Motor-evoked potentials (MEPs) vary in size from one stimulus to the next. The objective of this study was to determine the cause and source of trial-to-trial MEP size variability. In two experiments involving 10 and 14 subjects, the variability of MEPs to cortical stimulation (cortical-MEPs) in abductor digiti minimi (ADM) and abductor hallucis (AH) was compared to those responses obtained using the triple stimulation technique (cortical-TST). The TST eliminates the effects of motor neuron (MN) response desynchronization and of repetitive MN discharges. Submaximal stimuli were used in both techniques. In six subjects, cortical-MEP variability was compared to that of brainstem-MEP and brainstem-TST. Variability was greater for MEPs than that for TST responses, by approximately one-third. The variability was the same for cortical- and brainstem-MEPs and was similar in ADM and AH. Variability concerned at least 10-15% of the MN pool innervating the target muscle. With the stimulation parameters used, repetitive MN discharges did not influence variability. For submaximal stimuli, approximately two-third of the observed MEP size variability is caused by the variable number of recruited alpha-MNs and approximately one-third by changing synchronization of MN discharges. The source of variability is most likely localized at the spinal segmental level.

SummaryThe analgesic effect of subanaesthetic concentrations of ether, trichloroethylene, methoxyflurane and halothane has been investigated previously using either clinical assessment or pain threshold measurements, but with conflicting results. The purpose of the present study was to evaluate the analgesic effect of isoflurane using experimental pain models. We studied 12 healthy volunteers at three randomly chosen subanaesthetic isoflurane concentrations: low (0.10-0.14 vol %), middle (0.16-0.20 vol %) and high (0.22-0.26 vol %). We used thermal pain detection and pain tolerance thresholds to argon laser stimulation, pressure pain detection and pain tolerance thresholds, immersion of the hand in ice water, and the nociceptive reflex to single and repeated (temporal summation) electrical stimulations, as experimental models to assess analgesia. There were no significant changes in the response to heat, cold or mechanical pressure at any of the subanaesthetic concentrations of isoflurane used. The nociceptive reflex thresholds to single stimulations, but not the thresholds for repeated stimulations, were significantly increased in all three isoflurane groups compared with baseline values. The difference between the different isoflurane concentrations was not statistically significant. In experimental pain models, subanaesthetic isoflurane concentrations have little or no analgesic potency. (Br. J. Anaesth. 1995; 75: 55-60)

SummaryThe aim of this study was to see if an analgesic effect of subanaesthetic concentrations of isoflurane could be detected with evoked potentials elicited by nociceptive stimuli. We studied 10 healthy volunteers breathing three steady-state subanaesthetic concentrations of isoflurane (0.08, 0.16 and 0.24 vol% end-tidal). Reaction time, subjective pain intensities and evoked vertex potentials to laser (LEP) and electrical (SEP) stimuli were recorded and compared with auditory evoked potentials (AEP). Compared with baseline, the subanaesthetic concentrations of isoflurane did not change the latencies of the evoked potentials, but caused a significant reduction in the amplitudes of the LEP and SEP at 0.16 and 0.24 vol% and of the AEP at all three concentrations. There were no changes in perceived pain intensity, and isoflurane produced similar reductions in evoked potentials elicited by both nociceptive and non-nociceptive stimuli. The reaction time was increased significantly at 0.24 vol% isoflurane. The results demonstated that subanaesthetic isoflurane concentrations caused similar changes in evoked potentials with both painful and non-painful stimuli, with no effect on perceived pain intensity. (Br.