What is the correct formula for Oxygen Index (OI)?

What is the correct formula for Oxygen Index (OI)?

• A. Paw x FiO2 / PaO2
• B. Paw x PaO2 / FiO2
• C. Paw x FiO2 / PaCO2
• D. Paw / FiO2 x PaO2

Answer: (B)

Oxygen Index measures how much ventilatory support is needed to achieve a given level of arterial oxygenation. The correct formula combines mean airway pressure (Paw) and the fraction of inspired oxygen (FiO2) in the numerator and arterial oxygen tension (PaO2) in the denominator: Paw × FiO2 ÷ PaO2. Why this makes sense: Paw and FiO2 quantify the amount of support being provided to push oxygen into the blood. PaO2 shows how well that oxygen is actually getting into the bloodstream. If Paw or FiO2 go up, the index increases, indicating worse oxygenation despite more support. If PaO2 goes up, the index goes down, indicating better oxygenation with less relative burden. Choosing a formula that places PaO2 in the numerator (and FiO2 in the denominator) would invert this relationship and misrepresent how much support is needed to maintain oxygenation.

Oxygen Index measures how much ventilatory support is needed to achieve a given level of arterial oxygenation. The correct formula combines mean airway pressure (Paw) and the fraction of inspired oxygen (FiO2) in the numerator and arterial oxygen tension (PaO2) in the denominator: Paw × FiO2 ÷ PaO2.

Why this makes sense: Paw and FiO2 quantify the amount of support being provided to push oxygen into the blood. PaO2 shows how well that oxygen is actually getting into the bloodstream. If Paw or FiO2 go up, the index increases, indicating worse oxygenation despite more support. If PaO2 goes up, the index goes down, indicating better oxygenation with less relative burden.

Choosing a formula that places PaO2 in the numerator (and FiO2 in the denominator) would invert this relationship and misrepresent how much support is needed to maintain oxygenation.