Metabolic failure precedes intracranial pressure rises in traumatic brain injury: a microdialysis study

Background: Cerebral microdialysis (MD) is able to detect markers of tissue damage and cerebral ischaemia and can be used to monitor the biochemical changes subsequent to head injury. In this prospective, observational study we analysed the correlation between microdialysis markers of metabolic impairment and intracranial pressure (ICP) and investigated whether changes in biomarker concentration precede rises in ICP. Methods: MD and ICP monitoring was carried out in twenty-five patients with severe TBI in Neurointensive care. MD samples were analysed hourly for lactate:pyruvate (LP) ratio, glutamate and glycerol. Abnormal values of microdialysis variables in presence of normal ICP were used to calculate the risk of intracranial hypertension developing within the next 3h. Findings: An LP ratio > 25 and glycerol > 100 mu mol/L, but not glutamate > 12 mu mol/L, were associated with significantly higher risk of imminent intracranial hypertension (odds ratio: 9.8, CI 5.8-16.1; 2.2, CI 1.6-3.8; 1.7, CI 0.6-3, respectively). An abnormal LP ratio could predict an ICP rise above normal levels in 89% of cases, whereas glycerol and glutamate had a poorer predictive value. Conclusions: Changes in the compound concentrations in microdialysate are a useful tool to describe molecular events triggered by TBI. These changes can occur before the onset of intracranial hypertension, suggesting that biochemical impairment can be present before low cerebral perfusion pressure is detectable. This early warning could be exploited to expand the window for therapeutic intervention.