BJA Advance Access originally published online on January 27, 2006
British Journal of Anaesthesia 2006 96(3):346-352; doi:10.1093/bja/ael017
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NEUROSCIENCES AND NEUROANAESTHESIA |
The Narcotrend ‘depth of anaesthesia’ monitor cannot reliably detect consciousness during general anaesthesia: an investigation using the isolated forearm technique
Department of Anaesthesia, Hull Royal Infirmary, Anlaby Road, Hull HU3 2JZ, UK
* E-mail: i.f.russell{at}hull.ac.uk
Accepted for publication November 6, 2005.
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Abstract |
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 TopFootnotes Abstract IntroductionMethodResultsDiscussionReferences |
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Background. During general anaesthesia in the presence of neuromuscular blocking drugs clinical criteria cannot detect the presence of consciousness. Various ‘depth of anaesthesia’ monitors are available which claim to prevent consciousness and/or reduce anaesthetic drug use. This study uses the Narcotrend anaesthesia brain monitor to guide anaesthetic administration but at the same time checks for the presence of intra-operative consciousness by using the ‘isolated forearm’ technique throughout the whole surgical/anaesthetic procedure.
Methods. Twelve women presenting for major gynaecological surgery under general anaesthesia, which included the use of neuromuscular blocking drugs, had a target controlled infusion of propofol adjusted according to the anaesthetic ‘stage’ indicated by a Narcotrend ‘Depth of Anaesthesia Monitor’. Throughout surgery the isolated forearm technique was used to detect for the presence of consciousness at 1 min intervals.
Results. Isolated forearm responses to commands occurred in all 12 patients at some time during surgery, frequently in the absence of any significant changes in the usually monitored clinical variables. Overall, the 12 patients responded a total of 92 times during surgery. Only 41 (45%) responses were associated with an increase in the Narcotrend stage to a level suggesting consciousness (above stage C0). For the remaining responses, either there was no significant increase in the Narcotrend stage (above C0) or there was no change at all in the Narcotrend stage before, during, or after the patient responded to the taped command.
Conclusions. The Narcotrend was unable to differentiate reliably between conscious and unconscious patients during general anaesthesia when neuromuscular blocking agents were used.
Keywords: anaesthesia, depth; anaesthesia, general; equipment, brain monitors; monitoring, depth of anaesthesia
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Introduction |
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TopFootnotesAbstractIntroductionMethodResultsDiscussionReferences |
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In the presence of neuromuscular blocking drugs determining the presence or absence of consciousness during general anaesthesia using the usually monitored clinical indices is imprecise and may be quite misleading.1–6 Neither handwritten clinical records5 nor automatic printouts6 (or data collected automatically from cases of awareness with recall in my hospital) when viewed retrospectively can be used to differentiate patients who have been awake with recall during surgery from those who had no such experience.
As early as 1960, Parkhouse1 stated ‘what is required for the paralysed patient is not so much an index of anaesthetic "depth" as an index of consciousness’. As regards consciousness, the main site of action of general anaesthetic drugs lies within the brain and, consequently, many have sought changes within the EEG with which to define consciousness and ‘depth’ of anaesthesia. Several ‘depth of anaesthesia’ brain monitors are now commercially available and many studies demonstrate that the output of such monitors correlates with anaesthetic concentrations7 or scores for sedation/consciousness.8 9 Some research groups have used these monitors as guides to depth of anaesthesia and have demonstrated that by adjusting ‘depth of anaesthesia’ according to the brain monitor output it is possible to reduce the amount of anaesthetic drugs used, resulting in shorter wake up times, shorter times in the post anaesthesia recovery unit and a faster patient turn over.10 11 When using ‘depth of anaesthesia’ as the main determinant of unconsciousness/anaesthesia, it presupposes that for an individual patient the brain monitor can identify accurately the ‘depth of anaesthesia’ and, in particular, can identify reliably when patients are conscious.
This latter point is debatable. There are wide intra- and inter-individual differences in brain monitor output at loss of consciousness during induction of anaesthesia and return of consciousness when anaesthesia is terminated and, because of the overlap between conscious and unconscious values, brain monitor output can only be interpreted as a probability that a particular patient may be conscious or unconscious.12–14 Because of this, it is essential that all ‘anaesthesia brain monitors’ be adequately validated before being used as guides to the administration of anaesthetic agents for individual patients.15 16 To date, no anaesthesia brain monitor has been adequately validated in the presence of neuromuscular blocking drugs for the duration of surgery with reference to individual patients. When using neuromuscular blocking drugs, all studies to date have looked only at specific points during anaesthesia—for example loss of consciousness at induction or return of consciousness at the end of anaesthesia, although some have extended the loss of consciousness and return of consciousness data by deliberately awakening the patients and reinducing anaesthesia, either before surgery begins15 or in the presence of effective spinal anaesthesia.13
When using the ‘isolated forearm’ technique617 and deliberately awakening patients after tracheal intubation Schneider and colleagues15 noted that for individual patients the Narcotrend monitor was unable to differentiate between ‘just conscious’ and ‘just unconscious’. Such findings cast serious doubts on the ability of the Narcotrend to function as a clinically useful depth of anaesthesia monitor at the lighter stages of anaesthesia. However, detecting the EEG changes from ‘just conscious’ to ‘just unconscious’ may be difficult and perhaps an easier challenge would be to differentiate between consciousness and a deeper level of anaesthesia. This study is the first investigation of levels of consciousness, as determined by isolated forearm responses, during Narcotrend guided general anaesthesia in the presence of neuromuscular blocking drugs for the whole duration of surgery and anaesthesia.
