Within-patient variability of myogenic motor-evoked potentials to multipulse transcranial electrical stimulation during two levels of partial neuromuscular blockade in aortic surgery

Intraoperative recording of myogenic motor responses evoked by transcranial electrical stimulation (tcMEPs) is a method of assessing the integrity of the motor pathways during aortic surgery. To identify conditions for optima l spinal cord monitoring, we investigated the effects of manipulating the l evel of neuromuscular blockade (T1 response of the train-of-four (TOF) stim ulation 5%-15% versus T1 response 45%-55% of baseline), as well as the numb er of transcranial pulses (two versus six stimuli) on the within-patient va riability and amplitude of tcMEPs. Ten patients (30-76 yr) scheduled to und ergo surgery on the thoracic and thoracoabdominal aorta were studied. After achieving a stable anesthetic state and before surgery, 10 tcMEPs were rec orded from the right extensor digitorum communis muscle and the right tibia lis anterior muscle in response to two-pulse and six-pulse transcranial ele ctrical stimulation with an interstimulus interval of 2 ms during two level s of neuromuscular blockade. The right thenar eminence was used for recordi ng the level of relaxation. The tcMEP amplitude using the six-pulse paradig m was larger (P < 0.01; leg and arm) compared with the amplitude evoked by two-pulse stimulation during both levels of relaxation. The within-patient variability, expressed as median coefficient of variation, was less when si x-pulse stimulation was used. At a TI response of 45%-55% of baseline, larg er, less variable tcMEPs were recorded than at a T1 response of 5%-15%. Our results suggest that the best quality of tcMEP signals (tibialis anterior muscle) is obtained when the six-pulse paradigm is used with a stable level of muscle relaxation (the first twitch of the TOF-thenar eminence-at 45%-5 5% of baseline). Implications: This study shows that six-pulse (rather than two-pulse) transcranial electrical stimulation during a stable anesthetic state and a stable neuromuscular blockade aimed at 45%-55% (rather than 5%- 15%) of baseline provides reliable and recordable muscle responses sufficie ntly robust for spinal cord monitoring in aortic surgery.