Multiparametric continuous monitoring of brain metabolism and substrate delivery in neurosurgical patients.

Brain function and tissue integrity are highly dependent on continuous oxygen supply and clearance of CO2. Aerobic metabolism is the major energy source to normal brain, however, during hypoxia and ischemia, lactate accumulation may sometimes be seen, indicating anaerobic glycolysis after severe head injury. Current monitoring techniques often fail to detect such events which can affect substrate delivery to the injured brain. We have recently adapted a method for continuous monitoring of brain tissue pO2, pCO2, pH and temperature, using a single sensor. The multiparameter sensor is inserted into brain tissue, via a new three lumen bolt, together with a standard ventriculostomy catheter and a microdialysis probe. The system has been left in place as long as needed, but never more than 7 days. All readings were compared to clinical parameters, and outcome. Stable measurements could be obtained in the first group of 20 patients, after calibration and rigid fixation, using the new bolt. Severely head injured patients had brain oxygen levels of less than 25-30 mmHg for the first hours after injury. Thereafter two patterns could be seen. Patients with favorable outcome had a slow increase in brain oxygen, and brain CO2 decreased to normal values, as long as the cerebral perfusion pressure (CPP) was kept above 70 mmHg. However, in those patients with secondary ischemic events, and bad outcome, a further decline in brain oxygen to anaerobic levels (< 20 mmHg) was seen. For these patients, both decreased and increased brain CO2 levels could be seen. Brain CO2 levels of 90-150 mmHg were consistently seen after brain death. Brain pH was inversely related to brain CO2 for all patients. Brain glucose and lactate in patients with poor outcome were 639 microM l-1 +/- 330, and 1642 microM l-1 +/- 788, whereas patients with good outcome had brain glucose levels of 808 microM l-1 +/- 321 and lactate levels of 1001 microM l-1 +/- 417. Extended neuromonitoring using a combined sensor for brain oxygen, CO2, pH and temperature measurements, as well as a microdialysis probe for glucose and lactate analysis may optimize the management of comatose neurosurgical patients in the future, by allowing a fuller understanding of dynamic factors affecting brain metabolism.