Motor-evoked potential facilitation during progressive cortical suppression by propofol

We characterized the effects of various stimulation patterns on motor-evoked potentials (MEPs) elicited by repetitive transcranial magnetoelectric stimulation at different levels of cortical suppression by propofol, In 20 patients undergoing lumbar disk surgery, propofol target plasma concentrations (PTPCs) were increased incrementally by target plasma-level controlled infusion during the induction of anesthesia. MEPs were recorded from the muscles of the upper extremities after single, double, and quadruple magnetoelectric stimulation at 500, 200, and 100 Hz. The mean PTPC during loss of responsiveness to verbal instructions (CP50) was 3 mug/mL (CP95 mug/mL). At PTPCs <3 mug/mL, maximal MEP amplitudes were elicited by quadruple stimulation at 100 Hz. At PTPCs greater than or equal to3 mug/mL, four pulses at 200 Hz yielded peak MEP amplitudes. Therefore, quadruple magnetoelectric stimulation at 100 Hz yields peak myogenic responses in awake patients. With progressive cortical suppression resulting from PTPCs beyond 3 mug/mL, the most effective stimulation frequency shifts to 200 Hz. This may be explained by a differential dose-dependent action of propofol on GABAergic cortical interneurons, corresponding to the clinically observed state of vigilance. Recording of spinal cord evoked potentials will aid in further elucidation of the modulatory effects of general anesthesia on intracortical facilitation.