Some Effects of Isoflurane on I Waves of the Motor Evoked Potential

Abstract

We have investigated the effects of isoflurane anaesthesia on the motor evoked potential recorded in the extradural space during corrective spinal surgery in 15 patients. Isoflurane was added to a nitrous oxide in oxygen mixture supplemented with fentanyl and a neuromuscular blocking agent. Isoflurane was administered to achieve end-tidal concentrations of 2%, 1% and 0% in all patients, and also of 1.5% and 0.5% in nine patients. Transcranial electrical stimulation of the motor cortex was used to elicit descending volleys in corticospinal axons (the motor evoked potential). With stimuli of 450-750 V and no isoflurane, multiple I waves were always seen following the D wave. In all patients the number of I waves decreased and individual I waves became smaller in amplitude the greater the isoflurane concentration, but there were only minor changes in the D wave. The greatest depressant effect on I waves occurred at an end-tidal concentration of 0.5%. Given that I waves are an index of synaptic transmission, anaesthetic-induced changes in I waves may provide a useful model for the neuronal events underlying anaesthesia-induced unconsciousness.

Depressive effect of isoflurane on motor evoked potentials in the Nubian goat

“…[6][7][8][9][10][11]13 But the range of concentrations differs remarkably: 0.25% -1% studied Yamada et al, 13 while Schmidt et al 9 tested from 0.5 to 3%, the others remained between these values. 6,7,[9][10][11] In contrast, Toda 1 2 investigated a single concentration, an approach similar to our study, in which 2% were used throughout the experiment. C) Four authors added nitrous oxide to isoflurane at concentrations > 50%.…”

“…2,4,5 Since the goat has never been used in MEP experiments, the choice of an appropriate anesthetic agent was required to perform this study. Isoflurane suppresses MEP in humans, [6][7][8] rats, 9 , 1 0 rabbits 1 1 and cats. 12,13 However, methods and results differed to a large extent in these studies.…”

“…12,13 However, methods and results differed to a large extent in these studies. [6][7][8][9][10][11][12][13] Therefore, we designed this animal experiment to investigate how the agent would act in a new animal model and to compare the findings obtained with results of similar studies.…”

“…We did not examine the effect of incremental concentration increases, much of which has all ready been reported. [6][7][8][9][10][11]13 Nitrous oxide and fentanyl were used in combination with isoflurane, and care was taken to exclude their additional depressive effects by keeping their concentrations below critical values. 15,16 Our results confirmed the findings of similar studies: as with the other reports, 6-13 we observed dose-and time-dependent effects on amplitudes and latencies of MEP under isoflurane anesthesia.…”

“…[6][7][8][9][10][11][12][13] These differences consist of: A) the model (several species, humans), B) the different concentrations of isoflurane, C) the use of nitrous oxide in concentrations which contribute to the depressive effect of isoflurane, D) the stimulation method and stimulation site (magnetic or electrical, transcranial or directly after craniotomy), E) the recording site (spinal cord, muscles) and, as a result, F) the large variety of the described results. [6][7][8][9][10][11][12][13] (Table II) A) The choice of a convenient model is mostly a matter of availability and price. In our case, however, the special anatomy of the cervical vertebral column 5 led us to use the goat because no other animal model commonly used in medical experiments enables such non-traumatic exposure of spinal roots.…”