Acute respiratory distress syndrome

Acute respiratory distress syndrome

Acute respiratory distress syndrome

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Pathophysiology - The Leaky Lung

  • Insult (e.g., sepsis, trauma, pancreatitis) triggers a massive pulmonary inflammatory cascade.
  • Key Mediators: Activated neutrophils release proteases and reactive oxygen species, fueled by cytokines (TNF-α, IL-1, IL-6).
  • Core Injury: Diffuse alveolar damage (DAD) to both the capillary endothelium and alveolar epithelium.
    • This breakdown of the alveolar-capillary barrier leads to ↑ permeability.
    • Damage to Type II pneumocytes causes ↓ surfactant, leading to alveolar collapse (atelectasis).

⭐ The hallmark is non-cardiogenic pulmonary edema. This is confirmed by a normal or low Pulmonary Capillary Wedge Pressure (PCWP) of <18 mmHg, distinguishing it from heart failure.

Histology of ARDS: Normal, Exudative, and Organizing Phases

Etiology - The Usual Suspects

  • Direct Lung Injury (Insult directly to the lungs)

    • Pneumonia: Viral or bacterial.
    • Aspiration of gastric contents.
    • Pulmonary contusion or fat embolism.
    • Near-drowning.
  • Indirect Lung Injury (Systemic inflammation affecting lungs)

    • Sepsis: The #1 overall cause.
    • Severe Trauma with shock.
    • Acute Pancreatitis.
    • Transfusion-Related Acute Lung Injury (TRALI).

Sepsis is the single most common trigger for the development of ARDS, accounting for approximately one-third of all cases.

Diagnosis - Berlin's Big Criteria

📌 BERLIN Criteria for ARDS diagnosis:

  • Bilateral Opacities: On chest imaging (X-ray/CT), not fully explained by other causes (e.g., effusion, collapse).
    • Chest X-ray: ARDS vs. Cardiogenic Edema
  • Edema Origin: Respiratory failure not primarily from cardiac failure or fluid overload.
      • Objective assessment (e.g., echo) needed if no ARDS risk factor.
  • Respiratory Timing: Acute onset within 1 week of a known insult or new/worsening symptoms.
  • Low Oxygenation (Hypoxemia): PaO₂/FiO₂ ratio (P/F ratio) on PEEP ≥ 5 cmH₂O defines severity.
      • Mild: P/F 201-300 mmHg
      • Moderate: P/F 101-200 mmHg
      • Severe: P/F ≤ 100 mmHg

⭐ The PaO₂/FiO₂ ratio is a key prognostic indicator. A lower ratio signifies more severe hypoxemia and is associated with higher mortality.

Management - Lungs Under Pressure

Primary Goal: Treat the underlying cause + supportive care with lung-protective ventilation.

  • Low Tidal Volume (LTV) Ventilation:
    • Tidal Volume (V_T): $4-8 \text{ mL/kg}$ of ideal body weight.
    • Plateau Pressure (P_plat): Keep <30 cm H₂O to minimize barotrauma.
    • Permissive Hypercapnia: Tolerate ↑PaCO₂ and mild acidosis as a trade-off for LTV.
  • Positive End-Expiratory Pressure (PEEP):
    • Use high PEEP (>5 cm H₂O) to recruit collapsed alveoli and improve oxygenation.
  • Prone Positioning:
    • For moderate-to-severe ARDS (P/F ratio <150).
    • Improves ventilation-perfusion (V/Q) matching; done for >12 hours/day.
  • Conservative Fluid Management:
    • Diuresis or restricted fluids after shock resolves to reduce pulmonary edema.

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High-Yield: Low tidal volume ventilation is the cornerstone intervention shown to decrease mortality in ARDS by reducing ventilator-induced lung injury (VILI).

  • ARDS is non-cardiogenic pulmonary edema from alveolar-capillary damage, not heart failure.
  • Key triggers include sepsis (most common), pneumonia, trauma, and pancreatitis.
  • Presents with rapid-onset, severe hypoxemia refractory to supplemental oxygen.
  • CXR shows bilateral, diffuse pulmonary infiltrates resembling a "white-out."
  • PaO₂/FiO₂ ratio ≤ 300 mmHg is a hallmark diagnostic criterion.
  • Pulmonary capillary wedge pressure (PCWP) is normal (≤ 18 mmHg), ruling out cardiogenic causes.
  • Histology classically shows diffuse alveolar damage (DAD) with hyaline membranes.

Practice Questions: Acute respiratory distress syndrome

Test your understanding with these related questions

A previously healthy 35-year-old woman is brought into the emergency department after being found unresponsive by her husband. Her husband finds an empty bottle of diazepam tablets in her pocket. She is stuporous. At the hospital, her blood pressure is 90/40 mm Hg, the pulse is 58/min, and the respirations are 6/min. The examination of the pupils shows normal size and reactivity to light. Deep tendon reflexes are 1+ bilaterally. Babinski sign is absent. All 4 extremities are hypotonic. The patient is intubated and taken to the critical care unit for mechanical ventilation and treatment. Regarding the prevention of pneumonia in this patient, which of the following strategies is most likely to achieve this goal?

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Flashcards: Acute respiratory distress syndrome

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Pneumoconioses requires chronic exposure to small particles that are _____

TAP TO REVEAL ANSWER

Pneumoconioses requires chronic exposure to small particles that are _____

fibrogenic

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