An infant with severe respiratory failure is on high frequency oscillatory ventilation with a mean airway pressure of 20 cm H2O in 100% oxygen. A recent ABG shows PaO2 = 40 mm Hg. Which oxygenation index (OI) is most likely?

• A. 40
• B. 50
• C. 60
• D. 70

Answer: (B)

This question tests how to quantify oxygenation efficiency with the oxygenation index (OI), which combines ventilator support and blood gas into a single number to gauge how well oxygen is being transferred.

OI is calculated as: OI = (MAP × FiO2 × 100) / PaO2. Here, the infant is on high-frequency oscillatory ventilation with a mean airway pressure (MAP) of 20 cm H2O and FiO2 of 100% (FiO2 = 1.0). The arterial oxygen tension is PaO2 = 40 mm Hg. Plugging in the numbers: OI = (20 × 1.0 × 100) / 40 = 2000 / 40 = 50.

An OI of 50 indicates substantial oxygenation failure despite maximal support, which aligns with the given scenario of severe respiratory failure on high MAP and 100% oxygen but a low PaO2. The value 50 is the computed OI, and it reflects why this level of ventilatory support is insufficient to achieve adequate oxygenation.