Middle latency auditory evoked potentials during repeated transitions from consciousness to unconsciousness

Abstract

SummaryWe have investigated the relationship between changes in the middle latency auditory evokedpotentials during alternating periods of consciousness and unconsciousness produced by propofol infusion combined with spinal anaesthesia for total knee replacement. Eleven patients completed the study, of whom two had recollection of events after the onset of the anaesthetic. There were no SigniJicant diyerences in heart rate or systolic arterial pressure between any conscious and unconscious period. With the first change from consciousness to unconsciousness, latencies 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 from unconsciousness to consciousness, awake latencies were slightly higher than those of baseline awake, whereas anaesthetised latencies were similar to the ones obtained during the first period of unconsciousness. The consistent changes demonstrated, suggest that the auditory evoked potentials could represent a reliable indicator of potential awareness during anaesthesia. Key wordsMeasurement techniques; auditory evoked potentials. Anaesthetics, intravenous; propofol.The objective measurement of anaesthetic depth and its implications for the production of a reliable monitor of awareness, remains a desirable but elusive goal in anaesthesia [I]. Previous attempts to obtain an index of anaesthetic depth have included the use of the spontaneous electroencephalogram [2] and its processed derivatives, the cerebral function monitor [3], the cerebral function analysing monitor [4], fast Fourier transformation (FFT) and aperiodic waveform analysis [5]. More recently, the change in lower oesophageal contractility associated with anaesthesia has been investigated and shown to be related to the end-tidal concentration of volatile anaesthetics [6]. It was, however, insufficiently discriminating at the interface between consciousness and unconsciousness to be used as a monitor of awareness [7], and proved to be unrelated to blood concentration of intravenous anaesthetics [8].The most promising area of investigation has been the auditory evoked potential (AEP), which appears to show specific changes in its early cortical components related to depth of anaesthesia. These changes are independent of the anaesthetic agent used [8-131 and are partially reversed by surgical stimulation [ 141, properties thought to be necessary to qualify as an indicator of anaesthetic depth [IS]. In a recent study [16], the early cortical AEP was shown to be 'able to demonstrate potential awareness' under isoflurane anaesthesia. During some of those studies, a partial recovery of the AEP was observed after temporary discontinuation of the anaesthetic before the start of surgery [8-131. However, the patients were not allowed to recover completely and no quantitative data were published.Using a purpose-built, computer-based system, capable of processing the raw EEG and obtaining the AEP in real time,...

Median nerve evoked responses and explicit memory during recovery from isoflurane/nitrous oxide anesthesia

“…Similarly, it is known from the midlatency auditory evoked responses that Pa and Nb components, occurring approximately 30 and 40 msec post-stimulus, are promising markers for changes in consciousness during anesthesia. 26 However, the generators of the later MnSSER components are still debated and the nomenclature differs in relation to different recording techniques. 27 However, they have been shown to be early markers for cognitive impairment in patients with hepatic encephalopathy for example.…”

Evoked potential monitoring in anaesthesia and analgesia

“…When propofol infusion produces unconsciousness (as judged by loss of both the eyelash reflex and the response to command), Davies et al [88] observed an increase in the latencies of Na, Pa and Nb, which is reversible and reproducible during alternating periods of consciousness and unconsciousness. Schwender et al [89] also observed that during anaesthesia for Caesarean section, a prominent MLAEP is associated with intra-operative wakefulness, purposeful movements and postoperative recall of surgical manipulation.…”

Effects of cardiopulmonary bypass and hypothermia on electroencephalographic variables

“…The AEP were obtained using a similar system to that described in our previous studies [15,20] from three electrodes (Zipprep) placed on the right mastoid (þ), middle forehead (¹) and Fp2 as the reference. The amplifier was custom-built with a 5-kV medical grade isolation.…”

“…Middle latency auditory evoked potential (MLAEP) has been reported to correlate well with anaesthetic depth [14] and to be able to demonstrate potential awareness [15,16]. However, the MLAEP are usually obtained intermittently and the waveforms are difficult to use in the clinical situation.…”