Fraction of inspired oxygen

Fraction of inspired oxygen(F_IO₂), correctly denoted with a capitalI,^[1]is the molar or volumetric fraction ofoxygenin the inhaled gas. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmosphericF_IO₂.Natural air includes 21% oxygen, which is equivalent toF_IO₂of 0.21. Oxygen-enriched air has a higherF_IO₂than 0.21; up to 1.00 which means 100% oxygen.F_IO₂is typically maintained below 0.5 even with mechanical ventilation, to avoidoxygen toxicity,^[2]but there are applications when up to 100% is routinely used.

Often used inmedicine,theF_IO₂is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, theF_IO₂may remain constant while thepartial pressureof oxygen changes with the change in barometric pressure.

Equations

Abbreviated alveolar air equation: $P_{A}{\ce {O2}}={\frac {P_{E}{\ce {O2}}-P_{I}{\ce {O2}}{\frac {V_{D}}{V_{t}}}}{1-{\frac {V_{D}}{V_{t}}}}}$

$P A O 2$ , $P E O 2$ ,and $P I O 2$ are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and $.mw-parser-output .sfrac{white-space:nowrap}.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tion{display:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center}.mw-parser-output .sfrac .num{display:block;line-height:1em;margin:0.0em 0.1em;border-bottom:1px solid}.mw-parser-output .sfrac .den{display:block;line-height:1em;margin:0.1em 0.1em}.mw-parser-output .sr-only{border:0;clip:rect(0,0,0,0);clip-path:polygon(0px 0px,0px 0px,0px 0px);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px}⁠VD/Vt⁠$ is the ratio of physiologic dead space over tidal volume.

Medicine

In medicine, theF_IO₂is the assumed percentage of oxygen concentration participating ingas exchangein thealveoli.^[3]

Uses

TheF_IO₂is used in theAPACHE II(Acute Physiology and Chronic Health Evaluation II) severity of disease classification system forintensive care unitpatients.^[4]ForF_IO₂values equal to or greater than 0.5, thealveolar–arterial gradientvalue should be used in the APACHE II score calculation. Otherwise, thePaO₂will suffice.^[4]

The ratio betweenpartial pressureofoxygeninarterial blood(PaO2) andF_IO₂is used as an indicator ofhypoxemiaper the American-European Consensus Conference onlung injury.A highF_IO₂has been shown to alter the ratio of P_aO₂/F_IO₂.^[3]

P_aO₂/F_IO₂ratio

The ratio ofpartial pressure arterial oxygenand fraction of inspired oxygen, known as theHorowitz indexorCarrico 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.^[5]A sample of arterial blood is collected for this test.^[6]With a normal P_aO₂of 60–100 mmHg and an oxygen content ofF_IO₂of 0.21 of room air, a normal P_aO₂/F_IO₂ratio ranges between 300 and 500 mmHg. A P_aO₂/F_IO₂ratio less than or equal to 200 mmHg is necessary for the diagnosis ofacute respiratory distress syndromeby theAECC criteria.^[7]The more recent Berlin criteria defines mildARDSat a ratio of less than 300 mmHg.

A P_aO₂/F_IO₂ratio less than or equal to 250 mmHg is one of the minor criteria for severecommunity acquired pneumonia(i.e., possible indication for inpatient treatment).

A P_aO₂/F_IO₂ratio less than or equal to 333 mmHg is one of the variables in theSMART-COPrisk 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_aO₂is found to be 100 mmHg. Since the patient is receiving oxygen-saturated air resulting in aF_IO₂of 50% oxygen his calculated P_aO₂/F_IO₂ratio would be 100 mmHg/0.50 = 200 mmHg.

Related mathematics

Alveolar air equation

Thealveolar air equationis the following formula, used to calculate the partial pressure of alveolar gas:

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)

References

^Wagner, Peter D. (2021-03-01)."i stands for internet (and other things), not for inspired O 2 concentration".American Journal of Physiology. Lung Cellular and Molecular Physiology.320(3): L467.doi:10.1152/ajplung.00610.2020.ISSN 1040-0605.PMID 33750222.
^Bitterman H (2009)."Bench-to-bedside review: oxygen as a drug".Crit Care.13(1): 205.doi:10.1186/cc7151.PMC2688103.PMID 19291278.
^^a ^bAllardet-Servent J, Forel JM, Roch A, Guervilly C, Chiche L, Castanier M, et al. (2009). "FIO2 and acute respiratory distress syndrome definition during lung protective ventilation".Crit Care Med.37(1): 202–7, e4-6.doi:10.1097/CCM.0b013e31819261db.PMID 19050631.
^^a ^b"APACHE II Score".mdcalc.com.MDCalc.Retrieved21 September2017.
^Toy P, Popovsky MA, Abraham E, Ambruso DR, Holness LG, Kopko PM, et al. (2005)."Transfusion-related acute lung injury: definition and review".Crit Care Med.33(4): 721–6.doi:10.1097/01.ccm.0000159849.94750.51.PMID 15818095.
^Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.
^Mason, R. Murray and Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia, PA 2010

External links

FiO2 by Delivery Device Archived2015-09-20 at theWayback Machine- Shows FiO₂by common oxygen deliver systems.

[1] Wagner, Peter D. (2021-03-01)."i stands for internet (and other things), not for inspired O 2 concentration".American Journal of Physiology. Lung Cellular and Molecular Physiology.320(3): L467.doi:10.1152/ajplung.00610.2020.ISSN 1040-0605.PMID 33750222.

[2] Bitterman H (2009)."Bench-to-bedside review: oxygen as a drug".Crit Care.13(1): 205.doi:10.1186/cc7151.PMC2688103.PMID 19291278.

[pmid19050631-3] Allardet-Servent J, Forel JM, Roch A, Guervilly C, Chiche L, Castanier M, et al. (2009). "FIO2 and acute respiratory distress syndrome definition during lung protective ventilation".Crit Care Med.37(1): 202–7, e4-6.doi:10.1097/CCM.0b013e31819261db.PMID 19050631.

[mdcalc_APACHE_II-4] "APACHE II Score".mdcalc.com.MDCalc.Retrieved21 September2017.

[pmid818095-5] Toy P, Popovsky MA, Abraham E, Ambruso DR, Holness LG, Kopko PM, et al. (2005)."Transfusion-related acute lung injury: definition and review".Crit Care Med.33(4): 721–6.doi:10.1097/01.ccm.0000159849.94750.51.PMID 15818095.

[6] Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.

[7] Mason, R. Murray and Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia, PA 2010

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