Increased inspired oxygen concentration as a factor in improved brain tissue oxygenation and tissue lactate levels after severe human head injury

Abstract

Object. Early impairment of cerebral blood flow in patients with severe head injury correlates with poor brain tissue O 2 delivery and may be an important cause of ischemic brain damage. The purpose of this study was to measure cerebral tissue PO 2 , lactate, and glucose in patients after severe head injury to determine the effect of increased tissue O 2 achieved by increasing the fraction of inspired oxygen (FiO 2 ). Methods. In addition to standard monitoring of intracranial pressure and cerebral perfusion pressure, the authors continuously measured brain tissue PO 2 , PCO 2 , pH, and temperature in 22 patients with severe head injury. Microdialysis was performed to analyze lactate and glucose levels. In one cohort of 12 patients, the PaO 2 was increased to 441 ± 88 mm Hg over a period of 6 hours by raising the FiO 2 from 35 ± 5% to 100% in two stages. The results were analyzed and compared with the findings in a control cohort of 12 patients who received standard respiratory therapy (mean PaO 2 136.4 ± 22.1 mm Hg). The mean brain PO 2 levels increased in the O 2 -treated patients up to 359 ± 39% of the baseline level during the 6-hour FiO 2 enhancement period, whereas the mean dialysate lactate levels decreased by 40% (p < 0.05). During this O 2 enhancement period, glucose levels in brain tissue demonstrated a heterogeneous course. None of the monitored parameters in the control cohort showed significant variations during the entire observation period. Conclusions. Markedly elevated lactate levels in brain tissue are common after severe head injury. Increasing PaO 2 to higher levels than necessary to saturate hemoglobin, as performed in the O 2 -treated cohort, appears to improve the O 2 supply in brain tissue. During the early period after severe head injury, increased lactate levels in brain tissue were reduced by increasing FiO 2 . This may imply a shift to aerobic metabolism.