Individualized Thresholds of Hypoxemia and Hyperoxemia and their Effect on Outcome in Acute Brain Injured Patients: A Secondary Analysis of the ENIO Study

Background: In acute brain injury (ABI), the effects of hypoxemia as a potential cause of secondary brain damage and poor outcome are well documented, whereas the impact of hyperoxemia is unclear. The primary aim of this study was to assess the episodes of hypoxemia and hyperoxemia in patients with ABI during the intensive care unit (ICU) stay and to determine their association with in-hospital mortality. The secondary aim was to identify the optimal thresholds of arterial partial pressure of oxygen (PaO₂) predicting in-hospital mortality.

Methods: We conducted a secondary analysis of a prospective multicenter observational cohort study. Adult patients with ABI (traumatic brain injury, subarachnoid aneurysmal hemorrhage, intracranial hemorrhage, ischemic stroke) with available data on PaO₂ during the ICU stay were included. Hypoxemia was defined as PaO₂ < 80 mm Hg, normoxemia was defined as PaO₂ between 80 and 120 mm Hg, mild/moderate hyperoxemia was defined as PaO₂ between 121 and 299 mm Hg, and severe hyperoxemia was defined as PaO₂ levels ≥ 300 mm Hg.

Results: A total of 1,407 patients were included in this study. The mean age was 52 (±18) years, and 929 (66%) were male. Over the ICU stay, the fractions of patients in the study cohort who had at least one episode of hypoxemia, mild/moderate hyperoxemia, and severe hyperoxemia were 31.3%, 53.0%, and 1.7%, respectively. PaO₂ values below 92 mm Hg and above 156 mm Hg were associated with an increased probability of in-hospital mortality. Differences were observed among subgroups of patients with ABI, with consistent effects only seen in patients without traumatic brain injury.

Conclusions: In patients with ABI, hypoxemia and mild/moderate hyperoxemia were relatively frequent. Hypoxemia and hyperoxemia during ICU stay may influence in-hospital mortality. However, the small number of oxygen values collected represents a major limitation of the study.