Propofol and Sevoflurane in Subanesthetic Concentrations Act Preferentially on the Spinal Cord: Evidence from Multimodal Electrophysiological Assessment

Background: Animal experiments in recent years have shown that attenuation of motor responses by general anesthetics is mediated at least partly by spinal mechanisms. Less is known about the relative potency of anesthetic drugs in suppressing cortical and spinal electrophysiological responses in vivo in humans, particularly those but not only those connected with motor responses. Therefore, we studied the effects of sevoflurane and propofol in humans using multimodal electrophysiological assessment. Methods: We studied 9 healthy volunteers in two sessions during steady-state sedation with either 0.5, 1.0, and 1.5 µg/l (targeted plasma concentration) propofol or 0.2 and 0.4 vol (end-tidal) sevoflurane. Following a 15 min equilibration period, motor responses to transcranial magnetic stimulation (TMS) and peripheral (H-reflex, F-wave) stimulation were recorded, while electroencephalography and auditory evoked responses were recorded in parallel. Results: At concentrations corresponding to 2/3 of C50awake motor responses to TMS were reduced by about 50, H-reflex amplitude by 22, F-wave amplitude by 40, and F-wave persistence by 25. No significant differences between sevoflurane and propofol were found. At this concentration the bispectral index was reduced by 7 and the middle-latency auditory evoked responses were attenuated only mildly (Nb latency increased by 11, amplitude PaNb did not change). In contrast, the postauricular reflex was suppressed by 77. Conclusions: The large effect of both anesthetics on all spinal motor responses, compared to the small effect on electroencephalography and middle-latency auditory evoked responses – assuming that they represent cortical modulation, may suggest that the suppression of motor responses to TMS is largely due to submesencephalic effects