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.

Motor-evoked potential monitoring for intramedullary spinal cord tumor surgery: correlation of clinical and neurophysiological data in a series of 100 consecutive procedures

“…[12,14,16,30,32] It is our experience that this drug together with narcotic agents is required when reliable MEP monitoring is attempted. The fact that no muscle relaxants are used, however, has been subject to considerable criticism in many discussions.…”

Intraoperative monitoring of blood flow insufficiency in the anterior choroidal artery during aneurysm surgery

“…Transcranial electrostimulation 18,25 and magnetic stimulation 24,25 have been used to stimulate the brain; however, these methods are not suitable for aneurysm surgery. The area activated by transcranial electrostimulation remains uncertain.…”

“…If the stimulation reaches the medulla oblongata, this method cannot be used to detect deficits in either the motor cortex or the internal capsule. Transcranial magnetic stimulation is also unsuitable, because it is difficult to achieve constant stimulation of the motor cortex 24 and because the stimulated area remains uncertain. In addition, the effect of metal clips on magnetic stimulation cannot be ignored.…”

“…Induced by transcranial magnetic stimulation, the MEPs were abolished in 43 to 86% of patients during induction of anesthesia with propofol, etomidate, methohexital, or thiopental. 24 Monitoring of MEPs in patients in a state of anesthesia is hampered by the conduction block of lower motor neurons by anesthetic agents. Anesthetic gases, such as nitrous oxide, halothane, and isoflurane, block conduction at the synapse in the anterior horn of the spinal cord.…”

Does ketamine affect intraoperative electrophysiological monitoring in children undergoing selective posterior rhizotomy?

“…Maintenance of anesthesia was achieved with sufentanil (0.3–0.6 mcg·kg·h −1 ) and propofol (4–10 mg·kg·h −1 ) infusions, and with N 2 O. Because propofol appears to depress motor evoked responses in a dose‐response fashion (11,12) continuous infusion of low dose propofol (4–6 mg·kg·h −1 ) was used during the stimulation phase. No muscle relaxants or potent volatile anesthetics were used during the maintenance phase.…”