[Cerebral Blood Flow Monitoring and Motor Evoked Potentials]

Near infra-red spectroscopy (NIRS) and motor evoked potentials (MEP) have been used to monitor brain and spinal cord ischemia. NIRS calculates oxygen saturation of hemoglobin, based on the modified Beer-Lambert law. It correlates with the change in regional tissue blood flow. However, the technology is not matured enough for the measured value to be used as an index of tissue oxygenation, so that relative change should be carefully followed. Myogenic MEP has widely been used to monitor spinal cord ischemia, since the introduction of pulse train transcranial electrical stimulation. It evaluates motor pathways from the cortex to the muscle. Therefore it is influenced by non-spinal cord factors such as peripheral nerve ischemia. It is highly sensitive and shows changes in the early phase of spinal cord ischemia. On the other hand, its vulnerability to anesthesia requires special anesthetic consideration, and baseline amplitude fluctuation is common. Specificity is thus low, and the results should be interpreted together with the operative findings.