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