Acute Respiratory Distress Syndrome (ARDS): Mechanism

Overview

• Direct: Pneumonia, aspiration of gastric contents, pulmonary contusion, inhalation injury
• Indirect: Sepsis (most common), pancreatitis, major trauma, transfusion-related lung injury (TRALI), burns

Phases of ARDS

1. Exudative Phase (Days 1-7)

• Type I pneumocytes (which cover ~95% of the alveolar surface) are particularly vulnerable. Their death disrupts barrier integrity, allows flooding of the alveolar space with proteinaceous fluid, and eliminates the cells responsible for alveolar fluid clearance.
• Type II pneumocytes are also damaged, impairing surfactant synthesis.
• Multiple mechanisms drive epithelial cell death: necrosis, apoptosis, coagulation cascade activation, and mechanical stretch.
• Loss of functional ENaC (epithelial Na+ channels) and Na+/K+-ATPase on alveolar epithelial cells impairs active sodium - and thus fluid - reabsorption from the alveolar space, perpetuating edema.

• Loss of pulmonary vascular endothelial barrier integrity is both necessary and sufficient for ARDS development.
• Disruption of tight junctions and adherens junctions allows plasma proteins and fluid to leak into the interstitium and alveoli.
• Elevated angiopoietin-2 (Ang2) - released from endothelial cells - competitively antagonizes the barrier-stabilizing angiopoietin-1/Tie2 axis, amplifying vascular leak. Genetic variants in Ang2 are associated with higher ARDS risk.

The Role of Neutrophils and Inflammatory Mediators

1. Sequestration: Activated neutrophils become "stiff" (actin cytoskeleton remodeling) and cannot deform through narrow pulmonary capillaries (diameter ~5-8 µm, smaller than a neutrophil). This causes transient leukopenia - one of the earliest manifestations of ARDS, preceding hypoxemia. Sequestered neutrophils then induce endothelial barrier breakdown, facilitating their own transmigration.
2. Transmigration: PMNs cross the alveolar-capillary membrane, where they release:
   • Reactive oxygen species (ROS) - oxidative damage to epithelial and endothelial cells
   • Neutrophil elastase (NE) - degrades epithelial/endothelial cadherins (adherens junction components), destroys surfactant protein A, and cleaves growth factors/cytokines
   • Matrix metalloproteinases - proteolytic degradation of the extracellular matrix
   • TNF-α and IL-1β - amplify the inflammatory cascade
   • IL-8 - potent neutrophil chemoattractant, driving further PMN recruitment via PI3K-γ signaling
3. Neutrophil extracellular traps (NETs): Activated neutrophils release web-like structures of DNA, histones, myeloperoxidase, and NE. In ARDS, large quantities of NETs cause direct endothelial damage and thrombus formation. Animal models show that DNase treatment (degrading NETs) attenuates ARDS severity and reduces IL-6 and TNF levels.

Surfactant Dysfunction

• Damage to Type II pneumocytes reduces surfactant synthesis.
• The composition of surfactant changes: the ratio of dipalmitoylphosphatidylcholine (DPPC) and phosphatidylglycerol is altered, and the proportion of large (active) to small (inactive) surfactant aggregates decreases.
• Plasma proteins leaking into alveoli directly inhibit surfactant function.
• Neutrophil elastase degrades surfactant protein A in vitro, consistent with degraded SP-A found in BAL fluid from ARDS patients.
• Consequence: alveolar units collapse (atelectasis), reducing functional residual capacity (FRC) and worsening ventilation-perfusion mismatch and intrapulmonary shunting.

2. Proliferative Phase (Days 7-21)

• Alveolar exudates are organized and cleared.
• Inflammatory infiltrate shifts from neutrophil-predominant to lymphocyte-predominant.
• Type II pneumocytes proliferate along denuded alveolar basement membranes, synthesizing new surfactant and differentiating into Type I cells to restore barrier function.
• Some patients develop early pulmonary fibrosis: elevated N-terminal procollagen peptide III (a marker of collagen synthesis) can be detected in BAL fluid as early as 24 hours after onset, suggesting fibroproliferation may begin in parallel with, not after, the inflammatory phase.

3. Fibrotic Phase (>3 weeks in some patients)

• Alveolar-duct and interstitial fibrosis replaces the earlier inflammatory exudate.
• Emphysema-like changes and large bullae develop from acinar architectural destruction.
• Intimal fibroproliferation in pulmonary microcirculation causes vascular occlusion and pulmonary hypertension.
• Lung compliance decreases, dead space increases, and pneumothorax risk rises.

Ventilator-Induced Lung Injury (VILI) - An Amplifying Mechanism

• Volutrauma: High tidal volumes cause overdistension of relatively normal alveoli (ARDS is heterogeneous - dependent regions are consolidated/atelectatic, non-dependent regions are relatively normal). Overdistension increases epithelial and endothelial permeability.
• Atelectrauma: Repetitive opening and closing of collapsed terminal lung units generates high shear stresses at the interface of collapsed and aerated tissue.
• Biotrauma: Mechanical stretch activates alveolar macrophages and epithelial cells to release cytokines (TNF-α, IL-1β, IL-6, leukotrienes, prostaglandins, ROS), which can spill into the systemic circulation and contribute to multi-organ dysfunction.

Summary: Pathophysiologic Cascade

Precipitating insult (direct or indirect)
        ↓
Activation of innate immunity (macrophages, Toll-like receptors, complement)
        ↓
Cytokine storm (TNF-α, IL-1β, IL-6, IL-8)
        ↓
Neutrophil sequestration & transmigration into alveoli
        ↓
Release of proteases (elastase, MMPs), ROS, NETs, cytokines
        ↓
Disruption of alveolar-capillary barrier
(endothelial tight junction breakdown + Ang2↑ + epithelial death)
        ↓
Protein-rich edema floods alveoli → hyaline membranes
Surfactant dysfunction → alveolar collapse (atelectasis)
Na+ transport failure → impaired fluid clearance
Coagulation activation → microvascular fibrin thrombi
        ↓
↓ FRC, ↑ intrapulmonary shunt, V/Q mismatch
        ↓
Refractory hypoxemia (hallmark of ARDS)
        ↓
(+/- VILI amplification on mechanical ventilation)
        ↓
Recovery (proliferative) OR fibrosis progression