In addition to these measurements, taken directly and indirectly from the patient, there is one vital piece of information that holds the key to interpretation of the results. The fractional inspired oxygen concentration (FiO₂) is the percentage of oxygen in the patient’s inspired air. If they are breathing room air, this is simply 21% (or 0.21).

If the patient is taking supplementary oxygen, the FiO₂ will increase proportionately with the inspired flow rate of oxygen. It is imperative you add this value and the mode of oxygen delivered (nasal cannula or mask) into the blood gas machine when analysing a sample as the inspired oxygen has a profound effect on the results.

This table provides the approximate Fractional Inspiration concentration of oxygen for each delivery modality and flow rate of oxygen.

The FiO₂ and modality with which oxygen is delivered must be recorded on the blood gas report. Without this observation, the results from a blood gas measurement are meaningless.