Changes of motor evoked potentials in global and focal ischemic models of cats

In order to evaluate the significance of motor evoked potentials (MEPs) in central nervous system monitoring, the authors conducted two sets of experiments using feline ischemic models. Twenty-three adult mongrel cats were divided into two groups: global (n = 9) and focal ischemic (n = 14) groups. In the case of global ischemia, which was induced by hypovolemic hypotension due to blood letting, deterioration of the D wave began when the mean arterial blood pressure (MABP) approached 45 mmHg, and regional cerebral blood flow (rCBF) dropped to 40% of control. Complete disappearance of the D wave was observed below 30 mmHg in MABP and 20% of control in rCBF. In the case of focal ischemia, which was induced by transorbital occlusion of the middle cerebral artery, the percentages of rCBF at which the D wave disappeared ranged from 9 to 20%. Changes in the amplitude of the D wave--an increase and following decrease--preceded the prolongation of its latency. In contrast with the D wave I waves were too easily affected by ischemia. Moreover, rCBF at the point of disappearance of the I wave varied greatly. In conclusion, the D wave is stable in mild ischemia and is a reliable indicator of critically profound ischemia (%rCBF < 40%). Monitoring the D wave of MEPs seems to be a useful method for avoiding the deterioration of motor function by ischemic insult.