Background: This study examines the effect of an increase in the inspired fraction of oxygen (FiO2) on brain tissue oxygen (PbO2) in children with severe traumatic brain injury (TBI).
Methods: A prospective observational study of patients who underwent PbO2 monitoring and an oxygen challenge test (temporary increase of FiO2 for 15 min) was undertaken. Pre- and post-test values for arterial partial pressure of oxygen (PaO2), PbO2, and arterial oxygen content (CaO2) were examined while controlling for any changes in arterial carbon dioxide tension and cerebral perfusion pressure during the test. Baseline transcranial Doppler studies were done. Outcome was assessed at 6 months.
Results: A total of 43 tests were performed in 28 patients. In 35 tests in 24 patients, the PbO2 monitor was in normal-appearing white matter and in eight tests in four patients, the monitor was in a pericontusional location. When catheters were pericontusional or in normal white matter the baseline PbO2/PaO2 ratio was similar. PaO2 (P < 0.0001) and PbO2 (P < 0.0001) significantly increased when FiO2 was increased. The magnitude of the PbO2 response (PbO2) was correlated with PaO2 (P < 0.0001, R(2) = 0.37) and CaO2 (P = 0.001, R(2) = 0.23). The PbO2/PaO2 ratio (oxygen reactivity) varied between patients, was related to the baseline PbO2 (P = 0.001, r = 0.54) and was inversely related to outcome (P = 0.02, confidence interval 0.03-0.78).
Conclusion: Normobaric hyperoxia increases PbO2 in children with severe TBI, but the response is variable. The magnitude of this response is related to the change in PaO2 and the baseline PbO2. A greater response appears to be associated with worse outcome.