oxygenation index oxygenation index 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. equation o i = f i o 2 × m paw p a o 2 \times m_ f i o 2 : fraction of inspired oxygen, in percent; m paw : mean airway pressure, in mmhg; p a o 2 : partial pressure of oxygen in arterial blood, in mmhg.