Summary of Acute Respiratory Distress Syndrome (ARDS) | Clinical Medicine

Definition and Diagnosis of ARDS

• Acute Hypoxemia: Develops within one week, indicated by:
  • SpO2 < 90% or PaO2 < 60 mmHg.
• PF Ratio: The ratio of arterial oxygen partial pressure (PaO2) to the fraction of inspired oxygen (FiO2) must be < 300.
• Imaging Findings: Diffuse bilateral infiltrates visible on chest X-ray or CT scan.
• Exclusion of Cardiogenic Pulmonary Edema: Confirmed via Swan-Ganz catheter showing pulmonary capillary wedge pressure < 18 mmHg.

Pathophysiology

• Diffuse Alveolar Damage (DAD): Injury to Type 1 and Type 2 alveolar cells leads to:
  • Fluid and protein accumulation in alveoli.
  • Decreased surfactant production, resulting in alveolar collapse.
• Ventilation-Perfusion (V/Q) Mismatch: Poor ventilation with normal perfusion leads to significant hypoxemia.

Causes of ARDS

• Direct Lung Injury:
  • Pneumonia (including SARS-CoV-2).
  • Aspiration of gastric contents.
  • Inhalation of toxic substances.
• Indirect Lung Injury:
  • Systemic infections (sepsis).
  • Inflammatory conditions (e.g., pancreatitis).
  • Transfusion-related lung injury.

Complications

• Pulmonary Hypertension: Due to hypoxic vasoconstriction, leading to increased pulmonary vascular resistance and potential right heart failure.
• Ventilator-Associated Pneumonia (VAP): Risk increases with prolonged intubation.
• Ventilator-Induced Lung Injury: Due to excessive airway pressures or volumes, leading to barotrauma or volutrauma.

Management and Treatment

• Ventilation Strategies:
  • Low tidal volume ventilation (< 6 cc/kg).
  • Appropriate PEEP settings to prevent alveolar collapse.
• Sedation and Neuromuscular Blockade: To synchronize patient breathing with the ventilator.
• Proning: Improves ventilation and oxygenation in severe cases.
• Extracorporeal Membrane Oxygenation (ECMO): Considered in refractory hypoxemia.

Diagnostic and Management Steps

1. Confirm Acute Hypoxemia and PF Ratio < 300.
2. Assess imaging for bilateral infiltrates.
3. Rule out cardiogenic causes using echocardiogram or Swan-Ganz catheter.
4. Initiate low tidal volume ventilation and monitor plateau pressures.
5. Consider proning and ECMO for severe cases.

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