Một số nhà điều tra cho thấy một sự thay đổi đáng kể xảy ra khi cường độ kích thích kinh tế đã được tăng lên trong trường hợp để gợi ra cùng một phản ứng. Những người khác cho thấy một sự thay đổi đáng kể chỉ xảy ra khi phản ứng là hoàn toàn bị mất, bất kể cường độ kích thích. | Neurophysiologic Intraoperative Monitoring FIGURE . Intraoperative MEP monitoring during posterior spinal fusion for scoliosis showing stable responses in both upper extremities and the right lower extremity but a transient loss of the MEP response in the left lower extremity. Unlike BAEP and SEP there is disagreement as to what is a significant MEP change. Some investigators suggest that a significant change occurs when the stimulus intensity has to be increased during the case to elicit the same response. Others suggest a significant change occurs only when the response is completely lost regardless of the stimulation intensity. In the author s experience a significant response is one in which the response disappears completely or by at least 90 . In the example above at the start of the case MEP responses are noted in both upper first two columns of each graph thin arrows and lower last column in each graph thick arrows extremities. With distraction there was loss of the left lower extremity MEP dashed arrow . The surgeon was notified and the distraction was relaxed with return of the MEP dotted arrow . 249 CHAPTER 7 FIGURE . Intraoperative MEP monitoring in a patient undergoing spinal cord tumor biopsy showing the initial absence of the MEP responses in the lower extremities due to administration of neuromuscular blocking agents during induction and the subsequent return with drug cessation. MEPs are very sensitive to inhalational anesthetics and neuromuscular-blocking agents. When these drugs are given in boluses . during induction the affect on MEPs is striking. Maintaining low doses of both agents may be compatible with MEP monitoring. In the above example initially the upper extremity MEP first column were seen thin arrows but lower extremity responses were absent thick arrows during induction with neuromuscular- blocking agents. When further boluses of neuromuscular-blocking agents were not administered after a few minutes robust MEPs were seen
