Approach: We will conduct a multicenter interventional study (FiO2 optimization protocol) of adult patients resuscitated from cardiac arrest. We will record data pertaining to oxygenation parameters and other factors and measure biomarkers of oxidative stress [isoprostanes (IsoPs) and isofurans (IsoFs)] in the plasma at 0 and 6 hours after ROSC using gas chromatography negative ion chemical ionization mass spectrometry. We will compare plasma IsoPs/IsoFs at each time point between subjects enrolled in REOX II (i.e. receive the study intervention, rapid FiO2 optimization) and REOX I (i.e. do not receive the study intervention) using t-test or Mann-Whitney U as appropriate with corrections for multiple comparisons.