Explain the mechanism of acute respiratory distress syndrome

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Mechanism of Acute Respiratory Distress Syndrome (ARDS)

Definition and Core Concept

ARDS is a syndrome of non-cardiogenic pulmonary edema caused by increased alveolar-capillary permeability rather than elevated hydrostatic pressure (as in heart failure). The edema fluid is protein-rich and exudative, flooding the alveoli and causing profound hypoxemia and decreased lung compliance. — Murray & Nadel's Textbook of Respiratory Medicine

Triggers and Risk Factors

ARDS is initiated by either direct (pulmonary) or indirect (non-pulmonary) injury:

Direct Injury (Pulmonary)	Indirect Injury (Non-pulmonary)
Pneumonia (bacterial, viral, COVID-19)	Sepsis
Aspiration of gastric contents	Major trauma
Pulmonary contusion	Multiple blood transfusions
Toxic inhalation	Pancreatitis
Near-drowning	Cardiopulmonary bypass
Reperfusion injury	Drug overdose

Sepsis, trauma, aspiration, and massive transfusions carry the highest risk. Presence of more than one risk factor further increases incidence.

Pathological Phases: Diffuse Alveolar Damage (DAD)

The pathologic hallmark of ARDS is diffuse alveolar damage (DAD), which progresses through three overlapping stages:

1. Exudative Phase (Days 1–7)

Protein-rich fluid and hyaline membranes (composed of cellular debris, proteins, and surfactant components) fill alveolar spaces
Widespread epithelial disruption
Intense neutrophil infiltration of the interstitium and airspaces
Impaired gas exchange → profound hypoxemia

2. Proliferative Phase (~Day 7 onwards)

Hyaline membranes are reorganized
Early fibrosis appears
Obliteration of pulmonary capillaries
Deposition of interstitial and alveolar collagen
Reduction in neutrophil numbers

3. Fibrotic Phase (>2 weeks)

Pulmonary fibrosis in a subset of patients with persistent ARDS
Elevated N-terminal procollagen peptide III in BAL fluid (detectable as early as 24 hours after onset) suggests fibroproliferation begins simultaneously with — not after — inflammatory injury

Cellular and Molecular Mechanisms

Step 1: Alveolar-Capillary Barrier Breakdown

Damage to the alveolar epithelium is considered the key precipitating event. Multiple mechanisms contribute to epithelial cell death:

Direct cytotoxicity
Apoptosis
Necrosis triggered by inflammatory mediators

The microvascular endothelium is also damaged, and together these two layers form a compromised barrier that allows protein-rich fluid to flood the alveoli.

Step 2: Neutrophil Sequestration and Activation

Role of neutrophils in ARDS pathogenesis — PMNs exit the bloodstream and transmigrate across the alveolar-capillary membrane, releasing cytokines, proteases, reactive oxygen species, and neutrophil extracellular traps (NETs)

Figure: Role of neutrophils in ARDS. PMNs transmigrate from the capillary lumen through the endothelial and epithelial barriers into the alveolar space, releasing destructive mediators at each step. — Murray & Nadel's Textbook of Respiratory Medicine

One of the earliest manifestations of ARDS — even before hypoxemia — is a transient leukopenia from neutrophil sequestration in the pulmonary microvasculature:

Pulmonary capillaries are narrower than neutrophils; passage requires cell deformation
Activated neutrophils become "stiff" (due to actin cytoskeletal changes) and become trapped
Sequestered neutrophils then migrate into lung parenchyma, disrupting endothelial barrier integrity

Once in the interstitium and alveoli, activated neutrophils release:

Reactive oxygen species (ROS) — oxidative injury to cells
Proteolytic enzymes (e.g., leukocyte elastase) — digest structural proteins
Cationic peptides (e.g., defensins)
Eicosanoids
TNF-α and IL-1β — amplify the inflammatory cascade
Neutrophil extracellular traps (NETs) — chromatin fibers that trap pathogens but also damage host tissue

Step 3: Cytokine Storm and Amplification

The inflammatory response is amplified by a network of cytokines and chemokines:

TNF-α and IL-1β upregulate adhesion molecules, recruit more neutrophils, and promote vascular permeability
Macrophages and epithelial cells further release pro-inflammatory mediators
The transcription factor NF-κB is a master regulator of this inflammatory amplification
Heat shock protein 70 (HSP70) normally suppresses NF-κB; loss of HSP70 is associated with increased lung injury

Step 4: Surfactant Dysfunction

Injured type II pneumocytes produce abnormal surfactant:

Surfactant composition is altered
Surfactant proteins (SP-A, SP-B, SP-C, SP-D) are reduced
Phospholipase A2 (released in pancreatitis-associated ARDS) enzymatically degrades surfactant
Loss of surfactant → alveolar collapse → worsened V/Q mismatch and shunting

Step 5: Impaired Alveolar Fluid Clearance

Normally, Na⁺ channels on type II pneumocytes drive fluid reabsorption from alveoli. In ARDS:

Hypoxia impairs expression of epithelial Na⁺ channel subunits
Hypoxia also directly inhibits apical Na⁺ channel activity and basolateral Na⁺/K⁺-ATPase
Increased nitric oxide (e.g., from hemorrhagic shock) impairs β-adrenergic–mediated fluid clearance
Result: edema fluid accumulates and cannot be cleared

Step 6: Endothelial Dysfunction — Angiopoietin Imbalance

Angiopoietin 2 (Ang2), released by injured endothelial cells, promotes vascular leak:

Elevated Ang2 is found in patients with sepsis and ARDS
Ang2 competitively antagonizes Angiopoietin 1 (Ang1), which normally maintains endothelial barrier integrity via the Tie2 receptor
Genetic variants in Ang2 are associated with increased ARDS risk
This imbalance further compromises endothelial barrier function

Physiological Consequences

Consequence	Mechanism
Profound hypoxemia	Alveolar flooding → intrapulmonary right-to-left shunt + low V/Q regions
Decreased compliance	Alveolar edema, atelectasis, surfactant loss
Increased dead space	Vascular obliteration → elevated minute ventilation requirement
Pulmonary hypertension	Hypoxic vasoconstriction, intravascular fibrin deposition, compression by positive-pressure ventilation
Hypercapnia (in severe cases)	Massively increased dead space overwhelms compensatory hyperventilation

Summary Cascade

Trigger (sepsis, pneumonia, trauma, etc.)
        ↓
Direct or indirect lung injury
        ↓
Alveolar epithelial + endothelial damage
        ↓
Neutrophil sequestration → transmigration
        ↓
Release of ROS, proteases, NETs, cytokines (TNF-α, IL-1β)
        ↓
Increased alveolar-capillary permeability
        ↓
Protein-rich exudate floods alveoli → Hyaline membrane formation (DAD)
        ↓
Surfactant dysfunction + impaired Na⁺-driven fluid clearance
        ↓
Alveolar collapse, shunt, hypoxemia, ↓ compliance, pulmonary hypertension
        ↓
ARDS

Sources: Murray & Nadel's Textbook of Respiratory Medicine; Goldman-Cecil Medicine; Sabiston Textbook of Surgery

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