An Eeg Investigation of Awareness During Anaesthesia

Abstract

Previous studies of awareness during anaesthesia have used patients undegoing surgery. In an attempt to overcome the complications of anxiety and specific stimuli of surgery, this study used healthy volunteers in a laboratory setting. No evidence of recall of events taking place during light anaesthesia was found.Over the years since the advent of modern anaesthetic practice, there have been accounts of patients being ' conscious ' of events during surgery. Assuming adequate anaesthesia, these reports are surprising, since there have been several studies demonstrating that anaesthetic drugs have an inhibitory effect on learning and memory (McGaugh & Petrinovich, 1965). The present investigation was undertaken to inquire further into the problem of awareness during anaesthesia without the complications of premedication and anxiety due to, and stimuli from, surgery. METHOD Ten young adults (six male, four female) volunteered to take part in the study when the procedure was explained in detail to them. Two females were rejected because of past medical history. Subjects reported to the laboratory at 09.30 hr., having starved from at least 22.00 hr.the previous night.Prior to the anaesthetic induction, EEG electrodes were attached with collodion in a lateral ring montage using the International '10/20' system. Electrodes 0, and 0, were omitted since they would be disturbed by the face-mask harness. Electrodes were also attached about 1 cm lateral to the outer canthi to monitor eye-movements. These electrodes were used aa the subjects were encouraged to sleep after the anaesthesia, when it was hoped that subsequent dreams could be obtained by waking t,he subject from a period of paradoxical sleep (Dement & Kleitman, 1957). After a pair of earphones had been put in position and the subject had been recumbent for several minutes, baseline heart rate, blood pressure and EEG measures were obtained.Anaesthesia was induced in a sound-attenuated room with thiophentone (mean dose 3.5 mg/kg) and maintained with N,O and 0, (mean N,O concentration, 76.4 per cent). In three cases 0-5 per cent halothane was administered for 30 sec. No other drugs were administered. All the subjects appeared to be in a very light plane of anaesthesia on clinical assessment as they moved head and limbs, and mumbled. During anaesthesia, but after barbiturate-induced fast activity on the EEG was minimal, a pre-recorded tape containing the reading of a poem, the ringing of a fire-bell (with a voice shouting 'That's the fire-bell') and list of 15 low-frequency words (taken from the ThorndikeLorge list) was played to the subject through the earphones. The order of these stimuli was changed randomly. This material was chosen to reduce the probability of random recall since reported recall in patients is related to conversation that had taken place during surgery (e.g. Wilson & Turner, 1969). Following this, anaesthesia was stopped and an attempt was made to 'force' the subjects to recover as speedily as possible -their names shouted, their faces slapped (g...

The Assessment of Postoperative Mental Function

“…Evidence of selective attention or awareness of "threatening" auditory input, as reported by Levinson (1965) on the basis of e.e.g. evidence, and recall using hypnosis, has not been supported by later investigations of possible selective auditory sensibility (Terrell et al, 1969;Lewis, Jenkinson and Wilson, 1973).…”

Learning during anaesthesia: A review

“…However, this was only the case when the stimuli had been presented during surgery patients who were played the words only during the post-operative period showed no evidence of learning. The need for surgical stimulation for learning during anaesthesia may also account for the null result obtained by Lewis, Jenkinson & Wilson (1973). They played 15 words and the sound of a firebell to volunteers under 'very light ... [nitrous oxide] anaesthesia'.…”

Drug-induced amnesia: Implications for cognitive neuropsychological investigations of memory.

“…Studies of memory for the intraoperative stage of surgery have produced mixed, and even contrary, results (see Ghoneim & Block, 1992, for a recent review and Sebel, Bonke, & Winograd, 1993, for a debate on the subject). Some studies have produced evidence that patients can manifest a form of memory for intraoperative events (e.g., Bennett, Davis, & Giannini, 1985; Bogetz & Katz, 1984; Cheek, 1959; Furlong, 1990; Goldmann, Shah, & Hebden, 1987; Levinson, 1965; Millar & Watkinson, 1983), whereas other studies have failed to find any evidence for intraoperative memory (e.g., Dubovsky & Trustman, 1976; Faithfull, 1969; Lewis, Jenkinson, & Wilson, 1973; Loftus, Schooler, Loftus, & Glauber, 1985; Terrell, Sweet, Gladfelter, & Stephen, 1969). In an early review of the literature, Cherkin and Harroun (1971) noted this disparity and attributed it to poorly controlled experiments.…”

“…In a controlled, experimental context, Terrell et al (1969) had patients listen to audiotapes while they were anesthetized and found that postoperative recall failed in every case. Several other researchers have also failed to observe intraoperative memory using recall and recognition tests (e.g., Dubovsky & Trustman, 1976; Lewis et al, 1973; Loftus et al, 1985).…”