Recording the middle latency response of the auditory evoked potential as a measure of depth of anaesthesia. A technical note.

Institute of Sound and Vibration Research, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
BJA British Journal of Anaesthesia (Impact Factor: 4.35). 04/2004; 92(3):442-5. DOI: 10.1093/bja/aeh074
Source: PubMed

ABSTRACT The middle latency response of the auditory evoked potential may be useful as an indicator of the hypnotic state during anaesthesia. However, it is difficult to record in some circumstances. This communication provides some suggestions and guidance for those not familiar with the technique.

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    ABSTRACT: Acoustic violations in temporal regularity have been traditionally indexed by mismatch negativity (MMN). However, recent studies have demonstrated that humans can detect auditory changes in physical sound features, such as frequency, location and intensity, in the first 50ms after sound onset. Our aim was to examine if temporal regularity violations could be detected in the middle latency range. We used an oddball paradigm with 290ms as standard stimulus onset asynchrony (SOA) and 200ms as deviant SOA. We also employed a control paradigm that comprised of seven SOAs including 200 and 290ms, in order to control for differences due to refractoriness. In the middle latency range, temporal regularity violations led to enhanced Pa and Nb responses, which behaved differently to the corresponding SOAs in the control condition. In the long latency range, temporal regularity violations led to similar behaviours in both oddball and control paradigms. These findings suggest that with a fast presentation rate, human brains are capable to detect temporal regularity violations in the middle latency range. Together with previous studies that found early change detection responses, the current study emphasises that the human brain can encode simple regularity violation as early as approximately 50ms post-stimulus onset.
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    ABSTRACT: Early postoperative cognitive dysfunction (POCD) is commonly associated with major surgery and anesthesia, occurring in 7 to 71% of patients [1-3]. Advanced age, degree of surgical trauma, depth of anesthesia and inflammatory activation are some of the risk factors for POCD [1,4-7]. It has been proposed that systemic inflammation may contribute to postoperative cognitive deficits and there could be a relationship between interleukin response and impaired postoperative cognition [8-10]. Monitoring the depth of anesthesia using digital processing of the EEG makes it possible to reduce anesthetic requirements and doses of opioids perioperatively, which can also influence POCD [5,11-14]. Inflammatory response and opioids are two risk factors for development of POCD [4,15]. The aim of this study was to evaluate the role of depth of anesthesia on POCD after major ENT surgery and to assess changes in postoperative inflammatory markers in patients undergoing major surgery. A selected group of experienced anesthesiologists or nurse anesthetists, specially trained in guiding anesthesia depth using auditory evoked potential (AEP, A-line), performed the anesthesia. The postoperative personnel were blinded to group assignment, and all data were processed independently of group allocation and were blinded to the investigator until the finalisation of the study. Randomisation procedure and baseline characteristics Patients were randomly assigned to one of two study groups: AEP group (group A): Anesthesia was guided by AEP: A-line  ARX index (AAI), version 1.6. Mid-latency auditory evoked potential (MLAEP) was calculated using the A-line monitor (Danmeter A/S, Odense, Denmark) [16,17], AAI between 15 and 25 was regarded as adequate [17]. Control group (group C): Anesthesia was guided by clinical signs of depth of anesthesia including blood pressure, heart rate, pupil reaction, sweating and lacrimation at the discretion of the attending anesthesiologist or nurse anesthetist. AEP was recorded in all patients in the control group but was blinded to the attending anesthesiologist or nurse anesthetist. After surgery, the data were transferred to storage media for later analysis of AAI. Citation: Jildenstål PK, Hallén JL, Rawal N, Berggren L (2012) Does Depth of Anesthesia Influence Postoperative Cognitive Dysfunction or Inflammatory Re-sponse Following Major ENT Surgery? J Anesth Clin Res 3:220. doi:10.4172/2155-6148.1000220 Copyright: © 2012 Jildenstål PK, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits un-restricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract The aim of this study was to evaluate the role of depth of anesthesia on POCD after major ENT surgery and to assess changes in postoperative inflammatory markers in patients undergoing major ENT surgery. Thirty two patients aged 40 to 94 yrs, scheduled for surgery under general anesthesia were randomly assigned to one of two groups. In group A (AEP group) depth of anesthesia (DOA) was measured with auditory evoked potential (AEP). In the control group (group C) DOA was monitored according to clinical signs. Cognitive function was evaluated using Mini-Mental State Examination (MMSE), Confusion Assessment Method (CAM) and Cognitive Failure Questionnaire (CFQ). Inflammatory markers were measured before and after anesthesia. Perioperative requirements for desflurane and fentanyl were significantly lower in group A. On the first postoperative day MMSE changes indicating POCD were noted in 1 patient in group A and 7 patients in group C (P<0.03). One month follow up did not show any difference between the groups regarding POCD. Our study indicates that AEP-guided anesthesia allows dose reduction of anesthetic agents including opioids leading to better cardiovascular stability and less early POCD. Anesthesia depth did not influence the inflammatory response to surgery.
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