Relationship between systemic glucose and cerebral glucose is preserved in patients with severe traumatic brain injury, but glucose delivery to the brain may become limited when oxidative metabolism is impaired: implications for glycemic control

OBJECTIVE: To clarify the dynamics of glucose delivery to the brain and the effects of changes in blood glucose after severe traumatic brain injury. DESIGN: Retrospective analysis of a prospective observational cohort study. SETTING: Neurosurgical intensive care unit of a university hospital. PATIENTS: Seventeen patients with acute traumatic brain injury monitored with cerebral and subcutaneous microdialysis. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: For continuous, accurate systemic monitoring, glucose was measured in the interstitial space of subcutaneous adipose tissue using microdialysis, and 39 specific episodes of spontaneous rises in glucose were identified. During these episodes, there was a significant positive linear relationship between systemic glucose levels and brain glucose concentrations measured by microdialysis (p , .0001). The basal lactate/pyruvate ratio, with a threshold of 25, was adopted to distinguish between disturbed and presumably preserved cerebral oxidative metabolism. Using normal vs. elevated lactate/pyruvate ratio as variable factor, the relationship between brain and systemic glucose during the episodes could be described by two significantly distinct parallel lines (p 5 .0001), which indicates a strong additive effect of subcutaneous glucose and lactate/pyruvate ratio in determining brain glucose. The line describing the relationship under disturbed metabolic conditions was lower than in presumably intact metabolic conditions, with a significant difference of 0.648 6 0.192 mM (p 5 .002). This let us to accurately predict that in this situation systemic glucose concentrations in the lower range of normality would result in critical brain glucose levels. CONCLUSIONS: The linear relationship between systemic and brain glucose in healthy subjects is preserved in traumatic brain-injured patients. As a consequence, in brain tissue where oxidative metabolism is disturbed, brain glucose concentrations might possibly drop below the critical threshold of 0.8 mM to 1.0 mM when there is a reduction in systemic glucose toward the lower limits of the “normal” range.