The broad objective of this study is to test the association between hyperoxia exposure after resuscitation from cardiac arrest and outcome. After obtaining written informed consent subjects enrolled in REOX II will undergo a rapid faction of inspired oxygen (FiO2) optimization protocol to prevent exposure to hyperoxia. We will compare outcomes between subjects enrolled in REOX I (observational study only) and REOX II (intervention: rapid FiO2 optimization protocol). Our overarching hypothesis is that exposure to hyperoxia after return of spontaneous circulation (ROSC) is associated with increased oxidative stress and worsened neurological and cognitive outcomes.

Specific Aim 1: Test if initiation of the rapid FiO2 optimization protocol following ROSC from cardiac arrest is associated with the degree of in vivo oxidative stress during the post-resuscitation phase of therapy.

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.

Specific Aim 2: Test if initiation of the rapid FiO2 optimization protocol following ROSC from cardiac arrest is associated with a decrease in neurological disability at hospital discharge.

Approach: In the study described above, we will determine the Modified Rankin Scale (mRS) at hospital discharge. We will compare proportions of good neurological outcome [defined as a mRS ≤ 3] between subjects enrolled in REOX II (i.e. receive the study intervention, rapid FiO2 optimization) vs. those enrolled in REOX I (i.e. do not receive the study intervention), using binomial test.

Specific Aim 3: Test if initiation of the rapid FiO2 optimization protocol following ROSC from cardiac arrest is associated with neuropsychological outcomes among survivors at 180 days.

Approach: In the study described above, we will assess neuropsychological outcome among survivors at 180 days. Neuropsychological testing will use validated instruments across five cognitive domains (attention, Wechsler Adult Intelligence Scale-IV-digit span; (2) reasoning, Wechsler Adult Intelligence Scale-IV-similarities; (3) immediate and delayed memory, Wechsler Memory Scale-III-logical memory I and II; (4) verbal fluency, Controlled Oral Word Association Test; and (5) executive functioning, Hayling Sentence Completion Test). Among survivors, we will compare the 180-day neuropsychological measures (composite z-scores for each cognitive domain) between the same two groups using t-test or Mann-Whitney U as appropriate with corrections for multiple comparisons. We will also compare the proportions of patients able to return to work between the two groups using binomial test

Inclusion Criteria:

• Age >17 years
• Cardiac arrest
• Return of spontaneous circulation
• Not following commands immediately after ROSC
• Endotracheal intubation
• Clinician intent to treat with therapeutic hypothermia (or absence of clinician intent to withhold therapeutic hypothermia)

Exclusion Criteria:

• Presumed etiology of arrest is trauma
• Presumed etiology of arrest is hemorrhage
• Presumed etiology of arrest is sepsis
• Permanent resident of nursing home or other long-term care facility
• Any other condition, that in the opinion of the investigator, would preclude the subject from being a suitable candidate, e.g. end stage chronic illness with no reasonable expectation of survival to hospital discharge

1.5 years