Latency of the Cortical Component of the Somatosensory Evoked Potential in Relation to Cerebral Blood Flow Measured in the White Matter of the Cat Brain during Focal Ischemia

In a focal model of cerebral ischemia (middle cerebral artery (MCA) occlusion in the cat), the existence of a relationship between the latency from the onset of stimulation to the appearance of the cortical component of the somatosensory evoked response and blood flow was explored. White matter blood flow was determined in the left and right hemispheres under control conditions. During left MCA occlusion, the blood flow and the latency of the cortical component recorded from the right hemisphere deviated from their preexisting linear relationship obtained from plotting latency as a function of blood flow. In an effort to increase flow during focal ischemia, hemodilution and volume expansion with dextran 40 was used. Application of this technique only mildly increased blood flows and did not restore the correlation of latencies of the cortical component of the evoked response and blood flows observed before MCA occlusion. This finding suggests that latencies and blood flows are not correlated in the contralateral hemisphere during focal ischemia and only show a mild degree of relationship in the unperturbed brain. During ischemia, flow and electrical activities may not respond to the same stimuli and, if they do respond, it may not be unidirectionally. These findings are discussed in light of other reports on the Significance of the evoked response in monitoring cerebrovascular insults and the observation of diaschisis.