Within patient variability of lower extremity muscle responses to transcranial electrical stimulation with pulse trains in aortic surgery

Intraoperative recording of myogenic motor responses evoked by transcranial electrical stimulation is a method of controlling the integrity of the motor pathways during clamping of the aorta. It is important to know the within patient variability of the transcranial motor evoked potential (tcMEP), before changes within the variability range are interpreted as abnormal during the period of aortic cross clamping. Lower limb muscle responses were obtained in 11 patients, following transcranial electrical stimulation with pulse trains, of 4, 6 and 8 pulses. Under the conditions of partial neuromuscular blockade and a stable low dose propofol/fentanyl/nitrous oxide anaesthetic state, this study shows that multipulse transcranial electrical stimulation reliably produces muscle responses of the lower limb in all patients tested with a coefficient of variation (CV) of around 20%. Eight pulses in the stimulation train produce neurophysiological facilitation that exceeds a 4 pulse train in terms of area under the curve (AUC) and response duration. The use of multipulse stimulation rather than double or single pulse stimulation is recommended in order to increase the clinical efficacy of tcMEP monitoring in aortic surgery.