Assess severity of respiratory failure in ventilated patients.
OI = (FiO₂ × MAP) / PaO₂. OI accounts for both ventilator support (MAP) and oxygen requirement (FiO₂), making it superior to P/F ratio for assessing ventilated patients. OI > 25 in neonates is a criterion for ECMO consideration. In adults, OI > 40 suggests severe refractory hypoxemia.