Medicine
Pathophysiology of Acute Respiratory Distress Syndrome
Quick fact
ARDS can develop within hours of a triggering event like sepsis, pneumonia, or trauma, and it carries a mortality rate of 30–40%, even with advanced medical care.
Why this is interesting
Imagine your lungs are coated with a protective film that keeps them springy and dry. Now imagine the film is suddenly ripped apart, and fluid floods your air sacs instead of air. What happens to your breath?
Read the full explanation
Understanding Pathophysiology of Acute Respiratory Distress Syndrome
ARDS is not a disease itself but a pattern of lung injury. Think of the lung as a delicate sponge: air fills the sponge's holes, and blood flows through it. The sponge is separated from the blood by a thin barrier, the alveolar-capillary membrane. In ARDS, an insult like infection or shock triggers an overwhelming inflammatory response. White blood cells rush into the lung, releasing destructive chemicals. This damages the barrier, making it leaky. Protein-rich fluid from the blood pours into the air sacs (alveoli), drowning them. The fluid disrupts surfactant—the lubricant that keeps air sacs open—causing them to collapse (atelectasis). The result is stiff lungs that are hard to inflate, making it difficult to get oxygen into the blood and remove carbon dioxide. The patient experiences severe breathlessness and low blood oxygen levels that resist simple oxygen therapy.
A deeper explanation
Mechanistically, ARDS unfolds in three overlapping phases. In the exudative phase (days 1–7), damage to alveolar epithelial cells and capillary endothelial cells leads to increased permeability, causing edema rich in protein and neutrophils. The hyaline membranes composed of fibrin and cellular debris line the alveoli, further impairing gas exchange. This is followed by the proliferative phase (days 7–21), where the body attempts to repair; type II pneumocytes multiply to restore surfactant production, but fibrosis may begin as myofibroblasts deposit collagen. The fibrotic phase (after 21 days) can leave permanent lung scarring, reducing compliance. The key principle is that ARDS is driven by an unleashed inflammatory cascade, not solely the initial trigger. This pathophysiologic understanding is essential because treatments focus on supportive care: lung-protective ventilation (low tidal volumes) to avoid further injury, prone positioning to improve matching of ventilation and perfusion, and fluid management to avoid worsening edema. The concept also explains why ARDS is a major cause of hypoxemic respiratory failure in critical care.