PF Ratio Calculator

Created by Dominika Śmiałek, MD, PhD candidate
Reviewed by Dominik Czernia, PhD and Jack Bowater
Based on research by
Rice TW, Wheeler AP, Bernard GR, Hayden DL, Schoenfeld DA, Ware LB; National Institutes of Health, National Heart, Lung, and Blood Institute ARDS Network. Comparison of the SpO2/FIO2 ratio and the PaO2/FIO2 ratio in patients with acute lung injury or ARDS. CHEST (August 2007)
Last updated: Jul 25, 2022

This PF ratio calculator (also a PaO2 FiO2 ratio calculator) is designed to determine a patient's respiratory efficiency. Its primary purpose is as a clinical indicator of hypoxemia (an abnormally low level of oxygen in the blood). A low PF ratio may be present if a person is suffering from severe health problems, including, but not limited to, Multiple Organ Dysfunction Syndrome (MODS). You may also be wondering what is PaO2 and what is FiO2? - well, look no further, you will find the explanation in the article below.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.

What is PaO2?

PaO2 is the partial pressure of oxygen that is measured from arterial blood in Intensive Care Units (ICUs) and wards. A normal, healthy person should have a value between 75 mmHg and 100 mmHg (these values are only true at sea level, as the values change with altitude as the partial pressure of oxygen changes). The most common way to measure PaO2 is to take an arterial-blood gas test. This not only finds the blood gas values (oxygen partial pressure, PaO2, and carbon dioxide level, PaCO2), but also arterial blood pH.

Normal blood gas values (at sea level) range between 75 - 100 mmHg for PaO2 and 38 - 42 mmHg for PaCO2.

An alternative to a blood gas tests is pulse oximetry, but, unfortunately, this can't be used in PF ratio calculations. Pulse oximetry is a non-invasive, inexpensive, and safe method of monitoring oxygen saturation in the blood. However, as it doesn't require a blood sample, the results are not the same as those from a blood gas tests. The results for pulse oximetry are given in percentages, and a healthy results is between 95 and 100 percent.

What is FiO2?

FiO2 is the fraction of inspired oxygen. If you breathe regular air (you probably do - it's almost everywhere), your FiO2 is equal to 21 %. Patients with hypoxemia, breathing difficulties, or any other medical condition affecting the inspiration process, receive oxygen-enriched air. In these cases, their FiO2 is higher than 21 % but typically less than 50 % as to avoid oxygen toxicity.

How does the PF ratio calculator work?

For performing a calculation, you need PaO2 values (from a blood gas test) and FiO2 values (either 21 % by default or a higher value if the patient is being treated with oxygen).

Let's say our patient has PaO2 = 95 mmHg, which is a healthy value (between 75 - 100 mmHg) and has a FiO2 = 30 % (so this patient is most likely receiving oxygen-enriched air).

PaO₂ / FiO₂ = PF ratio

95 mmHg / 30 % = 316.67 mmHg

This result is pathological, as a healthy PF ratio is > 400 mmHg. Therefore our patient should consult a doctor.

Our PaO2 to FiO2 calculator will make quick work of this calculation. It easily converts the percentage and parses the equation, so you will immediately know if there is anything to worry about!

When is PF ratio calculator useful?

As mentioned before, PF ratio is handy when assessing hypoxemia, particularly in patients with conditions affecting their upper or lower airways. Hypoxemia may affect several organs simultaneously, and, if it is happening in two or more major organ systems, it affects homeostasis. Usually, this condition appears as a complication of sepsis. A patient's state can be easily evaluated with the qsofa scale, in which PaO2 to FiO2 ratio plays an important part. The SOFA scale helps doctors to estimate the risk of morbidity and mortality caused by sepsis. Patients are rated base on how they perform in six categories:

The results are used to help diagnose MODS - Multiple Organ Dysfunction Syndrome.

When is PF ratio reduced in day-to-day life?

If you feel frightened by the previous paragraph, don't worry! We will explain now the situations in which you may have a lower PaO2 to FiO2 ratio and still be healthy. A low PF ratio in people breathing regular air is caused by a change in normal blood gas values. As the partial pressure of oxygen in arterial blood decreases, the person enters a state known as hypoxemia.

Among the most common causes are some respiratory and cardiac disorders:

  • hypoventilation
    • central sleep apnea
    • chest deformities, such as scoliosis
    • drugs, like anesthetics
  • ventilation/perfusion mismatch
    • physical exercise
  • right to left cardiac shunt
    • anatomical abnormalities - patent ductus arteriosus
    • ARDS - acute respiratory distress syndrome; it may have various causes, such as sepsis, trauma or pneumonia. The symptoms of ARDS are shortness of breath, rapid breathing, and bluish skin coloration.
  • diffusion impairment - diffusion is the transfer of gas from the air in the lungs to the red blood cells in the blood vessels. Impairment can be caused by structural changes in the lung tissue, e.g., sarcoidosis, emphysema, idiopathic pulmonary fibrosis.
  • low PO2 (oxygen pressure in the atmosphere)

As you can see, there are lots of health conditions that affect PF ratio.

Extreme sports and PF ratio

The external partial pressure of oxygen decreases with altitude. For example, when climbing Mount Everest, you are exposed to partial pressure equal to only 43 mmHg at the top of the mountain. This results in cerebral hypoxia, resulting in mountain sickness. As a reminder, the partial pressure at sea level is 150 mmHg. Hypoxia may also occur after diving when the diver must suddenly surface (water blackout). Tissue oxygen concentration is sufficient to keep a person conscious below the water's surface, due to increased hydrostatic pressure. However, after surfacing, the oxygen saturation is insufficient at surface pressure.

So, do you have a patient that requires immediate treatment? Input the corresponding data into our PaO2/FiO2 ratio calculator and you'll have your results in a moment.

Dominika Śmiałek, MD, PhD candidate
PaO₂/FiO₂ ratio
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