Minimal PaO2 threshold after traumatic brain injury and clinical utility of a novel brain oxygenation ratio

OBJECTIVE Avoiding decreases in brain tissue oxygenation (PbtO(2)) after traumatic brain injury (TBI) is important. How best to adjust PbtO(2) remains unclear. The authors investigated the association between partial pressure of oxygen (PaO2) and PbtO(2) to determine the minimal PaO2 required to maintain PbtO(2) above the hypoxic threshold (> 20 mm Hg), accounting for other determinants of PbtO(2) and repeated measurements in the same patient. They also explored the clinical utility of a novel concept, the brain oxygenation ratio (BOx ratio = PbtO(2)/PaO2) to detect overtreatment with the fraction of inspired oxygen (FiO(2)). METHODS A retrospective cohort study at an academic level 1 trauma center included 38 TBI patients who required the insertion of a monitor to measure PbtO(2). Various determinants of PbtO(2) were collected simultaneously whenever a routine arterial blood gas was drawn. A PbtO(2)/PaO2 ratio was calculated for each blood gas and plotted over time for each patient. All patients were managed according to a standardized clinical protocol. A mixed effects model was used to account for repeated measurements in the same patient. RESULTS A total of 1006 data points were collected. The lowest mean PaO2 observed to maintain PbtO(2) above the ischemic threshold was 94 mm Hg. Only PaO2 and cerebral perfusion pressure were predictive of PbtO(2) in multivariate analysis. The PbtO(2)/PaO2 ratio was below 0.15 in 41.7% of all measures and normal PbtO(2) values present despite an abnormal ratio in 27.1% of measurements. CONCLUSIONS The authors' results suggest that the minimal PaO2 target to ensure adequate cerebral oxygenation during the first few days after TBI should be higher than that suggested in the Brain Trauma Foundation guidelines. The use of a PbtO(2)/PaO2 ratio (BOx ratio) may be clinically useful and identifies abnormal O-2 delivery mechanisms (cerebral blood flow, diffusion, and cerebral metabolic rate of oxygen) despite normal PbtO(2).