Acute Respiratory Distress Syndrome (ARDS)

Definition

• Robbins & Kumar Basic Pathology defines it as: "A clinical syndrome of respiratory insufficiency caused by diffuse alveolar damage."
• Sabiston Textbook of Surgery states: "ARDS is an inflammatory response of the lung to multiple inciting factors... characterized by decreasing ability to oxygenate and decreased compliance of the lung."

Berlin Definition (2012) - The Current Standard

2023 Update (proposed): The Berlin criteria have been updated to include non-intubated patients (using SpO2/FiO2 ≤315 if SpO2 ≤97% on high-flow nasal oxygen ≥30 L/min or CPAP/NIV), ultrasound as an alternative imaging modality, and criteria for resource-limited settings without PEEP requirements. - Harrison's 22E, 2025

Common Causes / Precipitating Factors

• Pneumonia (most common)
• Aspiration of gastric contents
• Pulmonary contusion
• Inhalation injury

• Sepsis (most common overall trigger)
• Major trauma
• Severe acute pancreatitis
• Blood product transfusions (TRALI)
• Burns, shock

Pathology

Gross Pathology

• Robbins, Cotran & Kumar Pathologic Basis of Disease

The Three Overlapping Pathologic Phases

Phase 1 - Exudative Phase (Days 0-7)

• Alveolar capillary endothelial cells and type I pneumocytes are injured
• Loss of the normally tight alveolar-capillary barrier to fluid and macromolecules
• Protein-rich exudate leaks into interstitial and alveolar spaces
• Pro-inflammatory cytokines (IL-1, IL-6, IL-8, TNF-α) and lipid mediators (leukotriene B4) are elevated
• Neutrophil recruitment into pulmonary interstitium and alveoli - neutrophils release proteases, ROS, and cytokines causing further injury

• Widespread alveolar and interstitial edema, inflammation, and hemorrhage
• Hyaline membranes - composed of precipitated plasma proteins, fibrin, necrotic epithelial cell debris, and dysfunctional surfactant components - these are the pathological footprint of DAD
• Type I alveolar epithelial cells more severely damaged than Type II cells
• Swollen, disorganized epithelium detached from basement membranes
• Widened interstitium with leukocytes, platelets, RBCs, fibrin
• Pulmonary vascular injury: microthrombi and fibrocellular proliferation
• Inactivated/lost surfactant -> alveolar collapse

• Decreased respiratory system compliance (stiff lungs)
• Right-to-left intrapulmonary shunting (ventilation-perfusion mismatch)
• Profound, refractory hypoxemia
• Increased pulmonary dead space (minute ventilation ~twice normal, ~12 L/min)
• Pulmonary hypertension (from hypoxic vasoconstriction, fibrin deposition, vascular compression)

Phase 2 - Proliferative Phase (Days 7-21)

• Follows the exudative phase in a subset of patients
• Type II pneumocytes proliferate (attempt at epithelial repair)
• Hyaline membranes are reorganized
• Granulation tissue forms in alveolar walls and spaces
• Fibrin prominent in alveoli and interstitium
• Infiltration with fibroblasts (activated early in injury)
• Air-blood barrier thickened by interstitial and epithelial enlargement
• Decrease in neutrophil numbers
• Some reabsorption of edema fluid

• Murray & Nadel's Textbook of Respiratory Medicine; Harrison's 22E

Phase 3 - Fibrotic Phase (Day 21+)

• Occurs only in a subset of patients with persistent ARDS (>2 weeks)
• Pulmonary fibrosis with obliteration of pulmonary capillaries
• Deposition of interstitial and alveolar collagen
• Bullae formation
• Greatly reduced lung volumes and compliance
• Elevated N-terminal procollagen peptide III in BAL fluid (marker of active collagen synthesis) can be detected as early as 24 hours after onset - suggesting fibroproliferation may begin simultaneously with inflammation
• Murray & Nadel's Textbook of Respiratory Medicine

DAD - The Pathological Correlate of ARDS

• Younger
• More severely ill
• Have poorer lung compliance and lower P/F ratios
• Have 5x higher mortality from hypoxemic respiratory failure

