ARDS is characterized by increased alveolar-capillary permeability causing pulmonary edema, ventilation-perfusion mismatch, and hypoxemia refractory to supplemental oxygen. Inflammatory mediators (cytokines, complement, neutrophils) damage the epithelial-endothelial barrier.
Study the Berlin definition (PaO2/FiO2 ratio, imaging, onset timing). Understand exudative and fibroproliferative phases. Review common triggers: sepsis, aspiration, transfusion, trauma.
ARDS is not a single disease entity—it is a syndrome with heterogeneous causes and trajectories. Low tidal volume ventilation reduces mortality not by improving oxygenation directly but by limiting barotrauma.
You have already studied the alveolar-capillary barrier — the ultra-thin interface where oxygen crosses from air into blood, and carbon dioxide crosses back. That barrier's integrity depends on tight junctions between type I pneumocytes on the air side and endothelial cells on the blood side. ARDS is what happens when inflammation destroys that barrier. Understanding ARDS means tracing the chain from initial insult to barrier collapse to clinical syndrome.
The trigger can be direct (pneumonia, aspiration, inhalation injury) or indirect (sepsis, trauma, pancreatitis). Either way, the lung mounts an acute inflammatory response. Neutrophils — which you studied as the first responders in acute inflammation — flood into the alveolar space and release proteases, reactive oxygen species, and inflammatory cytokines. This cytokine storm (including IL-1β, TNF-α, and IL-8) amplifies the response and recruits more neutrophils. The critical consequence is that the inflammatory mediators dissolve the tight junctions holding the alveolar-capillary barrier together. Protein-rich fluid from the capillaries — exudate — pours into alveoli that normally contain only air.
Now think through the respiratory consequences. Fluid-filled alveoli cannot participate in gas exchange, but blood continues to flow past them — a ventilation-perfusion mismatch your respiratory physiology background prepared you for. The result is a shunt: blood passes through the lung and returns to circulation without picking up oxygen. This is why ARDS produces hypoxemia that does not respond to supplemental oxygen the way ordinary hypoxia does — adding more oxygen to ventilated alveoli helps very little if the blood is mostly flowing past collapsed, fluid-filled ones. The hallmark PaO2/FiO2 ratio below 300 captures this: even with 100% inspired oxygen (FiO2 = 1.0), the partial pressure of oxygen in arterial blood remains severely depressed.
ARDS has two pathological phases. The exudative phase (days 1–7) involves the barrier breakdown and flooding just described, along with hyaline membrane formation from precipitated proteins. The fibroproliferative phase (days 7–21) involves type II pneumocyte proliferation, fibroblast activation, and collagen deposition — the lung's attempt at repair. In severe cases this fibroproliferative response is excessive, leaving behind stiff, scarred lung tissue that impairs mechanics long after the acute crisis resolves. Treatment strategy reflects this pathophysiology: mechanical ventilation with low tidal volumes (6 mL/kg) prevents the volutrauma and barotrauma that would worsen the already-fragile lung, even though smaller breaths mean accepting higher CO2 levels — a deliberate tradeoff called permissive hypercapnia.