Awareness Under Anaesthesia and Depth of Anaesthesia Monitoring
Published date : 31 January 2013
Article date : 31 January 2013
By Dr Ian Nesbitt, MBBS, FRCA, DICM(UK), FFICM, Senior Editor: Anaesthesia & Critical Care and Consultant in Anaesthesia & Critical Care, UK
Surgeon: “Is he good to go?”
Anaesthetist: “Sleeping like a baby”
Patient: “WHAT? Am I supposed to be asleep right now? I can still hear you! Wait - something’s wrong - oh, my God!”
From: Awake (MGM Movies, 2007)
Introduction
Awareness under anaesthesia (AUA) is a serious concern for patients and subject to frequent dramatization in movies such as Awake, Anesthesia, Under and in the serialisations Hannah Montana and Nip & Tuck. It is a complex condition, ranging from the most obvious and distressing extreme of explicit recall with pain through several gradations to unawareness without any recall (see below):
Aware, explicit recall: Pain
Aware, explicit recall: No pain
Unaware, implicit recall, psychological sequelae: No pain
Unaware, no recall, no psychological sequelae: No pain
Explicit awareness with recall is the conscious recollection of events, either spontaneously or as a result of direct questioning. Implicit awareness is where memories exist without conscious recall which may lead to psychological sequelae.
The vast majority of patients undergoing anaesthesia are unaware, have no recall and no psychological sequelae. The true incidence of awareness under anaesthesia is unknown although estimates vary from 1/700 through to 1/3000 [1,2]. However, observational studies in daily practice fail to demonstrate rates this high [3] and awareness features rarely in legal claims against anaesthetists (around 2% of malpractice claims against anaesthetists) [4]. However, given the large number of anaesthetics delivered, rare events will still affect many patients annually (one estimate is that 100 patients a day in the USA experience accidental awareness) [1].
Predicting Awareness
When only 50% of subjects exposed to a certain concentration of inhaled agent respond to a standard surgical stimulus, this is termed the minimum alveolar concentration (MAC) for that agent, and represents the potency of the drug. Hence 1.2% isoflurane has a similar potency to 2.1% sevoflurane. Likewise, at certain concentrations of agent, half of patients will be “awake”, and half “asleep”. However, there is significant variation between patients in their susceptibility to anaesthetic agents (due to age, obesity, gender and other- including unknown, factors), and certain types of surgery have historically been associated with higher risks of AUA. These include Caesarean section (low agent concentration used to avoid effects on the baby), trauma (to avoid significant hypotension) and cardiac surgery (equipment configuration during cardiopulmonary bypass).
More recently, Total Intravenous Anaestheisa (TIVA) has also been associated with AUA. This may be due to inaccurate calculations or algorithms of estimated agent blood concentration, or more prosaically, inadvertent extravasation of anaesthetic agent. Patients receiving neuromuscular blocking drugs frequently receive lower concentrations of anaesthetic so are also at higher risk of AUA particularly since they cannot communicate even if they are receiving inadequate levels of anaesthetic while paralysed.
Most assessments of high risk include the above groups but no accurate identifier is available for individual patients.
Why the Discrepancy Between Study Incidence and Real Life Incidence of AUA?
1. Historical studies with lower rates of end-tidal anaesthetic concentration (ETAC) monitoring
ETAC technology was developed in the 1980s, and only became widespread and affordable during the 1990s. Although ubiquitous in developed countries, it is still not mandatory in all (e.g. the American Society of Anesthesiologists standards do not include this)
Previously, depth of anaesthesia was estimated using imperfect clinical signs such as hypertension, lacrimation or tachycardia, and reference to dialled up agent concentrations on anaesthetic machines (none of which deliver anaesthetic gas in a WYSIWYG* manner)
2. Other technological and practice developments
i. Spinal anaesthesia for Caesarean Section (CS) is commonplace now compared with previous decades. In part this has been driven by safety concerns regarding emergency CS under general anaesthesia. CS patients constitute a high risk group for AUA, but the significantly reduced numbers undergoing this under general anaesthesia have decreased the numbers experiencing AUA for this group.
ii. Similarly, improvements in cardiopulmonary bypass technology have reduced the chances of inadvertent disconnection or failure of delivery of anaesthetic agent, and the historical technique of using only high dose opiates without inhaled agent has fallen from use (this was associated with AUA).
iii. General improvements in monitoring technologies (e.g. arterial lines) allow more accurate real time physiological measurements than previously. Combined with increased understanding of physiology (e.g. the benefits of hypotensive resuscitation during trauma) allows improved titration of anaesthetic agents. Thus, the use of very low inhaled agent concentrations to prevent hypotension may be less prevalent than previously.
*WYSIWYG= what you see is what you get.
