CORTICAL REACTIVITY DURING ISOFLURANE BURST-SUPPRESSION ANESTHESIA

We studied cortical reactivity to auditory, visual, and somatosensory stimuli during moderate and deep levels of isoflurane anesthesia at wh ich the electroencephalogram (EEG) showed burst suppression patterns, defined as alternating high amplitude bursts and periods of suppressed background activity. Fifteen patients scheduled for gynecologic surge ry were anesthetized with isoflurane until burst suppression appeared in the EEG. During steady state burst suppression at 1.5 end-tidal iso flurane concentration (ET(isof)), each patient was given a 5-min inter val each of episodes of visual, auditory, and somatosensory stimulatio n. During the 5-min interval of visual stimulation the patient was giv en 3-s episodes of 60 flashes, 4 ms duration each, at a 20-Hz frequenc y via redlight-emitting diode goggles. Corresponding auditory and soma tosensory stimulation consisted of 60 clicks (80 dB, 0.1 ms, 20 Hz) vi a earphones and 60 pulses to the median nerve at the wrist (20 mA, 0.2 ms, 20 Hz). The 3-s episodes of stimulation were given at irregular i ntervals ranging from 5 to 20 s. End-tidal isoflurane was then increas ed by 0.3 vol% and 15 min later the stimulation sequence was repeated. During anesthesia at 1.5 +/- 0.1 ET(isof) all stimulus modalities rea dily evoked bursts. One hundred percent of visual stimuli, 98% +/- 4% of somatosensory stimuli, and 94% +/- 9% of auditory stimuli, given du ring EEG suppression, evoked bursts. Somatosensory and visual stimulat ion evoked bursts at both onset and offset of the 3-s episodes of stim uli. The responses to auditory stimuli were related mainly to the endi ng of the 3-s episode of clicks. Stimulation decreased the duration of suppression and increased the total duration of bursts (P < 0.01). Du ring deeper anesthesia (1.8 +/- 0.1 ET(isof) 78% +/- 37% of visual sti muli, 72% +/- 31% of somatosensory stimuli, and 64% +/- 40% of auditor y stimuli evoked bursts. Deeper anesthesia was associated with longer suppressions, shorter bursts, and less reactivity (P < 0.05), and ther e was a predominance of stimulus offset-evoked bursts over onset-evoke d bursts. We conclude that burst suppression stimulation studies may b e useful in determining the sites and mechanisms of anesthetic action.