Fraction of inspired oxygen

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Fraction of inspired oxygen (FiO2) is the fraction or percentage of oxygen in the space being measured. 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.



Abbreviated alveolar air equation
P_A O_2 = \frac{P_E O_2 - P_i O_2 \frac{V_D}{V_t}}{1- \frac{V_D}{V_t}}

PAO2, PEO2, and PiO2 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.


In medicine the FiO2 is the assumed percentage of oxygen concentration participating in gas exchange in the alveoli[1].


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[1].

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.[2] A sample of arterial blood is collected for this test.[3] A PaO2/FiO2 ratio less than or equal to 200 is necessary for the diagnosis of Acute Respiratory Distress Syndrome. [4]

Related mathematics

Alveolar air equation

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

 P_A O_2 =  F_I O_2(PB-P H_2 0) - P_A CO_2 (F_I O_2 + \frac{1-FIO2}{R})

See also


  1. ^ a b 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. 
  2. ^ 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. PMID 15818095. 
  3. ^ Tietz NW (Ed): Clinical Guide to Laboratory Tests, 3rd ed. W. B. Saunders, Philadelphia, PA, 1995.
  4. ^ Mason, R. Murray and Nadel's Textbook of Respiratory Medicine, 5th ed. Philadelphia, PA 2010