The modifying effects of stimulation pattern and propofol plasma concentration on motor-evoked potentials

The quality of intraoperative motor-evoked potentials (MEPs) largely depends on the stimulation pattern and anesthetic technique. Further improvement in intraoperative MEP recording requires exact knowledge of the modifying effects of each of these factors. Accordingly, we designed this study to characterize the modifying effect of different stimulation patterns during different propofol target plasma concentrations (PTPCs) on intraoperatively recorded transcranial electrical MEPs. In 12 patients undergoing craniotomy, stimulation patterns (300-500 V; 100-1000 Hz; 1-5 stimuli) were varied randomly at different PTPCs (2, 4, and 6 microg/mL). Remifentanil was administered unchanged at 0.2 microg . kg(-1) . min(-1). MEPs were recorded from the thenar and hypothenar muscles. Analysis of MEPs was blinded to the PTPC. Three-way analysis of variance revealed significant main effects of increasing stimulation intensity, frequency, and number of stimuli on MEP amplitude (P < 0.05). Maximum MEP amplitudes and recording success rates were observed with three or more stimuli delivered at 1000 Hz and > or =150 V. A significant main effect of PTPC (2 vs 4 and 6 microg/mL) on MEP amplitude was observed at the thenar recording site only (P < 0.05). An amplitude ratio calculated from corresponding MEPs evoked by double and quadruple stimulation proved to be insensitive to changes in PTPC. In conclusion, MEP characteristics varied significantly in response to changes in stimulation pattern and less to changes in PTPC.