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Method |
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Following Local Research Ethics Committee approval, informed consent was obtained from women undergoing major gynaecological surgery to have the depth of their anaesthetic guided by the Narcotrend monitor. At the same time, the isolated forearm technique6 17 was used.
The NarcotrendTM (software version 4.0; MonitorTechnik, Bad Bramstedt, Germany) automatically quantifies depth of anaesthesia into stages using a six-letter system with a total of 14 substages.11 18 20 This software version also provides a dimensionless numerical output from 100 (awake) to 0, called the Narcotrend index. A graphical output of the stages (the cerebrogram), a digital output of the stage and the dimensionless number are also provided on the monitor screen: these are all updated every 5 s. The stages are as follows: A, awake; B0–2, sedation; C0–2, light anaesthesia; D0–2, general anaesthesia; E0–1, general anaesthesia with deep hypnosis; F0–1, general anaesthesia with increasing burst suppression.
No premedication was given. In the anaesthetic room the electrode sites on the head were prepared with an abrasive paste (Nuprep, Dow Weaver and Company, Aurora, CO, USA) and then following the manufacturer's recommendation standard ECG electrodes (Ambu Blue Sensor R; AMBU, Medicotest A/S, Olstykke, Denmark) were affixed to the patient's forehead. I.V. access was established in the left hand or forearm and, if previously consented for, a low thoracic epidural (T11/T12) was inserted. Patients were then taken into the operating room and the Narcotrend monitor was connected to the forehead electrodes while standard monitoring was applied (non-invasive blood pressure, ECG, pulse oximetery, inspired/expired oxygen and end tidal carbon dioxide; Datex Cardiocap 5, Instrumentarium Corp, Finland). Padding was wrapped around the right forearm and a tourniquet was placed over the padding before resting that arm on an arm board with the forearm strapped to the board such that the hand was in full view. Two ECG electrodes were placed on the skin over the ulnar and median nerves at the elbow and these electrodes were then connected to a peripheral nerve stimulator.
Before the patient arrived in the operating suite, an individualized minidisk had been prepared. Three tracks were recorded: track 1 consisted of 15 s of radio static; track 2 consisted of a 15 s message asking the patient to open and close the fingers of her right hand—‘name, name, this is Dr Russell speaking: if you can hear me, open and close the fingers of your right hand; open and close the fingers of your right hand’; track 3 consisted of a 15 s message giving the patient some information to remember—‘name, name, this is Dr Russell speaking: here are some special words I'd like you to remember; green pear, sharp lemon, sour gooseberry’. A play list was programmed into the minidisk player such that these three tracks were played to the patient in the order 2,1,3,1 with the minidisk player set in programme repeat mode. Thus, once the play button was pressed, these tracks played non stop in the above order until the minidisk player was switched off.
After a baseline reading on the Narcotrend monitor a bolus dose of remifentanil (1 µg kg–1) was administered, after which general anaesthesia was induced and maintained with a target controlled infusion of propofol set initially at a plasma concentration of 8 µg ml–1. Throughout surgery, analgesia was provided by the epidural (if in place) and/or a remifentanil infusion. If an epidural was being used the remifentanil infusion was set at 50 ng kg–1 min–1: if an epidural was not being used the remifentanil infusion was set initially at 200 ng kg–1 min–1 and adjusted as required.
After loss of consciousness (defined as no response to the command ‘name, name, open your eyes’) the forearm tourniquet was inflated to a pressure of 200 mm Hg and atracurium 10–15 mg administered via the indwelling venous cannula in the contralateral arm. After this, the patient's trachea was intubated and their lungs ventilated with an air–oxygen mixture.
