Approach: We will conduct a multicenter prospective observational study 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 and isofurans) in the plasma at 0 and 6 hours after ROSC using gas chromatography negative ion chemical ionization mass spectrometry. We will determine the exposure to post-ROSC hyperoxia by calculating the time-weighted average PaO2 and SaO2 for the post-resuscitation phase of therapy, and will test the association with plasma isoprostane and isofuran levels.