Acute Respiratory Distress Syndrome

ARDS Overview - Defining the Damage

• ARDS: Acute, diffuse, inflammatory lung injury; ↑pulmonary vascular permeability, alveolar flooding → severe hypoxemia.
• Berlin Definition (2012):
  • Timing: Within 1 week of insult or worsening respiratory symptoms.
  • Chest Imaging: Bilateral opacities (X-ray/CT) not due to effusions, collapse, nodules.
  • Origin of Edema: Respiratory failure not fully from cardiac failure/fluid overload (objective assessment if needed).
  • Oxygenation (PEEP/CPAP ≥ 5 cmH2O):
    • Mild: PaO2/FiO2 > 200 to ≤ 300 mmHg
    • Moderate: PaO2/FiO2 > 100 to ≤ 200 mmHg
    • Severe: PaO2/FiO2 ≤ 100 mmHg

⭐ PaO2/FiO2 ratio is key for ARDS severity classification & prognosis.

Pathophysiology & Causes - The Insult Cascade

Common Causes:

• Direct Lung Injury: Pneumonia, aspiration, pulmonary contusion, fat embolism, near-drowning, toxic inhalation.
• Indirect Lung Injury: Sepsis (most common overall), severe trauma (multiple transfusions - TRALI), pancreatitis, drug overdose.

Pathophysiology Phases (📌 Every Proliferating Fibroblast):

1. Exudative (Days 0-7):
   • Alveolar-capillary barrier damage by inflammatory mediators (e.g., TNF-α, IL-1, IL-6, neutrophils).
   • ↑ Permeability → protein-rich edema → surfactant dysfunction/inactivation → hyaline membranes.
   • Results: Impaired gas exchange, ↓ lung compliance, severe hypoxemia.
2. Proliferative (Days 7-21):
   • Repair initiation: Type II pneumocyte proliferation (surfactant production), fibroblast activity.
3. Fibrotic (Days >21, variable):
   • Extensive collagen deposition → lung fibrosis, cysts. Potential for chronic lung impairment.

⭐ Diffuse Alveolar Damage (DAD) is the hallmark histopathological finding in ARDS, characterized by edema, inflammation, and hyaline membrane formation.

Clinical Picture & Diagnosis - Spotting the Syndrome

• Onset: Acute dyspnea, tachypnea within 1 week of insult (e.g., sepsis, pneumonia, trauma).
• Signs: Refractory hypoxemia (despite ↑O2), bilateral crackles, accessory muscle use.
• Berlin Criteria (Key Elements):
  • Timing: Acute (within 1 week of known insult or new/worsening respiratory symptoms).
  • Imaging: Bilateral opacities on CXR/CT (not fully explained by effusions, lobar/lung collapse, or nodules).
  • Edema Origin: Respiratory failure not fully explained by cardiac failure or fluid overload (objective assessment like Echo needed if no risk factor).
  • Oxygenation ($P_aO_2/F_iO_2$ with PEEP/CPAP ≥ 5 cmH2O):
    • Mild: 201-300 mmHg
    • Moderate: 101-200 mmHg
    • Severe: ≤ 100 mmHg
• Key Investigations: ABG (↓$P_aO_2$, initial respiratory alkalosis), CXR, Echo (to exclude cardiogenic edema).

⭐ The $P_aO_2/F_iO_2$ ratio is a cornerstone for ARDS diagnosis, severity grading, and guiding ventilator strategies; it must be assessed with PEEP ≥ 5 cmH2O.

ARDS Management - Lung Savers

• Goal: Prevent VALI (Ventilator-Associated Lung Injury).
• Lung Protective Ventilation (LPV):
  • Tidal Volume ($V_T$): 4-6 mL/kg Predicted Body Weight (PBW).
  • Plateau Pressure ($P_{plat}$): < 30 cm H₂O.
  • Driving Pressure ($%Delta P = P_{plat} - \text{PEEP}$): < 15 cm H₂O.
  • PEEP: Titrate to $SpO_2$ 88-95% or $PaO_2$ 55-80 mmHg.
• Permissive Hypercapnia: Tolerate $\uparrow PaCO_2$ if pH > 7.20-7.25 (unless contraindicated).
• Prone Positioning:
  • If $PaO_2/FiO_2$ (P/F ratio) < 150.
  • Duration: 12-16 hours/day. 📌 P for Prone, P/F < 150.
• Conservative Fluid Management.
• Neuromuscular Blockers (NMBAs): Consider early for severe ARDS (P/F < 150).

⭐ Prone positioning for $\geq$ 12-16 hours/day significantly improves mortality in moderate-severe ARDS ($PaO_2/FiO_2 < 150$).

High‑Yield Points - ⚡ Biggest Takeaways

• Berlin Definition: Acute onset, bilateral opacities (non-cardiac), PaO2/FiO2 ≤ 300 mmHg (with PEEP ≥ 5 cmH2O).
• Patho: Diffuse Alveolar Damage (DAD) → ↑ permeability pulmonary edema.
• Causes: Sepsis (most common); others include pneumonia, aspiration, trauma, pancreatitis.
• Rx: Lung-protective ventilation (≤6 mL/kg predicted body weight, Plateau Pressure < 30 cmH2O).
• Prone positioning for severe ARDS (PaO2/FiO2 < 150 mmHg).
• Employ conservative fluid management and optimal PEEP.
• High mortality; no specific drug therapy significantly improves survival outcomes consistently.