VARIABILITY OF MOTOR-EVOKED POTENTIALS RECORDED DURING NITROUS-OXIDE ANESTHESIA FROM THE TIBIALIS ANTERIOR MUSCLE AFTER TRANSCRANIAL ELECTRICAL-STIMULATION

When recorded as a compound muscle action potential (CMAP), the motor- evoked potential (MEP) is affected by volatile anesthetics and nitrous oxide. However, MEPs recorded using epidural electrodes in the presen ce of nitrous oxide are highly reproducible from trial to trial. We wi shed to establish the reproducibility over time of the CMAP produced b y supramaximal transcranial electrical stimulation of the human motor cortex. Cascades of 100 successive CMAPs were recorded from the tibial is anterior muscles of six anesthetized patients undergoing scoliosis surgery, in response to transcranial electrical stimuli of >500 V. Sat isfactory CMAPs could be recorded in the presence of nitrous oxide, bu t not isoflurane. Latencies and amplitudes were reproducible in repeat ed sequences of 100 responses. However, amplitude and, to a lesser ext ent, latency, were highly variable within a sequence. In addition, occ asional individual stimuli, although rarely successive ones, failed to evoke a CMAP. CMAPs have a much higher trial-to-trial variability tha n corticospinal volleys recorded from the epidural space. Using the pr esent methodology it would be difficult to rely on CMAP recordings as an indicator of corticospinal function in the clinical monitoring situ ation.