Which mechanism best explains injury in ARDS?

• A. Bacterial infection alone
• B. Hypovolemia
• C. Cytokine-mediated injury to the pulmonary parenchyma causing noncardiogenic edema
• D. Allergic reaction

Answer: (C)

Acute respiratory distress syndrome is driven by inflammatory injury to the alveolar–capillary barrier that causes capillary leak and noncardiogenic pulmonary edema. Activated neutrophils release cytokines that increase permeability, leading to protein-rich fluid flooding the alveoli, surfactant dysfunction, and formation of hyaline membranes. This results in stiff lungs, poor gas exchange, and severe hypoxemia, independent of left-heart failure. While infection can trigger ARDS, the defining mechanism is cytokine-mediated vascular injury causing edema. It’s not primarily due to hypovolemia causing hydrostatic edema, nor is it a simple allergic reaction.