Intraoperative monitoring of tibialis anterior muscle motor evoked responses to transcranial electrical stimulation during partial neuromuscular blockade

We studied the feasibility of recording motor evoked responses to transcranial electrical stimulation (tc(e)MERs) during partial neuromuscular blockade (NMB). In 11 patients, compound muscle action potentials were recorded from the tibialis anterior muscle in response to transcranial electrical stimulation during various levels of vecuronium-induced NMB. The level of NMB was assessed by accelerometry of the adductor pollicis muscle after train-of-four stimulation of the ulnar nerve. The compound muscle action potential was also recorded from the tibialis anterior muscle after direct stimulation of the peroneal nerve (M-response) as an alternative means of assessing the degree of NMB. In all patients, tc(e)-MERs could be recorded reliably during anesthesia with N₂O and a continuous infusion of sufentanil (0.5 μg·kg^-1·h^-1). An intact train-of-four was present in all patients, and the amplitude of the first twitch was recorded and designated as the control value. Before administration of vecuronium, the M-response amplitude was 9.6 ± 3.6 (mean ± SD) mV, and the tc(e)-MER amplitude was 1.21 ± 0.66 mV. Although administration of vecuronium (0.05 mg/kg) resulted in loss of the mechanical adductor pollicis response in 8 of the 11 patients, the M-response and the tc(e)-MER remained recordable. Subsequently, during an infusion of vecuronium, adjusted to maintain one or two mechanical responses to train-of-four stimulation, the average M-response to peroneal nerve stimulation was 5.2 ± 2.5 mV (53% of the control value), and tc(e)-MER amplitude was 0.59 ± 0.36 mV (59% of the control value). During incomplete relaxation, there was good correlation between the amplitude of the M-response and that of the tc(e)-MER (r = 0.75; P < 0.01). The ratio of tc(e)-MER amplitude to M-response amplitude remained constant, suggesting that it may be feasible to use the amplitude of the M-response to 'calibrate' the tc(e)-MER amplitude during vecuronium administration. The data indicate that tc(e)-MER monitoring is feasible during clinically effective levels of NMB.