Brain tissue oxygenation and cerebral perfusion pressure thresholds of ischemia in a standardized pig brain death model.

BACKGROUND: Neurointensive care of traumatic brain injury (TBI) patients is currently based on intracranial pressure (ICP) and cerebral perfusion pressure (CPP) targeted protocols. Monitoring brain tissue oxygenation (BtipO2) is of considerable clinical interest, but the exact threshold level of ischemia has been difficult to establish due to the complexity of the clinical situation. The objective of this study was to use the Neurovent-PTO (NV) probe, and to define critical cerebral oxygenation- and CPP threshold levels of cerebral ischemia in a standardized brain death model caused by increasing the ICP in pig. Ischemia was defined by a severe increase of cerebral microdialysis (MD) lactate/pyruvate ratio (L/P ratio > 30).

METHODS: BtipO2, L/P ratio, Glucose, Glutamate, Glycerol and CPP were recorded using NV and MD probes during gradual increase of ICP by inflation of an epidural balloon catheter with saline until brain death was achieved.

RESULTS: Baseline level of BtipO2 was 22.9 ± 6.2 mmHg, the L/P ratio 17.7 ± 6.1 and CPP 73 ± 17 mmHg. BtipO2 and CPP decreased when intracranial volume was added. The L/P ratio increased above its ischemic levels, (>30)when CPP decreased below 30 mmHg and BtipO2 to <10 mmHg.

CONCLUSIONS: A severe increase of ICP leading to CPP below 30 mmHg and BtipO2 below 10 mmHg is associated with an increase of the L/P ratio, thus seems to be critical thresholds for cerebral ischemia under these conditions.