After tracheal intubation, surgery was allowed to commence as normal while anaesthesia was lightened by reducing the target plasma propofol concentration. The intention was to continue to reduce the target propofol concentration until the Narcotrend indicated the C0 level (light anaesthesia/light sleep), unless the patients responded appropriately with their isolated arm to the commands played from the minidisk before this. The minidisk player display indicates which track is being played and when a patient appeared to be responding to command the ‘mark’ key was pressed on the Narcotrend and an attempt was made to verify the response by stopping the minidisk player, lifting one headphone and speaking directly to the patient. Verification involved asking the patient by name to ‘squeeze my fingers with your right hand’ and, if this instruction was followed, to ascertain by hand squeezes whether the patient was comfortable or uncomfortable. After this the target propofol concentration was increased until responding ceased. Thereafter, the target propofol concentration was once more adjusted according to the Narcotrend stage. A significant response from the Narcotrend was an increase in the stage to above C0 within a period from 2 min before to 2 min after a verified response in the isolated forearm.
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Results |
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Twelve women, aged between 35 and 79 yr of age and weighing between 49 kg and 122 kg, who were to undergo major gynaecological surgery gave informed consent to participate in the study. All but one had a low thoracic epidural as part of the anaesthetic.
Isolated forearm responses to commands occurred in all 12 patients at some time during surgery, frequently in the absence of any significant changes in the standard clinical variables. Not counting the responses at the time the patients awoke at the end of surgery, the 12 patients responded a total of 92 times during surgery. The Narcotrend stages at the time of these 92 patient responses and the associated change in the Narcotrend stage are shown in Table 1. Table 1 demonstrates that of the 92 isolated forearm responses to command during surgery only 18 (20%) responses were predicted by the Narcotrend: that is, there was an increase in the Narcotrend stage to above C0 before there was an isolated forearm response. A further 23 (25%) isolated forearm responses were accompanied or followed within 2 min by an increase in the Narcotrend stage to above C0. This leaves 51 responses (55%) which were neither predicted nor detected by the Narcotrend: that is, there was no significant change in the Narcotrend stage between a period 2 min before and 2 min after the patient responded to the taped command.
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At the end of surgery the Narcotrend stage at return of consciousness varied from D2 to B2.
In 7 patients there was a total of 22 periods when the Narcotrend displayed either a blank screen or a message ‘>32 Hz’. The median duration of the blank screen was 55 s (interquartile range 55–120 s, range 40 s to 9 min). These blank screen periods were often related to surgical diathermy but at other times the cause was unknown. Occasionally, the blank screen occurred at critical times just before or in association with an isolated forearm response.
There were problems with electrode impedance in 6 of the 12 patients and in these 6 patients a total of 36 electrodes were used before there was an acceptable impedance at all the electrodes. These were the same batch of electrodes as were being used for the routine monitoring of the EEG; they caused no problems in that mode of use.
Some 60 min after the end of surgery, just before they were returned to the gynaecology ward, 9 of the 12 women were asked detailed questions relating to their memory for intra-operative events (Table 2). None of the women remembered anything about the surgery. Four women had explicit recall for parts of the ‘taped’ message. For all four of these patients it is not possible to say with any certainty when the memories for the tape occurred as all these patients responded to the tape towards the end of surgery and at this point the infusion pumps and the tape were switched off and the patients remained awake.
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Figure 1 is a printout of a complete Narcotrend cerebrogram trace for a 78 yr old woman undergoing abdominal hysterectomy and bilateral salpingo-oophorectomy with a low thoracic epidural as part of the anaesthetic technique. She had a past medical history of what were thought to be cerebral transient ischaemic attacks. Unfortunately, despite using seven different electrodes, the Narcotrend would not record initially and the induction period of the anaesthetic was missed. However, once the patient had lost consciousness the Narcotrend began recording. The cerebrogram indicates ‘deep’ or ‘surgical anaesthesia’ for the entire procedure despite the isolated forearm responses indicating that the patient was very lightly anaesthetized and was drifting in and out of consciousness.
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Discussion |
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A previous study15 demonstrated that the Narcotrend was unable to differentiate the ‘just conscious’ from the ‘just unconscious’ states and the authors described these EEG data as ‘challenging’. The data in the current study support these previous observations in relation to return of consciousness15 but go further and indicate that patients are capable of responding to commands during periods defined by the Narcotrend stage as ‘light anaesthesia’, ‘general anaesthesia’ and even ‘general anaesthesia with deep hypnosis’. Such findings cast serious doubt on the ability and reliability of the Narcotrend monitor to differentiate consciousness from unconsciousness in the individual patient during general anaesthesia in the presence of neuromuscular blocking drugs.
There is some confusion over the nomenclature used to define the Narcotrend stages. The stages C0 to C2 are described as ‘light anaesthesia’ in the manufacturer's literature but the manufacturer's recommendation is to aim for a stage of D0 or deeper. A previous study11 used the Narcotrend stage C1, as the target level during the last 15 min of surgery to ‘facilitate rapid awakening’. When using general anaesthesia, ‘anaesthesia’ is by definition ‘unconsciousness’3 thus any Narcotrend stage described in terms of ‘anaesthesia’, even ‘light anaesthesia’ implies an unconscious patient.
