Mechanism of Acute Respiratory Distress Syndrome (ARDS)

Definition and Core Concept

Triggers: Direct vs. Indirect Lung Injury

Phases of Pathology (Diffuse Alveolar Damage)

• Hyaline membrane formation (cellular debris + proteins + surfactant components)
• Protein-rich fluid fills alveolar spaces
• Widespread epithelial disruption
• Massive neutrophil infiltration of interstitium and airspaces

• Hyaline membranes are reorganized
• Fibrosis begins to appear
• Pulmonary capillary obliteration
• Interstitial and alveolar collagen deposition
• Neutrophil numbers decrease

• Frank pulmonary fibrosis in a subset of patients
• Elevated N-terminal procollagen peptide III detectable in BAL fluid as early as 24 hours — indicating that fibroproliferation may begin simultaneously with, rather than after, the inflammatory injury

The Alveolar-Capillary Barrier: Core Mechanism

• Normally, the endothelium is a semi-permeable barrier, but under inflammatory conditions it becomes highly permeable
• Neutrophil-derived oxidants, proteases, and cytokines directly injure endothelial cells
• Loss of endothelial integrity → protein-rich edema fluid leaks into the interstitium and alveoli

• Type I cells (covering ~95% of the alveolar surface) are exquisitely sensitive to injury; their death causes massive permeability failure
• Type II cells (normally 5% of surface) are more resilient; they are responsible for surfactant production and for regenerating Type I cells during repair
• In ARDS, Type II cell dysfunction leads to surfactant deficiency, causing alveolar collapse and worsening shunt
• Normally, Type II cells reabsorb alveolar fluid via active sodium transport (ENaC channels); in ARDS this fluid-clearance capacity is severely impaired

Neutrophil-Mediated Injury: The Central Effector

1. An initiating stimulus (e.g., LPS from gram-negative bacteria in sepsis, gastric acid in aspiration) activates alveolar macrophages
2. Activated macrophages release IL-1β, IL-6, IL-8 (CXCL8), and TNF-α
3. IL-8 is a potent neutrophil chemoattractant — it recruits circulating neutrophils to the lung
4. Neutrophils marginate against pulmonary capillary endothelium via upregulated adhesion molecules (E-selectin, ICAM-1)
5. Transmigrated neutrophils release:
   • Reactive oxygen species (ROS) — oxidize lipid membranes, DNA, and proteins
   • Proteases (elastase, matrix metalloproteinases) — degrade the extracellular matrix and tight junctions
   • Platelet-activating factor and leukotrienes — amplify the inflammatory response
6. This causes widespread alveolar epithelial and endothelial death → barrier failure

Coagulation and Vascular Mechanisms

• Intravascular fibrin deposition in pulmonary capillaries causes microvascular obstruction
• Fibrin is also deposited in alveolar spaces, contributing to hyaline membrane formation
• Activated neutrophils and platelets further promote a prothrombotic, anti-fibrinolytic state in the lung
• These vascular changes contribute to pulmonary hypertension, alongside hypoxic vasoconstriction and mechanical compression of vessels by positive-pressure ventilation

Surfactant Dysfunction

• Phospholipase A2 (released systemically in pancreatitis, and locally in other triggers) enzymatically degrades surfactant
• Protein-rich edema fluid in alveoli also directly inactivates surfactant
• Loss of surfactant increases surface tension, causing alveolar collapse, reduced compliance, and worsening intrapulmonary shunt

Physiological Consequences

Biotrauma and Ventilator-Induced Lung Injury (VILI)

• High tidal volumes overstretch the relatively small population of aerated alveoli (the "baby lung"), releasing cytokines (IL-6, IL-8, TNF-α) locally
• These cytokines translocate into the systemic circulation, driving multiorgan failure
• This is why most ARDS deaths are caused by sepsis and multiorgan failure, not by refractory hypoxemia per se — Murray & Nadel's

Summary Cascade

Trigger (sepsis, aspiration, trauma, etc.)
        ↓
Alveolar macrophage activation → proinflammatory cytokines (IL-1β, TNF-α, IL-8)
        ↓
Neutrophil recruitment → ROS + proteases + PAF
        ↓
Endothelial + epithelial barrier disruption
        ↓
Protein-rich exudate floods alveoli
        ↓
Surfactant dysfunction → alveolar collapse
        ↓
Hyaline membrane formation (DAD)
        ↓
Severe V/Q mismatch + shunt → refractory hypoxemia
        ↓
Pulmonary hypertension + dead space ventilation
        ↓
(Persistent) → fibroproliferation and fibrosis