The Effects of the Neuromuscular Blockade Levels on Amplitudes of Posttetanic Motor-Evoked Potentials and Movement in Response to Transcranial Stimulation in Patients Receiving Propofol and Fentanyl Anesthesia

BACKGROUND: Patient movement in response to transcranial stimulation during monitoring of myogenic motor-evoked potentials (MEPs) may interfere with surgery. We recently reported a new technique to augment the amplitudes of myogenic MEPs, called “post-tetanic MEPs (p-MEPs),” in which tetanic stimulation of a peripheral nerve was applied prior to transcranial stimulation. We conducted the present study to determine an appropriate level of neuromuscular blockade during the monitoring of p-MEPs with a focus on patient movement. METHODS: In 15 patients under propofol/fentanyl anesthesia, conventional MEPs (c-MEPs) and p-MEPs in response to transcranial electrical stimulation were recorded from the abductor hallucis muscle. For p-MEP recording, tetanic stimulation to the posterior tibial nerve at an intensity of 50 mA for 5 s was started 6 s prior to transcranial stimulation. The level of neuromuscular blockade was assessed by recording the amplitude of compound muscle action potentials (T1) from the abductor hallucis brevis muscle in response to supramaximal electrical stimulation of the median nerve at the wrist. After the baseline recordings of c-MEP and p-MEP at a T1 of 50% of control, 0.1 mg/kg of vecuronium was injected and the amplitudes of c-MEPs and p-MEPs were recorded. Patient movement was also assessed with the movement score ranging from 1 to 4 (1 = no movement, 4 = severe movement). RESULTS: T1, %T1, the amplitudes of c-MEPs and p-MEPs, and the movement score changed in parallel after the administration of vecuronium. The amplitudes of p-MEPs before and 15–45 min after the administration of vecuronium were significantly higher than those of c-MEPs. When T1 and %T1 were less than and equal to 1 mV and 10%, respectively, the movement score was 1 or 2 in all patients, indicating that microscopic surgery was possible without the interruption of surgical procedures. When T1 was around 1 mV (0.8–1.2 mV), the success rates of recording of c-MEPs and p-MEPs were 73% (11 of 15) and 100% (15 of 15), respectively. CONCLUSIONS: Under propofol/fentanyl anesthesia, p-MEP could be recorded at a T1 of 1 mV, in which patient movement in response to transcranial stimulation did not interfere with surgery. This technique may be used in patients without preoperative motor deficits, in which patient movement during surgical procedures is not preferable.