Effects of Four Intravenous Anesthetic Agents on Motor Evoked Potentials Elicited by Magnetic Transcranial Stimulation

Abstract

The influence of four intravenous anesthetic agents on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (magnetic MEP) was examined in 77 subjects. The patients were anesthetized by a continuous intravenous infusion of one of the following anesthetic agents: propofol, etomidate, methohexital, or thiopental. Comparable anesthetic effects among the four agents were achieved by computing an infusion scheme for each drug. Infusion rates were increased slowly in a step-wise manner in order to reach minimal anesthetic blood concentrations within 15 minutes. During anesthesia induction, magnetic MEPs were recorded every 2 minutes from the abductor pollicis brevis muscle. The patient's level of consciousness was assessed and documented in the alternating minutes. A dose-related reduction of the MEP amplitudes was seen in all drug groups, while the latencies remained constant. Reduction of the amplitude was occasionally so prominent that the MEP was completely abolished before adequate anesthesia was achieved. MEPs were obtainable at the end of anesthesia induction in 14% of the propofol group (n = 22), 57% of the etomidate group (n = 21), 53% of the methohexital group (n = 19), and 20% of the thiopental group (n = 15). Propofol and thiopental showed significantly stronger suppression of MEP, when compared to etomidate (both P < 0.01) and to methohexital (P = 0.01 and 0.05, respectively). Etomidate was the least detrimental anesthetic agent for intraoperative monitoring of magnetic MEP. Nonetheless, the low incidence of 57% of preserved MEP in subjects without motor deficits indicated the inadequacy of this technique for intraoperative monitoring. More effective transcranial stimulation techniques are required for successful intraoperative MEP monitoring.

Safety of single and repetitive focal transcranial magnetic stimuli as assessed by intracranial EEG recordings in patients with partial epilepsy

“…Several uses require the application of repetitive stimuli, such as MEP monitoring during orthopedic and neurosurgical operations [38]. Single transcranial magnetic stimuli during surgery are not sufficient for eliciting suprathreshold motor responses because anesthetic agents substantially suppress I waves induced by transcranial stimuli [20, 24,33,36,[38][39][40][41]. Therefore repetitive stimulation at high frequencies and intensities is used to obtain temporal summation of descending pyramidal tract volleys at spinal α-motoneurons and to achieve reliable MEP monitoring [18,35].…”

Intraoperative spinal cord monitoring of motor function with myogenic motor evoked potentials: a consideration in anesthesia

“…Propofol has been shown to be a potent suppressor of MEPs induced by electrical and magnetic stimulation with a single pulse [20,[36][37][38][39]. Kalkman et al [20] reported that sustained reduction of MEP amplitude occurred after a single dose of 2 mg·kg Ϫ1 propofol.…”

“…Kalkman et al [20] reported that sustained reduction of MEP amplitude occurred after a single dose of 2 mg·kg Ϫ1 propofol. Taniguchi et al [36] investigated the effects of propofol on MEPs induced by transcranial magnetic stimulation. They documented that the reduction of MEP amplitude was very large and MEPs were completely abolished before adequate anesthesia was achieved.…”

Neurophysiological basis of direct cortical stimulation and applied neuroanatomy of the motor cortex: a review

“…102 When a stimulus has been transmitted via the synapse, so-called EPSPs are registered in the postsynaptic membrane. 11 The EPSPs indicate the depolarization of the postsynaptic cell, which can last for up to 20 msec.…”