Fraction of inspired oxygen (''F
I''O
2), correctly denoted with a capital ''I'', is the molar or volumetric fraction of
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in the inhaled gas. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric ''F
I''O
2. Natural air includes 21% oxygen, which is equivalent to ''F
I''O
2 of 0.21. Oxygen-enriched air has a higher ''F
I''O
2 than 0.21; up to 1.00 which means 100% oxygen. ''F
I''O
2 is typically maintained below 0.5 even with mechanical ventilation, to avoid
oxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen () at increased partial pressures. Severe cases can result in cell damage and death, with effects most often seen in the central nervous system, lung ...
, but there are applications when up to 100% is routinely used.
Often used in
medicine
Medicine is the science and Praxis (process), practice of caring for patients, managing the Medical diagnosis, diagnosis, prognosis, Preventive medicine, prevention, therapy, treatment, Palliative care, palliation of their injury or disease, ...
, the ''F
I''O
2 is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the ''F
I''O
2 may remain constant while the
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
of oxygen changes with the change in barometric pressure.
Equations
;Abbreviated alveolar air equation
:
, , and are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and is the ratio of physiologic dead space over tidal volume.
Medicine
In medicine, the ''F
I''O
2 is the assumed percentage of oxygen concentration participating in
gas exchange
Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane, or a b ...
in the
alveoli
Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* M ...
.
Uses
The ''F
I''O
2 is used in the
APACHE II
APACHE II ("Acute Physiology and Chronic Health Evaluation II") is a severity-of-disease classification system, one of several ICU scoring systems. It is applied within 24 hours of admission of a patient to an intensive care unit (ICU): an in ...
(Acute Physiology and Chronic Health Evaluation II) severity of disease classification system for
intensive care unit
An intensive care unit (ICU), also known as an intensive therapy unit or intensive treatment unit (ITU) or critical care unit (CCU), is a special department of a hospital or health care facility that provides intensive care medicine.
An inten ...
patients.
For ''F
I''O
2 values equal to or greater than 0.5, the
alveolar–arterial gradient
The Alveolar–arterial gradient (A-, or A–a gradient), is a measure of the difference between the alveolar concentration (A) of oxygen and the arterial (a) concentration of oxygen. It is a useful parameter for narrowing the differential diagno ...
value should be used in the APACHE II score calculation. Otherwise, the ''Pa''O
2 will suffice.
The ratio between
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal g ...
of
oxygen
Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...
in
arterial blood
Arterial blood is the oxygenated blood in the circulatory system found in the pulmonary vein, the left chambers of the heart, and in the artery, arteries. It is bright red in color, while venous blood is dark red in color (but looks purple through ...
(
PaO2
Blood gas tension refers to the partial pressure of gases in blood. There are several significant purposes for measuring gas tension. The most common gas tensions measured are oxygen tension (PxO2), carbon dioxide tension (PxCO2) and carbon monoxi ...
) and ''F
I''O
2 is used as an indicator of
hypoxemia
Hypoxemia (also spelled hypoxaemia) is an abnormally low level of oxygen in the blood. More specifically, it is oxygen deficiency in arterial blood. Hypoxemia is usually caused by pulmonary disease. Sometimes the concentration of oxygen in the ...
per the American-European Consensus Conference on
lung injury
The lungs are the primary Organ (biology), organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the Vertebral column, backbone on either side of the heart. Their ...
. A high ''F
I''O
2 has been shown to alter the ratio of P
aO
2/''F
I''O
2.
PaO2/''FI''O2 ratio
The ratio of
partial pressure arterial oxygen and fraction of inspired oxygen, known as the
Horowitz index
The Horowitz index or Horovitz index (also known as the Horowitz quotient or the P/F ratio) is a ratio used to assess lung function in patients, particularly those on ventilators. Overall, it is useful for evaluating the extent of damage to the ...
or ''Carrico index'', is a comparison between the oxygen level in the blood and the oxygen concentration that is breathed. This helps to determine the degree of any problems with how the lungs transfer oxygen to the blood.
A sample of arterial blood is collected for this test. With a normal P
aO
2 of 60–100 mmHg and an oxygen content of ''F
I''O
2 of 0.21 of room air, a normal P
aO
2/''F
I''O
2 ratio ranges between 300 and 500 mmHg. A P
aO
2/''F
I''O
2 ratio less than or equal to 200 mmHg is necessary for the diagnosis of
acute respiratory distress syndrome
Acute respiratory distress syndrome (ARDS) is a type of respiratory failure characterized by rapid onset of widespread inflammation in the lungs. Symptoms include shortness of breath (dyspnea), rapid breathing (tachypnea), and bluish skin co ...
by the
AECC criteria.
[Mason, R. Murray and Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia, PA 2010] The more recent Berlin criteria defines mild
ARDS
Ards (or ARDS, ARDs) may refer to:
Medical
* ARDS, Acute respiratory distress syndrome
* ARDs, age-related diseases
Places
* Ards Peninsula, Northern Ireland
** Ards (territory), several historical territorial divisions on the Ards Peninsula
...
at a ratio of less than 300 mmHg.
A P
aO
2/''F
I''O
2 ratio less than or equal to 250 mmHg is one of the minor criteria for severe
community acquired pneumonia
Community-acquired pneumonia (CAP) refers to pneumonia contracted by a person outside of the healthcare system. In contrast, hospital-acquired pneumonia (HAP) is seen in patients who are in a hospital or who have recently been hospitalized in the ...
(i.e., possible indication for inpatient treatment).
A P
aO
2/''F
I''O
2 ratio less than or equal to 333 mmHg is one of the variables in the
SMART-COP risk score for intensive respiratory or vasopressor support in community-acquired pneumonia.
;Example calculation: After drawing an arterial blood gas sample from a patient the P
aO
2 is found to be 100 mmHg. Since the patient is receiving oxygen-saturated air resulting in a ''F
I''O
2 of 50% oxygen his calculated P
aO
2/''F
I''O
2 ratio would be 100 mmHg/0.50 = 200 mmHg.
Related mathematics
Alveolar air equation
The
alveolar air equation
The alveolar gas equation is the method for calculating partial pressure of alveolar oxygen (). The equation is used in assessing if the lungs are properly transferring oxygen into the blood. The alveolar air equation is not widely used in clini ...
is the following formula, used to calculate the partial pressure of alveolar gas:
:
References
External links
FiO2 by Delivery Device - Shows FiO
2 by common oxygen deliver systems.
{{authority control
Pulmonology
Mechanical ventilation