Correlation between Cerebral Blood Flow, Substrate Delivery, and Metabolism in Head Injury: A Combined Microdialysis and Triple Oxygen Positron Emission Tomography Study

Abstract

Summary:Microdialysis continuously monitors the chemistry of a small focal volume of the cerebral extracellular space. Conversely, positron emission tomography (PET) establishes metabolism of the whole brain, but only for the duration of the scan. The objective of this study was to apply both techniques to head-injured patients simultaneously to assess the relation between microdialysis (glucose, lactate, lactate/pyruvate [L/P] ratio, and glutamate) and PET (cerebral blood flow [CBF], cerebral blood volume, oxygen extraction fraction (OEF), and cerebral metabolic rate of oxygen) parameters. Microdialysis catheters were inserted into the frontal cerebral cortex and adipose tissue of the anterior abdominal wall of 17 severely headinjured patients. Microdialysis was performed during PET scans, with regions of interest defined by the location of the microdialysis catheter membrane. An intervention (hyperventilation) was performed in 13 patients. The results showed that combining PET and microdialysis to monitor metabolism in ventilated patients is feasible and safe, although logistically complex. There was a significant relation between the L/P ratio and the OEF (Spearman r ‫ס‬ 0.69, P ‫ס‬ 0.002). There was no significant relation between CBF and the microdialysis parameters. Moderate short-term hyperventilation appeared to be tolerated in terms of brain chemistry, although no areas were sampled by microdialysis where the OEF exceeded 70%. Hyperventilation causing a reduction of the arterial carbon dioxide tension by 0.9 kPa resulted in a significant elevation of the OEF, in association with a reduction in glucose, but no significant elevation in the L/P ratio or glutamate.

Lactate Shuttling and Lactate use as Fuel after Traumatic Brain Injury: Metabolic Considerations

“…Correspondence between local microdialysate metabolite levels and whole-brain arteriovenous measurements or regional positron emission tomographic metabolic assays of CMR glc or CMR O2 is difficult to evaluate owing to the small regions of interest surrounding a microdialysis probe and temporal relationships between data sets. 19,24,79 Critically low extracellular glucose levels are often associated with a non-ischemic/non-hypoxic metabolic crisis after TBI, but glucose and lactate levels are not correlated, nor are CMR glc and glucose levels; CMR glc was positively, linearly correlated with lactate and pyruvate levels in one study, 24 but not in another. 19 Arterial glucose is maintained within set ranges by critical care protocols, and the reason(s) for low brain glucose levels are not known; the TBI literature suggests hyperglycolysis and/or transport deficits.…”

Brain function in coma, vegetative state, and related disorders

“…[36][37][38][39] However, in patients with traumatic diffuse axonal injury both hyperglycolysis and metabolic depression have been reported. [40][41][42][43] In patients who recover from a postanoxic coma, cerebral metabolic rates for glucose are 75% of normal values. 44 Cerebral metabolism has been shown to correlate poorly with the level of consciousness, as measured by the Glasgow Coma Scale, in mild to severely head-injured patients studied within the first month following head trauma.…”

Metabolic Crisis without Brain Ischemia is Common after Traumatic Brain Injury: A Combined Microdialysis and Positron Emission Tomography Study

“…Microdialysis is a safe and effective monitoring technique that enables sampling of brain neurochemistry and inferential assessment of brain metabolism. Despite this growing body of knowledge (see also Hutchinson et al, 2002), the exact relationship between microdialysis markers of metabolism and independent measures of metabolism has yet to be clearly defined in various disease states. Hence, the validation of microdialysis markers of cellular metabolism in human subjects is of paramount priority to clinicians and researchers alike.…”