MIDDLE LATENCY AUDITORY-EVOKED POTENTIALS DURING REPEATED TRANSITIONSFROM CONSCIOUSNESS TO UNCONSCIOUSNESS

We have investigated the relationship between changes in the middle la tency auditory evoked potentials during alternating periods of conscio usness and uneonsciousness produced by propofol infusion combined with spinal anaesthesia for total knee replacement. Eleven patients comple ted the study, of whom two had recollection of events after the onset of the anaesthetic. There were no significant differences in heart rat e or systolic arterial pressure between any conscious and unconscious period. With the first change from consciousness to unconsciousness, l atencies of Na, Pa and Nb increased from mean (SD) starting values of 20.0 (1.4), 31.7 (1.0) and 42.8 (1.6) ms to 22.5 (2.0), 39.3 (2.1) and 57.8 (4.4) ms, respectively. During successive transitions front unco nsciousness to consciousness, awake latencies were slightly higher tha n those of baseline awake, whereeas anaesthetised latencies were simil ar to the ones obtained during the first period of unconsciousness. Th e consistent changes demonstrated, suggest that the auditory evoked po tentials could represent a reliable indicator of potential awareness d uring anaesthesia.