Classify ARDS severity from oxygenation parameters.
P/F ratio = PaO₂ / FiO₂. Normal ≥ 400. Berlin criteria for ARDS: Mild 200–300, Moderate 100–200, Severe < 100 (all with PEEP ≥ 5). P/F ratio is the standard oxygenation metric in critical care. FiO₂ is expressed as a fraction (e.g., 40% = 0.40). Mortality increases with ARDS severity: mild ~27%, moderate ~32%, severe ~45%.