A MEASURE OF CONSCIOUSNESS AND MEMORY DURING ISOFLURANE ADMINISTRATION: THE COHERENT FREQUENCY †

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Abstract
The coherent frequency (CF) of the auditory evoked response (AER) is derived using auditory clicks presented at frequencies in the range 5–47 Hz. CF and psychological performance were measured while seven subjects breathed isoflurane in doses increasing from 0% to 0.2%, 0.4% and 0.8% endtidal concentration and then decreasing to 0%. With increasing doses of isoflurane, CF decreased and there was a decrease in within-list recognition (WLR) and category recognition (CR) scores. There was a correlation between changes in CF and WLR (P < 0.05) and between CF and category recognition (CR) (P < 0.05). A painful stimulus given in conjunction with 0.4% isoflurane caused an increase in CF, WLR and CR in some subjects. This did not reach statistical significance for the group as a whole, apart from the short word interval scores in the WLR which indicated an increase in attention (P < 0.01). Subjects did not respond with 0.8% isoflurane, either before or after painful stimulation. Reduction of end-tidal isoflurane from 0.8% to 0% caused an increase in the CF and improved performance on the psychological tests. A category generation task on recovery showed no evidence of implicit learning of words presented in conjunction with 0.8% isoflurane. Memory testing after the trial showed a relationship between the CF obtained with isoflurane and subsequent memory: conscious awareness with explicit memory was associated with a median CF (25th–75th centile) of 32.8(28.4–36.7) Hz; conscious awareness without explicit memory was associated with a CF 24.8 (21.5–30.6) Hz; absence of both responsiveness and implicit memory was associated with a mean CF of 14.9 (14.2–17.3) Hz. There was a significant difference in CF associated with these different categories of memory (P < 0.02). We conclude that the coherent frequency of the AER reflects the likelihood of cognition during anaesthetic inhalation. (Br. J. Anaesth. 1993; 71: 633–641)

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