Relationship between the cortical evoked potential and local cortical blood flow following acute middle cerebral artery occlusion in the baboon

In most experimental studies of the effects of ischemia on CNS electrical activity, the ischemia produced has been total. The present experiments, however, were designed to establish quantitatively the changes in electrical activity corresponding to different degrees of cerebral ischemia. The somatosensory evoked potential was measured at various sites on the exposed postcentral gyrus of the anesthetized baboon, and cortical blood flow was assessed in the region of the evoked potential electrode by the highly focal method of hydrogen clearance. The technique of middle cerebral artery occlusion was used to reduce blood flow over the hemisphere. Following occlusion of the artery, the amplitude of the evoked potential typically diminished steadily at a rate depending on the level of residual local blood flow. The rate of depression of the evoked potential amplitude (expressed in units of percent of control per minute) was highly and significantly correlated with the residual flow ( r = −0.95, P < 0.001), which indicated a linear relationship between these variables, the regression line intercepting the flow axis at 15.2 ml/100g/min. The data also strongly suggested a threshold-type relationship between the amplitude of the evoked potential and the local blood flow: If the flow was greater than about 16 ml/100g/min the evoked potential was not affected, but at flows less than about 12 ml/100g/min the evoked potential was abolished.