Acute Respiratory Distress Syndrome — Mechanism

Definition and Diagnostic Criteria (Berlin 2012)

• Bilateral pulmonary infiltrates on chest X-ray or CT within 7 days of a known clinical insult
• PaO₂/FiO₂ (P/F) ratio <300 (mild <300, moderate <200, severe <100)
• Non-cardiogenic pulmonary edema (not explained by heart failure or fluid overload)
• Positive end-expiratory pressure ≥5 cm H₂O on mechanical or non-invasive ventilation

Precipitating Causes

Core Pathophysiologic Mechanism

Phase 1 — Exudative Phase (Hours → ~7 days)

Step 1: Initiating Injury — Pneumocyte and Endothelial Damage

• Resident alveolar macrophages detect pneumocyte injury via pattern-recognition receptors and release TNF-α, IL-1β, IL-6, IL-8, and other pro-inflammatory mediators.
• In systemic insults (sepsis, trauma), circulating mediators — LPS, damage-associated molecular patterns (DAMPs), complement fragments — activate pulmonary endothelium directly, independent of pneumocyte injury.

Step 2: Endothelial Activation and Neutrophil Recruitment

• Adhesion molecules (ICAM-1, E-selectin, P-selectin) → neutrophil margination and rolling
• Chemokines (IL-8/CXCL8) → neutrophil transmigration
• Procoagulant proteins → local microvascular thrombosis

• Proteases (elastase, matrix metalloproteinases) — destroy extracellular matrix, tight junctions, and the alveolar basement membrane
• Reactive oxygen species (ROS) — oxidative damage to lipid membranes and surfactant
• Neutrophil extracellular traps (NETs) — directly damage alveolar epithelium
• Additional cytokines — positive-feedback amplification of the inflammatory cascade

Step 3: Breakdown of the Alveolar–Capillary Barrier

1. The interstitium → interstitial edema, widened intercellular junctions (visible on electron microscopy as endothelial swelling)
2. The alveolar space → alveolar flooding with proteinaceous edema fluid, fibrin, and cellular debris

Step 4: Surfactant Inactivation and Alveolar Collapse

• Alveolar flooding washes out and inactivates surfactant (phospholipase A2 released during pancreatitis/systemic inflammation also directly degrades surfactant phospholipids)
• Loss of surfactant eliminates surface tension reduction → alveolar instability and collapse (atelectasis)
• Type II pneumocyte necrosis further depletes surfactant production capacity

Step 5: Hyaline Membrane Formation

The Diagram — Normal vs. Injured Alveolus (Robbins & Cotran)

Histopathology of DAD and CT Correlation

Gas Exchange Failure — The Physiology

1. Intrapulmonary Shunt (Dominant Mechanism)

• Flooded and collapsed alveoli receive no ventilation but continue to be perfused → V/Q ratio = 0 → blood passes through the lung without oxygenation (true right-to-left shunt)
• In early MIGET (multiple inert gas elimination technique) studies of ARDS, up to 48% of cardiac output flowed through shunt or very low V/Q areas
• Shunt is refractory to supplemental O₂ — increasing FiO₂ has little effect because the shunted blood never contacts the alveolar gas
• PEEP reduces shunt by recruiting collapsed/flooded alveoli, redistributing blood to ventilated zones

2. V/Q Mismatch

• Heterogeneous distribution of edema and atelectasis creates areas of very low (but non-zero) V/Q that add shunt-like contributions to hypoxemia

3. Reduced Lung Compliance

• Fluid-filled, atelectatic lung has dramatically decreased compliance (ΔV/ΔP)
• Increased work of breathing, tendency to small tidal volumes
• Gravity-dependent consolidation ("sponge lung" model): nondependent zones retain near-normal compliance while dependent zones are flooded/atelectatic

Phase 2 — Fibroproliferative (Organizing/Late) Phase (~7–14+ days)

• Macrophages clear intra-alveolar debris and release fibrogenic cytokines: TGF-β and PDGF
• TGF-β and PDGF stimulate fibroblast and myofibroblast proliferation → collagen deposition → alveolar wall fibrosis
• Type II pneumocytes proliferate to replace necrotic type I cells (type II hyperplasia is a histologic hallmark of this phase)
• Alveolar septal thickening, disordered angiogenesis, and pulmonary hypertension develop
• Dead-space fraction increases (pulmonary vascular obliteration + microthrombi)
• High minute ventilation requirement reflects increased dead space
• Lung compliance falls further

ARDS Endotypes (Modern Concept)

Coagulation Dysregulation

• Exposed tissue factor on damaged endothelium → thrombin generation → intravascular fibrin microthrombi
• Simultaneous depression of fibrinolysis (raised PAI-1)
• Intra-alveolar fibrin contributes to hyaline membrane formation and impairs resolution
• Small pulmonary artery in situ thrombi → pulmonary hypertension and increased dead space

Ventilatior-Induced Lung Injury (VILI) — A Secondary Mechanism

• Volutrauma / Barotrauma — overdistension of non-dependent alveoli at high tidal volumes or pressures
• Atelectrauma — repeated collapse-reopening of unstable alveoli
• Biotrauma — mechanical strain releases cytokines/mediators from alveolar cells into the bloodstream → distal organ dysfunction (kidney, gut, brain)

Summary Cascade

Triggering insult (sepsis/pneumonia/trauma/etc.)
        ↓
Alveolar macrophage activation → TNF-α, IL-1β, IL-8
        ↓
Endothelial upregulation of adhesion molecules
        ↓
Neutrophil adhesion → transmigration into alveoli
        ↓
Release of proteases + ROS + NETs
        ↓
Destruction of tight junctions + basement membrane
        ↓
Protein-rich flood into alveolus → surfactant washout/inactivation
        ↓
Alveolar collapse + hyaline membrane formation (DAD)
        ↓
Massive right-to-left shunt → refractory hypoxemia
        ↓
If sustained → fibroproliferative repair → fibrosis + pulmonary HTN

• Robbins, Cotran & Kumar Pathologic Basis of Disease (Fig. 15.3 and surrounding text)
• Fishman's Pulmonary Diseases and Disorders (Chapter 141)
• Murray & Nadel's Textbook of Respiratory Medicine (Chapters 44, 126)
• Goldman-Cecil Medicine (Chapters 79, 91)
• Sabiston Textbook of Surgery (Chapter 11)