Midlatency auditory evoked potentials and motor signs of wakefulness during anaesthesia with midazolam

We have studied midlatency auditory evoked potentials (MLAEP) and motor signs of wakefulness during anaesthesia with midazolam in 10 patients undergoing elective laparotomy under continuous extradural analgesia. Anaesthesia was induced with midazolam 0.3 mg kg-1 and maintained with midazolam 0.3-0.9 mg kg-1 h-1. Motor signs of wakefulness were documented as spontaneous movements and movements after simple commands (open eyes or move arms). MLAEP were recorded continuously awake, and during anaesthesia until the end of anaesthesia. Latencies of the peaks V, Na, Pa, Nb and P1 (ms) and amplitudes of the peaks Na/Pa, Pa/Nb and Nb/P1 (microV) were measured. Twenty-five movements were observed during anaesthesia; 15 movements in six patients were in response to commands. In two patients supplementary isoflurane was given. Latencies of the MLAEP peaks Pa, Nb and P1 increased slightly during anaesthesia. Amplitudes for Na/Pa, Pa/Nb and Nb/P1 did not change significantly. The high incidence of motor signs of wakefulness associated with preserved MLAEP indicated a high level of cortical neural activity and none of the MLAEP variables predicted movement during anaesthesia with midazolam.