 The PaO2FIO2 can be easily manipulated. Alterations in PEEP and FIO2 can dramatically change the PaO2FIO2. Despite recent reports on the effects of standardized ventilator settings on PaO2FIO2 and fulfillment of AECC definitions of ARDS, it is still a matter of debate whether the assessment of hypoxemia at 24 hours is the most appropriate tool for stratifying lung severity in patients with ARDS. 