Effect of isoflurane plus nitrous oxide versus propofol on intraoperative motor evoked potentials after high frequency repetitive electrical stimulation during neurosurgical procedures

ABSTRACT

The goal of this study was to test the influence of two widespread techniques of general anaesthesia on motor evoked potentials [MEP] in response to transcranial and cortical high frequency repetitive electrical stimulation. Total intravenous anaesthesia [TIVA] based on propofol and alfentanil was studied in 17 patients [Group A] and balanced anaesthesia [BA] based on nitrous oxide, isoflurane and alfentanil was studied in 13 patients [Group B]. Distinct motor responses were available in 15 of 17 patients [88%]] of [Group A] and in one of 13 patients [8%] of [Group B]. Amplitudes increased significantly with increasing stimulus intensity and number of pulses under conditions of TIVA. At the same time, latencies decreased significantly with increasing stimulus intensity and decreasing inter-stimulus intensity interval, but not with increasing number of pulses. It is hypothesized that propofol suppresses corticospinal I-waves at the cortical level resulting in a conduction block at the level of the alpha-motor neuron; this effect may be overcome by high frequency repetitive stimulation. In contrast, nitrous oxide and isoflurane seem to have an additional suppressive effect on corticospinal D-waves which may be overcome by higher stimulation intensity. It was concluded that transcranial high frequency repetitive stimulation and TIVA provide a feasible setting for intraoperative MEP monitoring, while higher doses of nitrous oxide and isoflurane are not compatible with recording of muscular activity elicited by the stimulation technique as described