Fraction of inspired oxygen

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Fraction of inspired oxygen (FiO₂) is the fraction or percentage of oxygen in the volume being measured. Medical patients experiencing difficulty breathing are provided with oxygen-enriched air, which means a higher-than-atmospheric FiO₂. Natural air includes 21% oxygen, which is equivalent to FiO₂ of 0.21. Oxygen-enriched air has a higher FiO₂ than 0.21; up to 1.00 which means 100% oxygen. FiO₂ is typically maintained below 0.5 even with mechanical ventilation, to avoid oxygen toxicity. If a patient is wearing a nasal cannula or a simple face mask, each additional liter/min of oxygen adds about 4 percentage points for each of the first 3 liters and only 3 percentage points for every liter thereafter to their FiO
₂ (for example, a patient with a nasal cannula with 4L/min of oxygen flow would have an FiO
₂ of 21% + (3 × 4%)+(1 × 3%) =36%).

Often used in medicine, the FiO₂ is used to represent the percentage of oxygen participating in gas-exchange. If the barometric pressure changes, the FiO₂ may remain constant while the partial pressure of oxygen changes with the change in barometric pressure.

Video Fraction of inspired oxygen

Equations

Abbreviated alveolar air equation

${\displaystyle P_{A}{\ce {O2}}={\frac {P_{E}{\ce {O2}}-P_{i}{\ce {O2}}{\frac {V_{D}}{V_{t}}}}{1-{\frac {V_{D}}{V_{t}}}}}}$

P_AO₂, P_EO₂, and P_iO₂ are the partial pressures of oxygen in alveolar, expired, and inspired gas, respectively, and VD/VT is the ratio of physiologic dead space over tidal volume.

Maps Fraction of inspired oxygen

Medicine

In medicine, the FiO₂ is the assumed percentage of oxygen concentration participating in gas exchange in the alveoli.

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Uses

The FiO₂ is used in the APACHE II (Acute Physiology and Chronic Health Evaluation II) severity of disease classification system for intensive care unit patients. For FiO₂ values equal to or greater than 0.5, the alveolar-arterial gradient value should be used in the APACHE II score calculation. Otherwise, the PaO₂ will suffice.

The ratio between partial pressure of oxygen in arterial blood (PaO2) and FiO₂ is used as an indicator of hypoxemia per the American-European Consensus Conference on lung injury. A high FiO₂ has been shown to alter the ratio of PaO₂/FiO₂.

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PaO2/FiO2 ratio

The ratio of partial pressure arterial oxygen and fraction of inspired oxygen, sometimes called the 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. A PaO2/FiO2 ratio less than or equal to 200 is necessary for the diagnosis of acute respiratory distress syndrome by the AECC criteria. The more recent Berlin criteria defines mild ARDS at a ratio of <300.

A PaO
2/FiO
2 ratio less than or equal to 250 is one of the minor criteria for severe community acquired pneumonia (i.e., possible indication for inpatient treatment).

A PaO
2/FiO
2 ratio less than or equal to 333 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 ABG from a patient the PaO2 is found to be 100mmHg. Since the patient is receiving O2-saturated air resulting in a FiO2 of 50% oxygen his calculated PaO
2/FiO
2 ratio would be 100 mmHg / 0.5 = 200.

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Related mathematics

Alveolar air equation

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

${\displaystyle P_{A}{\ce {O2}}=F_{I}{\ce {O2}}(PB-P{\ce {H2O}})-P_{A}{\ce {CO2}}\left(F_{I}{\ce {O2}}+{\frac {1-FI{\ce {O2}}}{R}}\right)}$

See also

• Oxygen toxicity

References

External links

• FiO2 by Delivery Device - Shows FiO2 by common oxygen deliver systems.

Source of the article : Wikipedia