Measurement of motor evoked potentials following repetitive magnetic motor cortex stimulation during isoflurane or propofol anaesthesia

Background. Isoflurane and propofol reduce the recordabilityof compound muscle action potentials (CMAP) following singletranscranial magnetic stimulation of the motor cortex (sTCMS).Repetition of the magnetic stimulus (repetitive transcranialmagnetic stimulation, rTCMS) might allow the inhibition causedby anaesthesia with isoflurane or propofol to be overcome. Methods. We applied rTCMS (four stimuli; inter-stimulus intervalsof 3, 4, 5 ms (333, 250, 200 Hz), output 2.5 Tesla) in 27 patientsand recorded CMAP from the hypothenar and anterior tibial muscle.Anaesthesia was maintained with fentanyl 0.5–1 µgkg^–1 h^–1 and either isoflurane 1.2% (10 patients)or propofol 5 mg kg^–1 h^–1 with nitrous oxide 60%in oxygen (17 patients). Results. No CMAP were detected during isoflurane anaesthesia.During propofol anaesthesia 333 Hz, four-pulse magnetic stimulationevoked CMAP in the hypothenar muscle in 75%, and in the anteriortibial muscle in 65% of the patients. Less response was obtainedwith 250 and 200 Hz stimulation. Conclusions. In most patients, rTCMS can overcome suppressionof CMAP during propofol/nitrous oxide anaesthesia, but not duringisoflurane anaesthesia. A train of four magnetic stimuli ata frequency of 333 Hz is most effective in evoking potentialsfrom the upper and lower limb muscles. The authors concludethat rTCMS can be used for evaluation of the descending motorpathways during anaesthesia. Br J Anaesth 2003; 91: 487–92