The FIO2 will then be varied in random steps to 0.35, 0.4, 0.45, 0.5, 0.55, 0.6 and 0.65. After changing the FIO2 a 3−4 minute equilibration period will be allowed prior to taking an arterial blood gas sample for analysis. In the event of a PaO2 < 8kPa being recorded, further reductions in FIO2 will not be undertaken.