PF Ratio Calculator

This PF ratio calculator (also a PaO2 FiO2 ratio calculator) calculates the PF ratio, which may be useful in classifying the severity of, among others, acute respiratory distress syndrome (ARDS) You may 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. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a healthcare provider.

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 finds not only the blood gas values (oxygen partial pressure, PaO2, carbon dioxide level, PaCO2) but also arterial blood pH, which is also useful in the blood pH calculator.

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

An alternative to blood gas tests is pulse oximetry, but 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 blood gas tests. The results for pulse oximetry are given in percentages, and healthy results are between 95 and 100 percent.

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% to avoid oxygen toxicity.

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

As mentioned before, the 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 score, in which the PaO2 to FiO2 ratio plays an important part. The SOFA scale helps doctors estimate the risk of morbidity and mortality caused by sepsis. Patients are rated based on how they perform in six categories:

• Respiration — calculable with this PaO2 FiO2 ratio calculator;
• Cardiovascular;
• Hepatic;
• Coagulation;
• Renal; and
• Neurological systems.

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

If you feel frightened by the previous paragraph, don't worry! We will now explain the situations where 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; and

  • Drugs, like anesthetics.

• Ventilation/perfusion mismatch:

  • Physical exercise.

• Right to left cardiac shunt:

  • Anatomical abnormalities — patent ductus arteriosus; and

  • 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, and idiopathic pulmonary fibrosis.

• Low PO2 (oxygen pressure in the atmosphere)

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

The external partial pressure of oxygen decreases with altitude (check out hydrostatic pressure calculator). 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). Due to increased hydrostatic pressure, the tissue oxygen concentration below the water's surface is sufficient to keep a person conscious. However, after surfacing, the oxygen saturation is insufficient at surface pressure.