Question special

The initial PaO2/FiO2 ratio used as a stratification tool is great as the intervention such as proning was based on this ratio. However, why was the FiO2 itself not used as a stratification tool as well given that the ratio can vary drastically in a 12 hour time period between intubation and enrollment into a trial due to the quick titration of FiO2 after initial intubation? For instance, after intubation, I would have a patient's FiO2 at 100% and by the 12th hour, the FiO2 may have been titrated down to 60-80%.

Was there a standardized recording of the PaO2/FiO2 ratio such as taking the initial PaO2/FiO2 at intubation or 1-2 hours after initial intubation or was this ratio calculated at time of enrollement which can be random within the 12 hour enrollment window period?