Acute Respiratory Distress Syndrome

Acute Respiratory Distress Syndrome - Spotting the Storm

• Acute, diffuse, inflammatory lung injury → severe hypoxemia & bilateral radiographic opacities.
  • Edema origin: Respiratory failure not fully explained by cardiac failure or fluid overload (objective assessment if no risk factors present).
• Berlin Definition (Key Criteria):
  • Timing: Within 1 week of a known clinical insult or new/worsening respiratory symptoms.
  • Chest Imaging (X-ray/CT): Bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules.
  • Oxygenation (on PEEP/CPAP ≥ 5 cmH₂O):

    Severity	$PaO_2/FiO_2$ (mmHg)
    Mild	>200 to ≤300
    Moderate	>100 to ≤200
    Severe	≤100

⭐ The Berlin Definition requires symptoms to have started within 1 week of a known clinical insult or new/worsening respiratory symptoms.

Acute Respiratory Distress Syndrome - Lung Under Siege

• Acute, diffuse inflammatory lung injury: ↑alveolar-capillary permeability, protein-rich non-cardiogenic edema, severe hypoxemia.
• Berlin Definition Key Criteria:
  • Timing: Acute onset (within 1 week of insult).
  • Chest Imaging: Bilateral opacities (non-cardiac).
  • Origin of Edema: Non-cardiac origin.
  • Oxygenation ($P_aO_2/F_iO_2$ ratio with PEEP $\geq$5 cmH2O):
    • Mild: >200 to $\leq$300 mmHg
    • Moderate: >100 to $\leq$200 mmHg
    • Severe: $\leq$100 mmHg

⭐ Diffuse alveolar damage (DAD) with hyaline membrane formation is the characteristic histopathological finding in ARDS.

• Management: Lung-protective ventilation (LPV): low tidal volumes (4-6 mL/kg predicted body weight), optimal PEEP.

Acute Respiratory Distress Syndrome - The Usual Suspects

ARDS: Triggered by direct or indirect lung injury.

• Direct Lung Injury (Insult directly to lungs):
  • Pneumonia: Severe bacterial, viral, or fungal infection.
  • Aspiration: Inhalation of gastric contents; near-drowning.
  • Pulmonary Contusion: Bruising of lung from trauma.
  • Fat Embolism Syndrome: After long bone fractures.
  • Inhalational Injury: From smoke, toxic gases, chemical irritants.
• Indirect Lung Injury (Systemic process affecting lungs):
  • Sepsis: Widespread infection; most common overall ARDS cause.
  • Severe Trauma/Shock: Often with multiple transfusions (TRALI).
  • Acute Pancreatitis: Severe pancreatic inflammation.
  • Drug Overdose: E.g., salicylates, opioids, TCAs.
  • Major Burns: Extensive thermal injury, systemic inflammation.

⭐ Sepsis (especially pulmonary) and pneumonia are the most common causes of ARDS.

📌 "P-SST" for Pneumonia, Sepsis, ASpiration, Trauma.

Acute Respiratory Distress Syndrome - Breathing Battles

• Key Management Strategies:
  • Lung Protective Ventilation (LPV): Tidal Volume (Vt) 4-6 mL/kg Predicted Body Weight (PBW), Plateau Pressure (Pplat) <30 cmH2O, Driving Pressure ($\Delta$P = Pplat - PEEP) <15 cmH2O; Permissive Hypercapnia (pH >7.20).
  • PEEP: Titrate for optimal oxygenation (e.g., $P_aO_2$ 55-80 mmHg or $S_pO_2$ 88-95%) and compliance.
  • Prone Positioning: If $P_aO_2/F_iO_2$ < 150 mmHg, for 12-16 hours/day.
  • Neuromuscular Blocking Agents (NMBAs): Consider early, short course for severe ARDS ($P_aO_2/F_iO_2$ < 150 mmHg).
  • Fluid Management: Conservative strategy.
  • ECMO (Extracorporeal Membrane Oxygenation): For refractory hypoxemia despite maximal conventional therapy.

⭐ Lung-protective ventilation using low tidal volumes (around 6 mL/kg predicted body weight) is a cornerstone of ARDS management and improves survival.

High‑Yield Points - ⚡ Biggest Takeaways

• Berlin Definition: Acute onset, bilateral opacities, hypoxemia (PaO2/FiO2 ≤300), non-cardiac origin.
• PaO2/FiO2 severity (PEEP ≥5): Mild (201-300), Moderate (101-200), Severe (≤100).
• Pathophysiology: Diffuse Alveolar Damage (DAD), ↑ permeability edema.
• Lung Protective Ventilation: Low tidal volumes (4-6 mL/kg PBW), Plateau pressure <30.
• Prone positioning (PaO2/FiO2 <150) for moderate-severe ARDS improves mortality.
• Common causes: Sepsis, pneumonia, aspiration.
• Treat underlying cause, supportive care; no specific drug improves survival.