Fraction of inspired oxygen

{{short description|Volumetric proportion of oxygen to other constituents in a breathing gas }} '''Fraction of inspired oxygen''' ('''''F_I''O₂'''), correctly denoted with a capital ''I'',<ref>{{Cite journal|last=Wagner|first=Peter D.|date=2021-03-01|title=i stands for internet (and other things), not for inspired O 2 concentration|journal=American Journal of Physiology. Lung Cellular and Molecular Physiology|language=en|volume=320|issue=3|pages=L467|doi=10.1152/ajplung.00610.2020|pmid=33750222 |issn=1040-0605|doi-access=free}}</ref> is the molar or volumetric fraction of oxygen in the inhaled gas. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric ''F_I''O₂. Natural air includes 21% oxygen, which is equivalent to ''F_I''O₂ of 0.21. Oxygen-enriched air has a higher ''F_I''O₂ than 0.21; up to 1.00 which means 100% oxygen. ''F_I''O₂ is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity,<ref>{{cite journal | pmc = 2688103 | pmid=19291278 | doi=10.1186/cc7151 | volume=13 | title=Bench-to-bedside review: oxygen as a drug | year=2009 | author=Bitterman H | journal=Crit Care | issue=1 | pages=205 | doi-access=free }}</ref> but there are applications when up to 100% is routinely used.

Often used in medicine, the ''F_I''O₂ is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the ''F_I''O₂ may remain constant while the partial pressure of oxygen changes with the change in barometric pressure.

== Equations == ;Abbreviated alveolar air equation :<math chem="">P_A \ce{O2} = \frac{P_E \ce{O2} - P_I \ce{O2} \frac{V_D}{V_t}}{1- \frac{V_D}{V_t}}</math> {{math|''P''{{sub|''A''}}O{{sub|2}}}}, {{math|''P''{{sub|''E''}}O{{sub|2}}}}, and {{math|''P''{{sub|''I''}}O{{sub|2}}}} are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and {{math|{{sfrac|''V''{{sub|''D''}}|''V''{{sub|''t''}}}}}} is the ratio of physiologic dead space over tidal volume.

== Medicine == In medicine, the ''F_I''O₂ is the assumed percentage of oxygen concentration participating in gas exchange in the alveoli.<ref name="pmid19050631">{{cite journal |vauthors=Allardet-Servent J, Forel JM, Roch A, Guervilly C, Chiche L, Castanier M, etal | title=FIO2 and acute respiratory distress syndrome definition during lung protective ventilation. | journal=Crit Care Med | year= 2009 | volume= 37 | issue= 1 | pages= 202–7, e4-6 | pmid=19050631 | doi=10.1097/CCM.0b013e31819261db }}</ref>

== Uses == The ''F_I''O₂ is used in the APACHE II (Acute Physiology and Chronic Health Evaluation II) severity of disease classification system for intensive care unit patients.<ref name="mdcalc APACHE II">{{cite web|title=APACHE II Score|url=https://www.mdcalc.com/apache-ii-score|website=mdcalc.com|publisher=MDCalc|access-date=21 September 2017}}</ref> For ''F_I''O₂ values equal to or greater than 0.5, the alveolar–arterial gradient value should be used in the APACHE II score calculation. Otherwise, the ''Pa''O₂ will suffice.<ref name="mdcalc APACHE II" />

The ratio between partial pressure of oxygen in arterial blood (PaO2) and ''F_I''O₂ is used as an indicator of hypoxemia per the American-European Consensus Conference on lung injury. A high ''F_I''O₂ has been shown to alter the ratio of P_aO₂/''F_I''O₂.<ref name="pmid19050631"/>

== P_aO₂/''F_I''O₂ ratio == The ratio of partial pressure arterial oxygen and fraction of inspired oxygen, known as the Horowitz index 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.<ref name="pmid818095">{{cite journal |vauthors=Toy P, Popovsky MA, Abraham E, Ambruso DR, Holness LG, Kopko PM, etal | title=Transfusion-related acute lung injury: definition and review. | journal=Crit Care Med | year= 2005 | volume= 33 | issue= 4 | pages= 721–6 | pmid=15818095 | doi= 10.1097/01.ccm.0000159849.94750.51}}</ref> A sample of arterial blood is collected for this test.<ref>Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.</ref> With a normal P_aO₂ of 60&ndash;100&nbsp;mmHg and an oxygen content of ''F_I''O₂ of 0.21 of room air, a normal P_aO₂/''F_I''O₂ ratio ranges between 300 and 500&nbsp;mmHg. A P_aO₂/''F_I''O₂ ratio less than or equal to 200&nbsp;mmHg is necessary for the diagnosis of acute respiratory distress syndrome by the AECC criteria.<ref>Mason, R. Murray and Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia, PA 2010</ref> The more recent Berlin criteria defines mild ARDS at a ratio of less than 300&nbsp;mmHg.

A P_aO₂/''F_I''O₂ ratio less than or equal to 250&nbsp;mmHg is one of the minor criteria for severe community acquired pneumonia (i.e., possible indication for inpatient treatment).

A P_aO₂/''F_I''O₂ ratio less than or equal to 333&nbsp;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_aO₂ is found to be 100&nbsp;mmHg. Since the patient is receiving oxygen-saturated air resulting in a ''F_I''O₂ of 50% oxygen his calculated P_aO₂/''F_I''O₂ ratio would be 100&nbsp;mmHg/0.50&nbsp;=&nbsp;200&nbsp;mmHg.

== Related mathematics ==

=== Alveolar air equation === The alveolar air equation is the following formula, used to calculate the partial pressure of alveolar gas: :<math chem> P_A \ce{O2} = F_I \ce{O2}(PB-P \ce{H2O}) - P_A \ce{CO2} \left(F_I \ce{O2} + \frac{1-F_I\ce{O2}}{R}\right)</math>

==References== {{reflist|30em}}

==External links== *[http://respwiki.com/FiO2_by_Delivery_Device FiO2 by Delivery Device] {{Webarchive|url=https://web.archive.org/web/20150920083019/http://respwiki.com/FiO2_by_Delivery_Device |date=2015-09-20 }} - Shows FiO₂ by common oxygen deliver systems.

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Category:Pulmonology Category:Mechanical ventilation