Fraction of inspired oxygen (F_IO₂), correctly denoted with a capitalI,^[1] is the molar or volumetric fraction ofoxygen in 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_IO₂ of 0.21. Oxygen-enriched air has a higherF_IO₂ than 0.21; up to 1.00 which means 100% oxygen.F_IO₂ is 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_IO₂ is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, theF_IO₂ may remain constant while thepartial pressure of oxygen changes with the change in barometric pressure.

Abbreviated alveolar air equation

${\displaystyle P_A{\text{O}}_2^{}=\frac{P_E{\text{O}}_2^{}-P_I{\text{O}}_2^{}\frac{V_D}{V_t}}{1-\frac{V_D}{V_t}}}$

P_AO₂,P_EO₂, andP_IO₂ are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and⁠V_D/V_t⁠ is the ratio of physiologic dead space over tidal volume.

In medicine, theF_IO₂ is the assumed percentage of oxygen concentration participating ingas exchange in thealveoli.^[3]

TheF_IO₂ is used in theAPACHE II (Acute Physiology and Chronic Health Evaluation II) severity of disease classification system forintensive care unit patients.^[4] ForF_IO₂ values equal to or greater than 0.5, thealveolar–arterial gradient value should be used in the APACHE II score calculation. Otherwise, thePaO₂ will suffice.^[4]

The ratio betweenpartial pressure ofoxygen inarterial blood (PaO2) andF_IO₂ is used as an indicator ofhypoxemia per the American-European Consensus Conference onlung injury. A highF_IO₂ has been shown to alter the ratio of P_aO₂/F_IO₂.^[3]

The ratio ofpartial pressure arterial oxygen and fraction of inspired oxygen, known as theHorowitz index orCarrico 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_aO₂ of 60–100 mmHg and an oxygen content ofF_IO₂ of 0.21 of room air, a normal P_aO₂/F_IO₂ ratio ranges between 300 and 500 mmHg. A P_aO₂/F_IO₂ ratio less than or equal to 200 mmHg is necessary for the diagnosis ofacute respiratory distress syndrome by theAECC criteria.^[7] The more recent Berlin criteria defines mildARDS at a ratio of less than 300 mmHg.

A P_aO₂/F_IO₂ ratio 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_IO₂ ratio less than or equal to 333 mmHg is one of the variables in theSMART-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 mmHg. Since the patient is receiving oxygen-saturated air resulting in aF_IO₂ of 50% oxygen his calculated P_aO₂/F_IO₂ ratio would be 100 mmHg/0.50 = 200 mmHg.

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

${\displaystyle P_A{\text{O}}_2^{}=F_I{\text{O}}_2^{}(PB-P{\text{H}}_2^{}\text{O})-P_A{\text{CO}}_2^{}\left(F_I{\text{O}}_2^{}+\frac{1-F_I{\text{O}}_2^{}}{R}\right)}$

• FiO2 by Delivery DeviceArchived 2015-09-20 at theWayback Machine - Shows FiO₂ by common oxygen deliver systems.

• Pulmonology
• Mechanical ventilation

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