Demographic data (such as age, sex, and presence of comorbidities), as well as clinical scores on admission (such as Charlson Comorbidity Index score, Sequential Organ Failure Assessment score on day 1, and the Simplified Acute Physiology Score III), were collected. Continuous variables were described either as median and interquartile range or mean and standard deviation. Because of the small number of patients and the asymmetrical distribution of variables, we used the Friedman nonparametric test for comparison between repeated measurements before, during, and after the hyperoxia challenge. Characteristics of the study population N = 29 Age, mean (± SD) 44 (± 14) Male sex, n (%) 20 (69) Comorbidities, n (%) Heart disease 2 (7) Arterial hypertension 13 (45) Diabetes 3 (10) Lung disease 2 (7) Liver cirrhosis 4 (14) Immunosuppression 2 (7) Cancer 2 (7) Previous neurological disease 1 (3) Corticosteroids 1 (3) Alcohol 15 (52) Charlson comorbidity index, median (IQR) 0 (0–2) SAPS, median (IQR) 58 (53–70) GCS score on admission, median (IQR) 8 (5–13) SOFA score day 1, median (IQR) 7 (4–10) Seizure on admission, n (%) 8 (28) Diagnosis and severity on admission Subarachnoid hemorrhage, n (%) 11 (38) WFNS score of 4 or 5, n (%) 7/11 (64) Traumatic brain injury, n (%) 13 (45) Marshall score, median (IQR) 3 (2–4) Intracerebral hemorrhage, n (%) 5 (17) NIHSS score, median (IQR) 22 (18–22) During the ICU stay, n (%) Infection 19 (66) Hyponatremia 7 (24) SIADH 3 (10) Vasopressor agents 26 (90) Inotropic agents 8 (28) Intracranial hypertension 23 (79) Outcomes GOS score at discharge, median (IQR) 3 (2–3) ICU length of stay, mean (± SD) 24 (± 9) Hospital length of stay, median (IQR) 37 (26–50) ICU mortality, n (%) 7 (24) Hospital mortality, n (%) 7 (24) GCS Glasgow coma scale, GOS Glasgow outcome scale, ICU Intensive care unit, IQR Interquartile range, NIHSS National institutes of health stroke scale, SAPS Simplified acute physiological score, SD Standard deviation, SIAHD Syndrome of inappropriate secretion of antidiuretic hormone, SOFA Sequential organ dysfunction assessment, WFNS World federation of neurological surgeons Table 2. Physiological variables assessed at different time points (T0 = baseline, T1 = after 30 min of hyperoxia, T2 = after 30 min of returning FiO2 at baseline values) T0 T1 T2 p value Systolic blood pressure (mm Hg) 154 (139–181) 158 (145–176) 152 (133–168) 0.66 Diastolic blood pressure (mm Hg) 81 (70–87) 79 (72–91) 79 (67–83) 0.11 Median blood pressure (mm Hg) 107 (97–124) 105 (97–114) 101 (92–112) 0.18 Heart rate (bpm) 85 (68–107) 81 (67–106) 76 (68–102) 0.71 Cerebral perfusion pressure (mm Hg) 97 (82–107) 90 (82–103) 85 (79–101) 0.20 Intracranial pressure (mm Hg) 12