The PaO2/FiO2 ratio, also called the PF ratio, is a useful clinical tool that can be used to determine the lung health of a patient with oxygen therapy. This ratio is often used to diagnose or monitor patients with pulmonary disease, as well as help treat patients in an ICU setting. Here’s what you need to know about the PF ratio, including how it’s calculated and when it should be used.

What is the PF ratio?

The PF ratio is the proportion of the time that a patient is breathing spontaneously without any mechanical ventilator support. It’s also known as respiratory muscle strength index (RMIS) and, in some countries, it’s called respiratory reserve capacity (RRC).

The interpretation of this value depends on what your normal ranges are for these values: if you have patients who are breathing spontaneously 100% of the time, your baseline will be 0%; if you have people who need to breathe manually 100% of their time, your baseline will also be 0%; and if you have people with severe disease who never breathe spontaneously at all but do so when given positive pressure ventilation (PPV) then your baseline will be 100%.

What is blood?

Blood is a fluid that carries oxygen and nutrients to the body’s cells. It is made up of red blood cells, white blood cells, platelets and plasma. Red blood cells carry oxygen from the lungs to all parts of your body. White blood cells fight infection and help with wound healing. Platelets help blood to clot so if you cut yourself or get a thorn in your finger it won’t bleed for long enough for you to die from it!

Arterial blood

Arterial blood is the fluid that carries oxygen and nutrients to the body’s cells. It contains red blood cells, white blood cells and platelets (cells in your body that help with clotting).

Blood is made up of:

  • red blood cells—responsible for carrying oxygen and carbon dioxide throughout the body
  • white blood cells—fight bacteria and viruses

To calculate your FPI ratio, you’ll need to know your arterial oxygen saturation levels. This is a measurement of how much oxygen is being carried by your arterial blood.


In the 1960s, Dr. Irving Horowitz developed the Horowitz Index to help him determine whether patients were receiving enough oxygen therapy. At the time, Dr. Horowitz was a medical student at Harvard University and later went on to become a pulmonologist. He created this tool as part of his dissertation and published it in 1963 in The New England Journal of Medicine. In this article, he described how he used this index during clinical practice with patients who were prescribed supplemental oxygen therapy but still had an elevated respiratory rate (RR).


FiO₂ is the fraction of oxygen that is dissolved in the blood. It is measured in a blood gas machine, which measures concentration by electrical conductivity. The FiO₂ value for pure O₂ gas at STP (standard temperature and pressure) is exactly 0.21%—in other words, one-fifth of an atmosphere or one-sixth of a kilogram per cubic meter at 1 atmosphere and 0 °C (273 K).

The extent to which O₂ dissolves in water depends on its partial pressure: i.e., how much there is compared to all other gases present. In humans and other mammals, this partial pressure is determined by breathing: when we inhale air containing high amounts of O₂ (for example at high altitudes), then our body absorbs more dissolved oxygen; likewise if we exhale air containing high levels of CO2 or other gasses then less oxygen will be absorbed from our bloodstream into our cells due to competition between gasses for space within capillaries.


PaO₂ (partial pressure of oxygen) is a direct measure of the amount of oxygen in your arterial blood. PaO₂ is measured in millimeters of mercury (mmHg), much like blood pressure. It can be used to help determine if a patient has adequate oxygenation and assess if an individual has chronic respiratory distress syndrome or acute lung injury.

How to use the PF ratio calculator

So, as we mentioned before, to calculate a person’s PF ratio, you need to know two things. Those two things are:

  • Partial pressure of arterial oxygen
  • Fraction of inspired oxygen


What is considered a normal PF ratio?

A normal PF ratio is above 400.

What is a good FiO2 level?

The FiO2 level is generally kept below 0.5.

What should PaO2 be on 100 oxygen?

It can be as high as 663 mmHg.