Acute Respiratory Distress Syndrome (ARDS): Mechanism

Definition and Overview

The Central Lesion: Alveolar-Capillary Barrier Breakdown

1. Pulmonary microvascular endothelium - loss of endothelial barrier integrity is both necessary and sufficient for ARDS to develop
2. Alveolar epithelium (particularly Type I pneumocytes) - damage disrupts barrier integrity, prevents alveolar fluid clearance, and abolishes normal epithelial repair

Step-by-Step Pathogenesis

1. Initiating Insult

• Direct: pneumonia, aspiration, inhalational injury, pulmonary contusion
• Indirect: sepsis, pancreatitis, trauma, transfusion reactions (TRALI)

2. Innate Immune Activation and Cytokine Storm

• Activate pulmonary vascular endothelium
• Upregulate adhesion molecules (selectins, beta-2 integrins)
• Recruit and activate circulating neutrophils into the pulmonary microvasculature

3. Neutrophil Sequestration and Transmigration (Central Step)

• Pulmonary capillaries are narrower than neutrophil diameter, so neutrophils must deform to pass through
• Activated neutrophils become "stiff" (via actin cytoskeletal remodeling), cannot negotiate these narrow segments, and become sequestered in pulmonary capillaries
• One of the earliest signs is transient leukopenia as neutrophils are trapped in the lung before hypoxemia even develops
• Sequestered neutrophils then transmigrate across the alveolar-capillary membrane into the interstitium and alveolar space - and notably, this migration can occur without the usual requirement for L-selectin and beta-2 integrins

4. Neutrophil-Mediated Tissue Destruction

5. Surfactant Failure

• Leaked plasma proteins inactivate surfactant by competing with phospholipids at the air-liquid interface
• Neutrophil elastase degrades surfactant protein A
• The proportion of large (active) to small (inactive) surfactant aggregates falls due to decreased production and increased conversion
• Consequence: alveolar collapse (atelectasis), reduced lung compliance, and worsening hypoxemia

6. Coagulation Dysregulation

• The inflammatory milieu activates the coagulation cascade within the alveolar space
• Intravascular fibrin deposition and microvascular thrombosis further contribute to pulmonary hypertension and dead-space ventilation
• Fibrin in the alveolar lumen contributes to hyaline membrane formation

Pathophysiological Consequences

Three Pathological Phases (Diffuse Alveolar Damage)

Phase 1 - Exudative (Days 1-7)

• Hyaline membranes (fibrin + cellular debris + surfactant remnants) line alveolar ducts
• Protein-rich edema floods the alveolar spaces
• Widespread epithelial disruption; necrosis of Type I pneumocytes
• Heavy neutrophil infiltration in interstitium and airspaces
• Macroscopic appearance: lungs are dark red, firm, airless, heavy

Phase 2 - Proliferative (Days 7-14)

• Type II pneumocytes proliferate vigorously to regenerate the alveolar lining
• Hyaline membranes are reorganized
• Neutrophils decrease; fibroblasts and procollagen appear
• Obliteration of pulmonary capillaries begins

Phase 3 - Fibrotic (>2 weeks)

• Fibrin-rich exudate undergoes organization
• Alveolar septal thickening and fibrosis
• Importantly, fibroproliferation may begin simultaneously with inflammatory injury (elevated N-terminal procollagen III peptide is detectable in BAL fluid as early as 24 hours after ARDS onset)

Ventilator-Induced Lung Injury (VILI) and Biotrauma

• Volutrauma/barotrauma: High tidal volumes cause stress fractures in the alveolar-capillary membrane and basement membrane
• Biotrauma: High tidal volumes and zero PEEP trigger local cytokine release (TNF, IL-1, IL-6) that spills into the systemic circulation, potentially driving multiorgan failure - a phenomenon called biotrauma. Switching from a protective to an injurious ventilation strategy increases proinflammatory cytokines in the lungs and plasma within one hour.

Neutrophil-Platelet Interactions

Summary Diagram of the Core Cascade

Initiating insult (sepsis, pneumonia, aspiration, trauma)
        ↓
Macrophage activation → IL-1, TNF-alpha
        ↓
Endothelial activation, neutrophil recruitment
        ↓
Neutrophil sequestration in pulmonary capillaries (stiffness → trapping)
        ↓
Transmigration into alveolar space
        ↓
ROS + proteases + cytokines released
        ↓
Alveolar epithelial + endothelial injury
        ↓
┌──────────────────────────────────────────────────────────────┐
│ Increased permeability → protein-rich edema                  │
│ Surfactant inactivation → atelectasis                        │
│ Coagulation activation → fibrin, hyaline membranes          │
│ Type I pneumocyte death → impaired fluid clearance           │
└──────────────────────────────────────────────────────────────┘
        ↓
Hypoxemia, reduced compliance, pulmonary hypertension (ARDS)
        ↓ (if ventilated injuriously)
Biotrauma → systemic cytokine dissemination → multiorgan failure