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Oxygenation Index Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Dechert RE, Park PK, Bartlett RH. Evaluation of the oxygenation index in adult respiratory failure.; Journal of Trauma and Acute Care Surgery; February 2014
Last updated: Jun 05, 2023


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 well-being 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, the 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 the use of ECMO.

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

Checking out the partial pressure calculator might come in handy to estimate and understand partial pressure.

What does FiO₂ stand for?

FiO₂ stands for a 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 must monitor it carefully.

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

Might we suggest taking a look at the rsbi calculator, short for rapid shallow breathing index – it counts a parameter used for patients put on mechanical ventilation for you.

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 runners calculator).

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 that 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

We also have a PF ratio calculator to help you determine a patient's respiratory efficiency.

Łucja Zaborowska, MD, PhD candidate
FiO₂
%
MAP
cmH₂O
PaO₂
mmHg
PaO₂/FiO₂ ratio
Oxygenation Index (OI)
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