Inspired oxygen fraction ($F_{\mathrm{I}{O}_2}$)

Inspired oxygen fraction, $F_{\mathrm{I}{O}_2}$, is the fraction of the inspired gas mixture attributable to oxygen, with the gas-condition convention specified. It is dimensionless: a fraction 0.40 is 40%, not an arterial oxygen partial pressure of 40 mmHg.

For a dry oxygen fraction entering a humidified gas mixture, the inspired oxygen pressure can be writtenwhere $P_B$ is total barometric pressure and $P_{{\mathrm{H}}_2\mathrm{O}}$ the water-vapor contribution. The primary technical account explicitly includes this distinction.^[ref] Using stipulated $P_B=760$ mmHg, $P_{{\mathrm{H}}_2\mathrm{O}}=47$ mmHg and $F_{\mathrm{I}{O}_2}=0.21$ gives about 149.7 mmHg. Changing the barometric pressure changes this partial pressure without changing the dry fraction.

An original ideal blending calculation uses two streams compared at equal temperature and pressure. If pure oxygen flows at $Q_O$ and air with oxygen fraction 0.21 flows at $Q_A$, conservation givesStipulated flows 2 and 8 L/min give $F_{\mathrm{mix}}=0.368$. This describes the closed two-stream model. Additional entrained air, leakage or unequal gas conditions changes the relation, so a source-flow setting alone need not establish the fraction actually inspired.

The inspired oxygen fraction specifies an input to gas exchange. Arterial oxygen partial pressure is an output influenced by ventilation, perfusion and transfer between alveolar gas and blood. The same inspired fraction consequently need not give the same arterial measurement in different physical or physiological systems.