmonary fibrosis develops in a subset of patients
• N-terminal procollagen peptide III (a collagen synthesis marker) can be detected in BAL fluid as early as 24 hours, suggesting fibroproliferation begins simultaneously with inflammatory injury, not after it
• Murray & Nadel's Textbook of Respiratory Medicine, p. 3074-3076

Important caveat: Only ~50% of ARDS patients show DAD on autopsy or biopsy. Patients with confirmed DAD tend to have worse compliance, lower PaO2/FiO2, and roughly 5x greater risk of death from hypoxemic failure.

4. The Alveolar-Capillary Barrier

• Type I pneumocytes cover ~95% of the alveolar surface; extremely vulnerable to injury
• Type II pneumocytes cover ~5% of the surface; produce surfactant and serve as progenitors for type I cells during repair

1. A more permeable epithelium allows flooding of the normally dry airspace
2. Type II cell dysfunction impairs surfactant production and active ion transport (Na⁺/K⁺-ATPase, ENaC), which is the primary mechanism for alveolar fluid clearance
3. The degree of type II cell impairment correlates with severity and outcome

• Murray & Nadel's Textbook of Respiratory Medicine, p. 3083+

5. Neutrophil-Mediated Injury

1. Recruitment: Inflammatory cytokines (IL-8/CXCL8, TNF-α, IL-1β, IL-6) and complement fragments (C5a) massively recruit neutrophils into the pulmonary capillaries, interstitium, and airspaces
2. Activation: Activated neutrophils release a destructive arsenal:
   • Neutrophil elastase (NE) - cleaves extracellular matrix proteins, tight junction proteins, and surfactant proteins; degrades structural integrity of the barrier
   • Matrix metalloproteinases (MMPs) - further degrade the basement membrane
   • Reactive oxygen species (ROS) - oxidative damage to lipid membranes, proteins, and DNA
   • Myeloperoxidase (MPO) - generates hypochlorous acid (bleach-like oxidant)
   • Platelet-activating factor (PAF) - amplifies inflammation and increases vascular permeability
3. NETosis: Neutrophils form neutrophil extracellular traps (NETs) which contribute to microvascular occlusion and barrier injury

• Murray & Nadel's Textbook of Respiratory Medicine, p. 3140-3151

6. Surfactant Dysfunction

• Surfactant normally reduces alveolar surface tension, prevents collapse, and has anti-inflammatory properties
• In ARDS: plasma proteins (fibrinogen, albumin, immunoglobulins) flooding the alveolus competitively inhibit surfactant function
• Phospholipase A2 (released during pancreatitis, among other conditions) enzymatically degrades surfactant
• Type II pneumocyte injury directly reduces surfactant synthesis and secretion
• Result: alveolar collapse at end-expiration, increased work of breathing, worsening hypoxemia, decreased lung compliance
• Murray & Nadel's Textbook of Respiratory Medicine

7. Coagulation and Fibrin Deposition

• Alveolar and intravascular coagulation is activated
• Fibrin deposition occurs in alveolar spaces (contributing to hyaline membrane formation) and in pulmonary microvessels
• Plasminogen activator inhibitor-1 (PAI-1) is markedly elevated, suppressing fibrinolysis and promoting fibrin persistence
• Elevated PAI-1 is a biomarker of the hyperinflammatory ARDS subphenotype and correlates with worse mortality
• Pulmonary microvascular occlusion from fibrin thrombi increases dead space ventilation and pulmonary hypertension

8. Angiopoietin Pathway

• Angiopoietin-1 (Ang-1): Produced by pericytes; binds Tie2 and maintains endothelial quiescence and barrier integrity
• Angiopoietin-2 (Ang-2): Stored in Weibel-Palade bodies; released rapidly by endothelial activation; acts as an Ang-1 antagonist; destabilizes the endothelium and promotes vascular permeability and neutrophil adhesion
• In ARDS: plasma Ang-2 is markedly elevated and correlates with severity and mortality
• Ang-2 also acts synergistically with TNF-α to upregulate endothelial adhesion molecules, amplifying neutrophil trafficking

9. Na⁺/Water Clearance Failure

• ENaC (epithelial sodium channel) absorbing Na⁺ from the lumen
• Na⁺/K⁺-ATPase on the basolateral surface pumping Na⁺ out
• Water follows osmotically through aquaporins

• Direct type II cell damage
• Hypoxia downregulates Na⁺ transporter expression
• Cytokines (TNF-α, TGF-β) inhibit ion transport
• Catecholamine-stimulated upregulation of transporters is blunted

10. Physiological Consequences

11. Ventilator-Induced Lung Injury (VILI)

• Volutrauma: Overdistention of non-collapsed alveoli (the lung in ARDS is "baby-sized" - only ~30% of normal alveoli remain open)
• Atelectrauma: Cyclic opening and closing of collapsed units generates shear stress
• Barotrauma: Excessive pressures rupture alveoli
• Biotrauma: Mechanical stretch induces cytokine release (mechano-transduction), amplifying systemic inflammation and contributing to multi-organ failure

• Goldman-Cecil Medicine, p. 1066-1067

12. ARDS Subphenotypes

• Murray & Nadel's Textbook of Respiratory Medicine, p. 3271-3272

Summary Diagram

Triggering insult (sepsis, pneumonia, trauma, etc.)
            ↓
 Systemic/local inflammatory cascade
  (TNF-α, IL-1β, IL-6, IL-8, C5a, PAF)
            ↓
  Neutrophil sequestration in lung
            ↓
 Release of elastase, ROS, MMPs, MPO
            ↓
 Endothelial + Epithelial barrier disruption
            ↓
 ┌──────────────────────────────────────┐
 │ Protein-rich exudate floods alveoli  │
 │ Surfactant inactivation              │
 │ Type II pneumocyte dysfunction       │
 │ Fibrin deposition (hyaline membranes)│
 │ Impaired Na⁺/fluid clearance         │
 └──────────────────────────────────────┘
            ↓
  DIFFUSE ALVEOLAR DAMAGE
  ↓ Compliance | ↑ Shunt | Hypoxemia
  Pulmonary hypertension | ↑ Dead space
            ↓
   ARDS (Berlin criteria met)
  ± VILI from mechanical ventilation
  ± Fibroproliferative phase → Fibrosis

• Murray & Nadel's Textbook of Respiratory Medicine (2-Volume), pp. 3064-3275
• Goldman-Cecil Medicine International Edition, pp. 1065-1068
• Sabiston Textbook of Surgery (Biological Basis of Modern Surgical Practice), p. 940