THE EFFECTS OF ISOFLURANE ON REPEATED NOCICEPTIVE STIMULI (CENTRAL TEMPORAL SUMMATION)

Central temporal summation of afferent nociceptive stimuli is involved in central hyperexcitability. This is assumed to bean important mecha nism in the nociceptive system which is probably activated during surg ery and trauma, The purpose of the present study was to investigate if isoflurane has a specific effect on central temporal summation in hum ans, Facilitation of the nociceptive reflex to repeated stimuli can be used to assess central summation in subjects unable to cooperate due to an anaesthetic procedure. The nociceptive reflex to single and repe ated (5 pulses delivered at 2 Hz) electrical surface stimuli of the su ral nerve were measured in 6 healthy volunteers anaesthetized with iso flurane, A reflex was defined as an EMG signal from the rectus and bic eps femoris exceeding 20 mu V for more than 10 msec in the 80-200 msec interval after the stimulus, The end-tidal isoflurane concentration w as increased in steps of 0.25 vol% from 0.25 to 1.50 vol%. For each co ncentration the thresholds for the nociceptive reflex were determined as the current intensity that could just elicit a reflex response to s ingle stimulations, and for the repeated stimulations as the current i ntensity that could just elicit a reflex response to the 4th and/or 5t h stimuli in the train of 5 stimuli. The nociceptive reflex to single stimuli was depressed at isoflurane concentrations producing sedation or light anaesthesia (0.25-0.50 vol% end-tidal), In contrast, 2-4-fold higher isoflurane concentrations (1.00-1.50 vol% end-tidal) that norm ally produce surgical anaesthesia were required to depress the nocicep tive reflex to repetitive stimuli. This indicates that central tempora l summation in the nociceptive system is a potent mechanism, and that isoflurane has a weak potency for depressing temporal summation in hum ans. As such isoflurane alone is not adequate for inhibiting surgicall y evoked hyperexcitability.