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

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% end-tidal 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.