Beyond the PaO2/FiO2 ratio in hypoxic respiratory failure, the Oxygenation Index (OI) is used in neonates, with 15/25/40 representing mild, moderate and severe illness. This retrospective study compared invasive with non-invasive measures of severity of respiratory failure.
OI = MAP × Fio2 × 100 / Pao2, where MAP indicates mean airway pressure.
Oxygen Saturation Index (OSI), a non-invasive counterpart is defined as OSI = MAP × Fio2 × 100 / Spo2
OI and OSI showed a strong correlation. Derived OI from OSI was in good agreement and strongly predictive of clinically relevant OI cutoffs from 5 to 25.
More studies in different disease severity are needed but OSI may provide a useful non-invasive alternative to OI in this setting.