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ARDS for PA

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Acute Respiratory Distress Syndrome (ARDS) for the Physician Assistant Licensing Exam
ARDS is a severe form of respiratory failure caused by non-cardiogenic pulmonary edema and hypoxemia. It results from direct or indirect lung injury, leading to inflammation and impaired gas exchange.
Pathophysiology
ARDS is caused by injury to the alveolar-capillary membrane, leading to fluid leakage into the alveoli.
  • Alveolar-Capillary Membrane Injury:
    • Inflammatory cytokines (e.g., TNF-α, IL-6) increase capillary permeability, causing fluid accumulation in the alveoli.
    • This leads to impaired oxygen exchange and hypoxemia due to ventilation-perfusion (V/Q) mismatch.
  • Phases of ARDS:
    • Exudative Phase (Days 1-7): Acute inflammation, fluid accumulation, and neutrophil infiltration.
    • Proliferative Phase (Days 7-21): Resolution of some alveolar edema and early lung repair.
    • Fibrotic Phase (After 21 Days): In some cases, fibrosis develops, leading to long-term lung dysfunction.
Causes
Direct Lung Injury
  • Pneumonia: The most common cause of ARDS, caused by bacterial, viral, or fungal infection.
  • Aspiration of gastric contents: Chemical pneumonitis damages alveoli.
  • Inhalation Injury: Smoke or toxic gases cause direct alveolar damage.
Indirect Lung Injury
  • Sepsis: The leading non-pulmonary cause of ARDS, resulting in systemic inflammation and lung damage.
  • Pancreatitis: Inflammatory mediators from the pancreas cause distant organ injury, including ARDS.
Clinical Presentation
  • Symptoms:
    • Severe dyspnea: Rapid onset of difficulty breathing.
    • Tachypnea: Fast breathing due to low oxygen levels.
    • Cyanosis: Hypoxemia that does not improve with oxygen therapy.
  • Physical Exam:
    • Diffuse crackles: Heard on lung auscultation.
    • Signs of respiratory distress: Use of accessory muscles and tachypnea.
Diagnosis
  • Berlin Criteria: Used to diagnose ARDS.
    • Timing: Onset within one week of a known insult.
    • Chest Imaging: Bilateral opacities on X-ray not due to heart failure or effusion.
Bilateral Opacities in Acute Respiratory Distress Syndrome
    • Oxygenation: PaO2/FiO2 ratio < 300 mmHg, indicating hypoxemia.
Treatment
Mechanical Ventilation
  • Low Tidal Volume Ventilation: 4-6 mL/kg of predicted body weight to prevent ventilator-induced lung injury.
  • PEEP (Positive End-Expiratory Pressure): Keeps alveoli open to improve oxygenation.
  • Prone Positioning: Helps improve oxygenation in severe ARDS.
Fluid Management
Conservative fluid management reduces the risk of worsening pulmonary edema.
Key Points
  • ARDS results from direct or indirect lung injury leading to non-cardiogenic pulmonary edema and severe hypoxemia.
  • Common causes include pneumonia, aspiration, and sepsis.
  • Diagnosis relies on the Berlin criteria, which includes timing, bilateral opacities, and hypoxemia.
  • Management focuses on lung-protective mechanical ventilation, prone positioning, and conservative fluid strategies to improve outcomes.

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