Brain Tissue Oxygen Response as Indicator for Cerebral Lactate Levels in Aneurysmal Subarachnoid Hemorrhage Patients

Background: Early detection of cerebral ischemia and metabolic crisis is crucial in critically ill subarachnoid hemorrhage (SAH) patients. Variable increases in brain tissue oxygen tension (PbtO(2)) are observed when the fraction of inspired oxygen (FiO(2)) is increased to 1.0. The aim of this prospective study was to evaluate whether a 3-minute hyperoxic challenge can identify patients at risk for cerebral ischemia detected by cerebral microdialysis. Methods: Twenty consecutive severe SAH patients undergoing continuous cerebral PbtO(2) and microdialysis monitoring were included. FiO(2) was increased to 1.0 for 3 minutes (the FiO(2) challenge) twice a day and PbtO(2) responses during the FiO(2) challenges were related to cerebral microdialysis-measures, ie, lactate, the lactate-pyruvate ratio, and glycerol. Multivariable linear and logistic regression models were created for each outcome parameter. Results: After predefined exclusions, 274 of 400 FiO(2) challenges were included in the analysis. Lower absolute increases in PbtO(2) ( increment PbtO(2)) during FiO(2) challenges were significantly associated with higher cerebral lactate concentration (P<0.001), and patients were at higher risk for ischemic lactate levels >4 mmol/L (odds ratio 0.947; P=0.04). Median (interquartile range) increment PbtO(2) was 7.1 (4.6 to 12.17) mm Hg when cerebral lactate was >4 mmol/L and 10.2 (15.76 to 14.24) mm Hg at normal lactate values (<= 4 mmol/L). Median increment PbtO(2) was significantly lower during hypoxic than during hyperglycolytic lactate elevations (4.6 vs. 10.6 mm Hg, respectively; P<0.001). Lactate-pyruvate ratio and glycerol levels were mainly determined by baseline characteristics. Conclusions: A 3-minute FiO(2) challenge is an easy to perform and feasible bedside diagnostic tool in SAH patients. The absolute increase in PbtO(2) during the FiO(2) challenge might be a useful surrogate marker to estimate cerebral lactate concentrations and might be used to identify patients at risk for impending ischemia.