• Murray & Nadel's Textbook of Respiratory Medicine

Key Pathological Summary Table

• Berlin ARDS Definition (JAMA 2012) - the current diagnostic standard
• Robbins, Cotran & Kumar Pathologic Basis of Disease, pp. 632-633
• Murray & Nadel's Textbook of Respiratory Medicine, Ch. 134 (Pathology section)
• Harrison's Principles of Internal Medicine 22E (2025), Ch. 312
• Fishman's Pulmonary Diseases and Disorders, Ch. 140

ARDS - Explained Simply 🫁

What is ARDS? (The Simple Version)

• Acute = happens suddenly, within hours to days
• Respiratory = it's a lung problem
• Distress = the lungs are in serious trouble
• Syndrome = a collection of signs and symptoms together

One-liner definition: ARDS is when the lungs get so severely inflamed and flooded that even giving the patient pure oxygen doesn't fix their low blood oxygen levels.

What Causes It?

• Severe pneumonia
• Breathing in vomit (aspiration)
• Smoke or toxic gas inhalation

• Sepsis - the #1 cause overall (a body-wide infection gone haywire)
• Severe pancreatitis
• Major trauma
• Blood transfusions gone wrong (TRALI)

Think of it this way: Sepsis is like a fire alarm going off in the whole building (your body), and the lungs are the room that gets the most smoke damage.

The Berlin Definition - How Doctors Diagnose ARDS

✅ 1. TIMING     → Symptoms appear within 1 week of a trigger
✅ 2. CHEST X-RAY → White patches (opacities) on BOTH sides of the lungs
✅ 3. NOT THE HEART → The fluid isn't there because of heart failure
✅ 4. LOW OXYGEN   → PaO2/FiO2 ratio ≤ 300 mmHg (even with breathing support)

The Severity Scale - Think of it like a Traffic Light

What is P/F ratio? It compares how much oxygen is in the blood (PaO2) versus how much oxygen you're breathing in (FiO2). A healthy person has a P/F ratio of ~400-500. In ARDS it crashes below 300.

The Pathology - What's Actually Happening Inside the Lung?

🔥 Phase 1 - Exudative Phase (Days 0 to 7)

INJURY
  ↓
Neutrophils rush into the lungs
  ↓
They release toxic chemicals (ROS, proteases, cytokines like TNF-α, IL-1, IL-8)
  ↓
These chemicals damage the walls between the air sacs and the blood vessels
  ↓
The "barrier" breaks down → protein-rich fluid LEAKS into the air sacs (alveoli)
  ↓
Air sacs fill with fluid instead of air → lungs can't oxygenate blood

• Hyaline membranes - a waxy pink lining inside the air sacs, made of fibrin, dead cell debris, and proteins. This is the hallmark finding - called Diffuse Alveolar Damage (DAD)
• Lots of neutrophils
• Flooded, collapsed air sacs
• Swollen, damaged Type I cells lining the alveoli
• Inactivated surfactant (the soap-like substance that keeps alveoli open)

Analogy: The hyaline membrane is like plastic wrap stuck to the inside of a balloon - air can't get in properly.

🔨 Phase 2 - Proliferative Phase (Days 7 to 21)

• Type II pneumocytes (the repair cells of the lung) start multiplying to replace the dead Type I cells
• Hyaline membranes start getting cleared away
• Fibroblasts (connective tissue cells) move in to patch up damage
• Fluid starts being reabsorbed
• Neutrophils decrease

🪨 Phase 3 - Fibrotic Phase (Day 21+)

• The lung starts scarring over (fibrosis)
• Air sacs get replaced by scar tissue
• Lungs become permanently stiff
• Can cause chronic lung disease and even cor pulmonale (right heart failure)

Analogy: Think of a cut on your skin. Most of the time it heals cleanly. Sometimes it heals with a thick, hard scar. ARDS fibrosis is the lung doing that - scarring instead of proper healing.

The Full Timeline at a Glance

Why Can't You Just Give More Oxygen?

• The air sacs are flooded - oxygen can't reach the blood even if you blow it in
• There's severe V/Q mismatch - blood is flowing to parts of the lung that can't ventilate
• This is called an intrapulmonary shunt - blood bypasses the functional lung entirely

Analogy: Imagine trying to dry a soaking wet sponge by blowing air at it. If the sponge is completely saturated, blowing more air doesn't help much.

Quick Summary Box