Explain the mechanism of acute respiratory distress syndrome
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"acute respiratory distress syndrome"[MeSH Terms] AND pathogenesis
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Acute Respiratory Distress Syndrome (ARDS) — Mechanism
Definition & Berlin Criteria
ARDS is a syndrome of acute hypoxemic respiratory failure characterised pathologically by diffuse alveolar damage (DAD). The 2012 Berlin Definition requires all four criteria:
| Criterion | Threshold |
|---|---|
| Timing | Onset within 7 days of a known clinical insult |
| Imaging | Bilateral opacities on CXR/CT, not explained by effusion, atelectasis, or cardiac failure |
| Origin | Not attributable to cardiac failure or fluid overload |
| Oxygenation (P/F ratio) | Mild: 200–300 mmHg · Moderate: 100–200 mmHg · Severe: <100 mmHg (all with PEEP ≥5 cmH₂O) |
Common Triggers
ARDS arises from direct (pulmonary) or indirect (extrapulmonary) insults to the alveolar-capillary membrane:
- Direct: pneumonia (35–45%), aspiration, inhalation injury, pulmonary contusion
- Indirect: sepsis (30–35%), severe pancreatitis, multiple trauma, massive transfusion, COVID-19
Core Pathophysiology
The central event is injury to both the alveolar epithelium and the vascular endothelium, driven by uncontrolled inflammation. This unfolds in overlapping phases:
Phase 1 — Initiating Insult & Innate Immune Activation
- A systemic or pulmonary insult activates alveolar macrophages (M1 phenotype) via pattern-recognition receptors (TLR/PRR).
- Macrophages release IL-1, TNF-α, IL-6, IL-8, and other proinflammatory cytokines.
- These cytokines activate pulmonary endothelial cells, upregulating adhesion molecules (ICAM-1, P-selectin) and triggering neutrophil sequestration within pulmonary capillaries.
Phase 2 — Neutrophil-Mediated Alveolar Injury
Neutrophils are central effectors. Histology in early ARDS shows increased neutrophils in capillaries, interstitium, and alveolar spaces. Activated neutrophils release:
- Reactive oxygen species (ROS) — oxidative membrane damage
- Proteases (elastase, MMPs) — degrade basement membrane and extracellular matrix
- Neutrophil extracellular traps (NETs) — promote further inflammation and coagulation
- Platelet-activating factor — amplifies vascular permeability
The combined assault on the type I pneumocytes (gas exchange cells) and the capillary endothelium destroys the alveolar-capillary barrier.
Phase 3 — Loss of Barrier Integrity & Alveolar Flooding
Disruption of the alveolar-capillary membrane causes:
- Increased vascular permeability → protein-rich fluid floods the alveolar space (noncardiogenic pulmonary oedema)
- Inactivation of surfactant (by phospholipase A₂ and edema fluid dilution) → alveolar collapse and reduced compliance
- Hyaline membrane formation — fibrin-rich exudate mixed with necrotic epithelial cell debris lines the alveolar ducts (the pathological hallmark)
Phase 4 — Ventilation-Perfusion Mismatch & Hypoxaemia
- Fluid-filled and collapsed alveoli receive blood flow but cannot oxygenate it → intrapulmonary shunt (V/Q = 0)
- Reduced functional residual capacity (FRC) and decreased lung compliance (stiff lung)
- Hypoxic vasoconstriction is overwhelmed, worsening shunt
- Result: refractory hypoxaemia unresponsive to supplemental O₂ alone
Cellular & Molecular Cascade (Summary Diagram)
Phases of Histological Evolution
| Phase | Timing | Pathology |
|---|---|---|
| Exudative | Days 1–7 | Alveolar oedema, neutrophil influx, hyaline membranes, necrosis of type I pneumocytes |
| Fibroproliferative | Days 7–21 | Type II pneumocyte proliferation (attempted repair), fibroblast infiltration, early collagen deposition |
| Fibrotic | >3 weeks | Alveolar septal thickening, interstitial fibrosis, loss of normal architecture |
Complete resolution is uncommon. More often, the fibrin-rich exudate undergoes organisation leading to fibrosis.
Gross/Microscopic Morphology
- Gross: Dark red, firm, heavy, airless ("liver-like") lungs
- Micro: Capillary congestion, interstitial and intra-alveolar oedema and haemorrhage, neutrophilic infiltrate, hyaline membranes lining alveolar ducts (hallmark), necrotic type I cells
- Resolving phase: Proliferating type II pneumocytes, fibroblasts, collagen — alveolar septal thickening
Ventilator-Induced Lung Injury (VILI) — A Secondary Mechanism
Because the lung is heterogeneously injured (dependent zones are consolidated; non-dependent zones remain aerated — the "baby lung" concept), mechanical ventilation can amplify injury through:
- Volutrauma/barotrauma: overdistension of aerated alveoli at high tidal volumes or pressures
- Atelectrauma: repetitive opening/collapse of unstable alveoli
- Biotrauma: mechanical stretch releases additional cytokines (IL-1β, IL-6, TNF-α) that spill into systemic circulation, contributing to multi-organ failure
This is why lung-protective ventilation (6 mL/kg tidal volume, plateau pressure ≤30 cmH₂O) is the cornerstone of management.
Key Physiological Consequences
| Consequence | Mechanism |
|---|---|
| Refractory hypoxaemia | Intrapulmonary shunt, V/Q mismatch |
| Reduced compliance | Surfactant loss, alveolar flooding, atelectasis |
| Reduced FRC | Alveolar collapse, oedema |
| Pulmonary hypertension | Hypoxic vasoconstriction, endothelial injury, microthrombi |
| Multi-organ failure | Biotrauma, systemic cytokine release |
Sources: Robbins & Kumar Basic Pathology, p. 444–445 · Barash Clinical Anaesthesia 9e, p. 4842–4843 · Murray & Nadel's Textbook of Respiratory Medicine · Goldman-Cecil Medicine, p. 1066–1067 · Sabiston Textbook of Surgery, p. 940
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