Accurate Identification of AUA
One effect of anaesthetic and sedative drugs is to impair short term memory and memory formation. Just as critically ill patients in intensive care units frequently develop delirium, in part drug induced, so patients emerging from anaesthesia may have similar impairment of memory formation. Thus, memories of the pre or post-operative recovery area become identified as being intra-operative memories. It can be extremely difficult to disentangle fact from perception.
There are several different assessment tools (for example, the Cochrane tool recognises eleven different states of anaesthesia or awareness) [5] and even expert interviewers may produce inconsistent results when investigating AUA. Thus, the same patient may be labelled as definite, probable or possible awareness depending on the interviewing technique, time of interview and assessment tool used.
So, historical studies and reports are of limited value for determining modern practice while current knowledge, monitoring and technology are of limited utility in accurately identifying patients with intraoperative awareness. A monitor that could do so would be of immense potential benefit.
Depth of Anaesthesia Monitors
Anaesthetic drugs reduce brain electrical activity, so theoretically, measuring brain activity allows one to diagnose whether a patient is sufficiently deeply anaesthetised to avoid awareness. Different areas of the brain are differentially affected by anaesthesia, with relative preservation of auditory inputs and processing (which explains why most (80%+) cases of AUA feature auditory symptoms, while pain or other symptoms occur in fewer than half of patients). Different agents also affect the brain in different ways (for example, ketamine tends to increase EEG activity compared with most other agents). This, combined with the natural inter-individual responsiveness to anaesthetic agents, makes for some difficulty in interpreting any brain activity monitor display.
Several depth of anaesthesia monitors are available, all using processed EEG activity to produce a display. The processing may be based on spectral edge analysis or a combination of EEG and EMG activity (typically using facial muscles). These proprietary processing algorithms are complex, but usually display a dimensionless number from 0 (no brain activity) to 100 (fully awake). Adequate anaesthesia is often regarded as being between 40—60.
The most frequently investigated device is the BIS Monitor (Covidien), while Narcotrend-Compact M (MT MonitorTechnik) and E-Entropy (GE Healthcare) have a comparatively small evidence base at present.
Although the literature is equivocal, with both reduced and increased rates of AUA being diagnosed with BIS monitors compared to standard monitoring [6], most studies of this topic are limited by differing definitions, low event rates, loss to follow up, and design weaknesses.
NICE has recently published guidance for the use of depth of anaesthesia monitoring [7]. The committee acknowledged the great uncertainties in the evidence base, but ruled out recommending additional studies on the basis of cost, time and feasibility.
Overall, NICE could not recommend the routine use of any depth of anaesthesia monitor, but did recommend that such monitors be considered for use in high risk patients.
The Royal College of Anaesthetists is currently undertaking study (NAP 5) to identify the incidence of AUA in the UK population.
In practical terms, a suitable monitor should have a sufficiently high sensitivity to demonstrate awareness. Even accepting the most optimistic estimates from (often industry sponsored) studies would require monitoring large numbers (perhaps thousands) of patients to prevent one case of awareness. The harsh fact may be that, given the current infrequent spontaneous reporting and legal consequence of AUA, there is little incentive for many institutions to fund an unproven technology that fails to completely eliminate an already rare condition.
Conclusion
Awareness under anaesthesia is a more complex phenomenon that it would appear at first sight. Certain clinical situations present higher risks of awareness. None of the currently available monitoring systems have sufficient evidence of benefit to strongly recommend their routine use.
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References
1: The incidence of awareness during anesthesia: a multicenter United States study. Sebel PS, Bowdle TA, Ghoneim MM, et al. Anesthesia and Analgesia 2004; 99(3): 833–9
2: Injuries associated with anaesthesia. A global perspective. A. R. Aitkenhead. British Journal of Anaesthesia 95 (1): 95–109 (2005)
3: Intraoperative awareness in a regional medical system: a review of 3 years' data. Pollard RJ, Coyle JP, Gilbert RL, Beck JE. Anesthesiology 2007. 106 (2): 269–74
4: Awareness during general anesthesia: ASA Closed Claims Database and Anesthesia Awareness Registry. Kent CD. ASA Newsletter 74(2): 14-16, 2010.
5: Anaesthetic interventions for prevention of awareness during surgery (Protocol) The Cochrane Collaboration. Messina AG, Ward MJ, Pace NL. 2010, Issue 10
6: More than an Abstract: Asleep, or just Napping? Kapoor A, Nesbitt I. CPD Anaesthesia, 2012; 12 (3):91-96.
8: Pandit JJ, Cook TM:National Institute for Clinical Excellence guidance on measuring depth of anaesthesia: limitations of EEG-based technology Br. J. Anaesth. 2013: 110: 325-328.
