Oxygenation Index Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Feb 15, 2022

This Oxygenation Index calculator (OI) gives us an accurate way of measuring the diffusion barrier of the lungs - that is, the ability of the lungs to transfer oxygen from the air we breathe into our blood. The Oxygen Index formula is frequently used to assess the wellbeing of neonates.

But how to interpret OI? Well, to put it as simply as possible: the larger the index is, the worse the state of the lungs. The OI calculator can be a useful tool in the hands of an experienced physician.

Why do we need the Oxygen Index formula?

The medical abbreviation "OI" can be found in documents or articles written by intensive care specialists. We use the Oxygenation Index to determine the breathing capacity of the patient (consult the What is oxygenation? section below). It's usually used alongside other formulas that calculate the parameters of mechanical ventilation. It gives the physicians a better view of the condition of the patient's pulmonary system and facilitates diagnostic and treatment decisions. It's also an efficient tool when used to predict the future outcome and performance of the patient.

A value of over 25 informs the ICU specialist that the patient needs increased attention, and that special measures need to be taken to stop the further deterioration of the patient's state (for example, use of the ECMO technique). If the value is over 40, we are obliged to use an ECMO device.

Oxygenation Index formula

In order to calculate the Oxygenation Index you will need the following data:

  • FiO₂ - fraction of inspired oxygen;
  • Mean Airway Pressure (MPaw); and
  • PaO₂ - partial pressure of the oxygen in the arterial blood.

OI = ( FiO2 * Mean Airway Pressure ) / PaO2

Results interpretation:

  • OI < 5 - healthy person;
  • OI 5 - 25 - indicates lung disease;
  • OI 25 - 40 - increased mortality; and
  • OI > 40 - consider use of ECMO.

As you can see, higher values of PaO₂ and lower values of FiO2 are always a good sign - it means that the patient can keep their blood oxygen levels high despite a low amount of inhaled oxygen.

What does FiO₂ stand for?

FiO₂ stands for the fraction of inspired oxygen, which is the amount of oxygen present in the air we breathe in. It's especially crucial for intensive care patients, where the physician can regulate this parameter artificially, and so must monitor it carefully.

You can use this Oxygenation Index calculator to calculate the FiO₂.

What is oxygenation?

In medicine, oxygenation is the process of adding oxygen to a person's blood. It depends on the amount of gas inhaled, the concentration of oxygen in it, as well as the cellular or fluid barrier present in the lungs. (You can check your own maximal O₂ capacity in our VO₂ max tool).

Oxygenation may happen the physiological way - through the lungs - or by use of a state-of-the-art machine - known as ExtraCorporeal Membrane Oxygenation (ECMO). You can think of ECMO as an artificial, short-term replacement of the human lungs and heart.

PaO₂ / FiO₂ ratio

The PaO₂ / FiO₂ ratio is another parameter we can obtain using the Oxygenation Index calculator. It informs us about the possibility of ARDS (Acute Respiratory Distress Syndrome), a sudden state of lung insufficiency which may result in a patient's death.

There are three stages of ARDS, each measured with the PaO₂ / FiO₂ ratio:

  • 200-300 = Mild
  • 100-200 = Moderate
  • 0-100 = Severe
Łucja Zaborowska, MD, PhD candidate
cm H₂O
PaO₂/FiO₂ ratio
Oxygenation Index (OI)
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