A metabolic threshold of irreversible ischemia demonstrated by PET in a middle cerebral artery occlusion-reperfusion primate model*

OBJECTIVE: to evaluate the predictive value of measurements of regional cerebral blood flow (CBF), oxygen metabolism (CMRO2) and oxygen extraction ratio (OER) for assessment of the fate of ischemic brain tissue. MATERIALS AND METHODS: Sequential PET measurements were performed during middle cerebral artery occlusion (MCAO; 2 h) and 12-24 h (mean 18 h) of reperfusion in a primate model (Macaca mulatta, n = 8). A penumbra region was delineated on the MCAO PET image (OER > 125% and CMRO2> or = 45% of the values observed in the contralateral hemisphere, respectively) and an infarction region was delineated on the last PET image (CMRO2 <45% of the values observed in the contralateral hemisphere). The penumbra regions delineated during MCAO and the infarction regions delineated at the final PET, were copied on to the images from all other PET sessions for measurements of CBF, CMRO2 and OER. Ratios were calculated by dividing the mean values obtained by the values of the corresponding contralateral region. RESULTS: Histopathology verified the adequacy of the criteria applied in the last PET for delineation of the infarction region. The penumbra region and infarction region were separated in all cases, except in two cases where a minimal overlap was seen. CBF and OER showed considerable variation over time and there was no consistent difference between the penumbra and infarction regions. CMRO2 showed a more stable pattern and the difference between penumbra and infarction regions was maintained from the time of MCAO throughout the entire reperfusion phase. With CMRO2 as predictor, all 50 observations could be correctly predicted as penumbra or infarction when using an optimal threshold ratio value estimated to be in the interval of 61% to 69% of the corresponding contralateral region. CBF and OER proved to have low power as predictors. CONCLUSIONS: The results indicate that CMRO2 is the best predictor of reversible or irreversible brain damage and the critical metabolic threshold level appears to be a reduction of oxygen metabolism to between 61% and 69% of the corresponding contralateral region.