The oxygenation index is a calculation used in intensive care medicine to measure the fraction of inspired oxygen (FiO2) and its usage within the body.

A lower oxygenation index is better - this can be inferred by the equation itself. As the oxygenation of a person improves, they will be able to achieve a higher PaO2 at a lower FiO2. This would be reflected on the formula as a decrease in the numerator or an increase in the denominator - thus lowering the OI. Typically an OI threshold is set for when a neonate should be placed on ECMO, for example >40.

${\displaystyle OI={\frac {Fi{\ce {O2}}\times M_{\text{PAW}}}{Pa{\ce {O2}}}}}$^[1]

• ${\displaystyle Fi{\ce {O2}}}$: Fraction of inspired oxygen, in percent;
• ${\displaystyle M_{\text{PAW}}}$: Mean airway pressure, in mmHg;
• ${\displaystyle Pa{\ce {O2}}}$: Partial pressure of oxygen in arterial blood, in mmHg.