What is the initial fraction of inspired oxygen (FIO2) in mechanical ventilation?

Updated: Sep 15, 2020
  • Author: Christopher D Jackson, MD; Chief Editor: Zab Mosenifar, MD, FACP, FCCP  more...
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The highest priority at the start of mechanical ventilation is providing effective oxygenation. For the patient's safety after intubation, the FIO2 should always be set at 100% until adequate arterial oxygenation is documented. A short period with an FIO2 of 100% is not dangerous to the patient receiving mechanical ventilation and offers the clinician several advantages. First, an FIO2 of 100% protects the patient against hypoxemia if unrecognized problems occur as a result of the intubation procedure. Second, using the PaO2 measured with an FIO2 of 100%, the clinician can easily calculate the next desired FIO2 and quickly estimate the shunt fraction. Nevertheless, FiO2 should be quickly titrated down to the minimal level required to maintain adequate oxygenation to avoid barotrauma.

The degree of shunt with 100% FIO2 can be estimated by applying this general rule: The measured PaO2 is subtracted from 700 mm Hg. For each difference of 100 mm Hg, the shunt is 5%. A shunt of 25% should prompt the clinician to consider the use of PEEP.

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