Acute Respiratory Distress Syndrome (ARDS): Mechanism

Overview

• Murray & Nadel's Textbook of Respiratory Medicine, p. 3069

Phases of ARDS

Phase 1: Exudative (Days 0-7)

The Alveolar-Capillary Barrier Fails

• Epithelial injury is considered the key precipitating event. Type I pneumocytes (which cover ~95% of the alveolar surface) die via necrosis and apoptosis, disrupting the barrier and eliminating the cells responsible for active fluid clearance (via Na+/K+-ATPase and ENaC channels). Type II pneumocytes, which produce surfactant and can differentiate into type I cells, are also injured.
• Endothelial injury increases microvascular permeability, which is both necessary and sufficient for edema formation. Tight junctions between endothelial cells are disrupted, allowing protein-rich fluid to flood the interstitium and alveoli.
• Murray & Nadel's Textbook of Respiratory Medicine, p. 3083

The Central Role of Neutrophils

1. Sequestration: One of the earliest signs of ARDS (even before hypoxemia) is a transient peripheral leukopenia as neutrophils are sequestered in the pulmonary microvasculature. Pulmonary capillaries (~5 µm) are narrower than neutrophils (~7-8 µm), so neutrophils must deform to pass through. Activated neutrophils become "stiff" (via actin cytoskeleton changes) and cannot deform, causing them to accumulate.
2. Transmigration: Sequestered neutrophils migrate across the alveolar-capillary membrane into the interstitium and airspaces. Importantly, in the lung this can occur without the adhesion molecules (L-selectin, β2-integrins) typically required elsewhere.
3. Cytotoxic payload release: Once in the alveolar space, activated neutrophils release:
   • Reactive oxygen species (ROS) - cause oxidative damage to lipids, proteins, and DNA
   • Neutrophil elastase (NE) - a serine protease that degrades epithelial and endothelial cadherins (components of adherens junctions), directly predisposing to alveolar flooding; also degrades surfactant proteins
   • Matrix metalloproteinases - degrade extracellular matrix
   • Neutrophil extracellular traps (NETs) - webs of chromatin and proteases that cause tissue damage
   • Cytokines (TNF-α, IL-1β, IL-8) that amplify the inflammatory cascade

• Murray & Nadel's Textbook of Respiratory Medicine, pp. 3111-3151

Macrophage and Cytokine Activation

• TNF-α and IL-1β act as "master cytokines," activating endothelial cells to upregulate adhesion molecules (ICAM-1, E-selectin), driving further neutrophil recruitment
• IL-8 (CXCL8) is the principal neutrophil chemoattractant - markedly elevated in BAL fluid of ARDS patients
• Platelet-activating factor (PAF) enhances vascular permeability and activates platelets
• Monocyte-derived macrophages are recruited from circulation and sustain the inflammatory response
• p38 MAP kinase and phosphatidylinositol 3-kinase pathways are central intracellular signaling nodes that drive these cytokine responses

Surfactant Dysfunction

• Surfactant production by type II pneumocytes is reduced
• Neutrophil elastase degrades surfactant proteins A, B, and C
• Plasma proteins that leak into the alveolar space inhibit surfactant function
• The ratio of large (active) to small (inactive) surfactant aggregates falls
• Net result: alveolar instability, widespread microatelectasis, and reduced compliance
• Murray & Nadel's Textbook of Respiratory Medicine, pp. 3092-3101

Coagulation Abnormalities

• Tissue factor is expressed on damaged epithelial cells and activated macrophages, triggering extrinsic coagulation
• Fibrin deposition occurs in alveolar spaces and within pulmonary microvessels (contributing to the hyaline membranes seen histologically and to microvascular occlusion)
• Plasminogen activator inhibitor-1 (PAI-1) levels are markedly elevated, suppressing fibrinolysis

Pulmonary Hypertension

• Hypoxic vasoconstriction
• Intravascular fibrin and platelet microthrombi occlude vessels
• Compression of blood vessels by positive-pressure ventilation
• Progressive vascular remodeling in prolonged disease

Phase 2: Proliferative (Days 7-21)

• Neutrophil-predominant infiltrate shifts to a lymphocyte-predominant pattern
• Hyaline membranes are reorganized and cleared
• Type II pneumocytes proliferate along alveolar basement membranes, synthesizing new surfactant and differentiating into type I cells to restore the epithelial barrier
• Alveolar exudates begin to organize (organizing pneumonia pattern)
• Early interstitial and alveolar fibrosis may appear in some patients

• Harrison's Principles of Internal Medicine 22E, p. 2688

Phase 3: Fibrotic (>21 Days)

• Alveolar edema and exudates are replaced by extensive alveolar-duct and interstitial fibrosis
• Disrupted acinar architecture creates emphysema-like changes with large bullae
• Intimal fibroproliferation in the pulmonary microcirculation causes progressive vascular occlusion and worsening pulmonary hypertension
• Physiologic consequences: reduced lung compliance, increased dead space, increased pneumothorax risk
• Elevated N-terminal procollagen peptide III in BAL fluid (a marker of collagen synthesis) can be detected as early as 24 hours after onset, indicating fibrogenesis begins very early
• Harrison's Principles of Internal Medicine 22E, p. 2690; Murray & Nadel's, p. 3074

Pathophysiologic Summary Table

Pathologic Definition: Diffuse Alveolar Damage (DAD)

• Murray & Nadel's Textbook of Respiratory Medicine, p. 3078

Recent Evidence

• Wick KD et al., BMJ 2024 (PMID 39467606) - comprehensive review confirming neutrophil-platelet interactions, NETs, and the importance of ARDS phenotyping (hyperinflammatory vs. hypo-inflammatory subtypes) as an emerging mechanistic/therapeutic distinction
• Al-Husinat L et al., Critical Care 2025 (PMID 39994815) - highlights the 2023 Global definition update and emphasizes evolving understanding of ARDS subphenotypes and tailored management based on biological mechanisms