On 54 occasions a patient was conscious and responding to command at ‘surgical’ depths of anaesthesia as indicated by the Narcotrend (D0 or deeper), or with the Narcotrend screen blank. Only 17 of these periods of consciousness were associated with an increase in the Narcotrend stage to above C0, the remainder of the responses to command either had no associated change in the Narcotrend stage, even when allowing several minutes for the monitor to process the events, or the screen was blank. Had the Narcotrend monitor been the only indicator of the ‘depth of anaesthesia’ then the fact that the patients were conscious at this time would not have been detected. Redefining the critical level as above D0 makes little difference as only two other responses would have been detected/predicted: both of these responses occurred as the Narcotrend stage was decreasing from a previously higher level (C0, C1) without any additional anaesthetic having been given.
In theory, if the Narcotrend stage at which an individual patient responded was consistent, then by establishing that stage early in the anaesthetic, the ‘depth of anaesthesia’ could be kept below that responsive threshold. Although an attractive theory, the current data suggest this may not be possible as, in most cases, patient responses occurred at widely different stages at different times with responses occurring at ‘deeper’ stages one moment, yet no responses at lighter levels at some other time. While using the Narcotrend to guide ‘depth’ of anaesthesia has been shown to reduce anaesthetic drug consumption and speed up awakening,11 such a practice, irrespective of the fact that no explicit recall was detected, may not guarantee that a patient was unconscious during surgery. Rapid wake-up times at the end of surgery may simply be an indication that patients were already conscious before the reversal of neuromuscular blocking drugs.
As with any anaesthesia brain monitor, at their current state of development, a particular stage/depth can only be associated with a probability that a patient is conscious or unconscious. Using the data from the current study, to be 100% certain that all 12 of the patients in this study were unconscious for the entire surgical procedure it would have been necessary to maintain anaesthesia at a Narcotrend stage below E0. For most patients this would represent very deep anaesthesia, equivalent to a Bispectral IndexTM (BISTM) level of <40.21 Aspect, the manufacturers of the BIS monitor do not recommend keeping the BIS under 40 as anaesthesia maintained at this level may be associated with poorer postoperative outcomes for up to 2 yr after surgery.22 23
To be clinically useful, not only must an anaesthesia brain monitor be 100% accurate but it must also be reliable. There were frequent periods when the screen of the Narcotrend went blank or displayed the message ‘>32 Hz’. Surgical diathermy for more than a few seconds usually resulted in a period with no monitor output. On several occasions the blank Narcotrend display occurred at a time when the patient began to respond to commands. Without using the isolated forearm technique to detect this, there is a potential for explicit or implicit recall to occur, both of which may have deleterious effects on a patient although the effects of implicit recall, would not normally be recognized.24 25 On one occasion, not related to diathermy, the blank screen lasted some 9 min. Periods with a blank screen lasting up to 10 min with the Narcotrend have been described by others.15
The data from this study do not allow a clear demonstration of what true Narcotrend-guided-anaesthesia would really be, as in reality, depth of anaesthesia was guided not only by the Narcotrend but also by the isolated forearm responses and the latter were frequently the indicator which determined an increase in anaesthetic administration. This has several consequences which could affect the sensitivity/utility of the Narcotrend: occasionally, an isolated forearm response had triggered an increase in the target propofol concentration before an increase in the Narcotrend trace and this treatment may have blunted the eventual Narcotrend response when it occurred some 30–90 s later; an associated Narcotrend and isolated forearm response would trigger an increase in the target propofol concentration but, because of the delay in the Narcotrend analysis of the EEG, the patient had often stopped responding to the isolated forearm commands and the propofol target concentration had been reduced again at a time when the Narcotrend stage was still rising—relying only on the Narcotrend stage to determine changes in anaesthetic administration would result in the continued administration of anaesthetic agents until the Narcotrend stage had returned to an ‘acceptable’ level: an isolated forearm response in the absence of a Narcotrend response was treated within a few minutes of its occurrence and it may be that by waiting a few minutes longer a Narcotrend response might have occurred. However, on several occasions there were repeated responses to command over several minutes with no indication of any change in the Narcotrend stage. This latter state, where repeated isolated forearm responses occur over several minutes with no indication of an increase in the Narcotrend stage are a cause for concern as the longer the period of responsiveness the greater the risk of explicit26 and probably implicit memory.
One of the difficulties when using Narcotrend guided anaesthesia was deciding when to intervene. While it is easy to look at a trace retrospectively it is not so easy in real time. Often there was no gradual increase from ‘anaesthetized’ to ‘conscious’ stages but rather a very rapid increase, with no warning, associated with an isolated forearm response. If the Narcotrend stage increases rapidly to above the C0 level the decision to treat is easy. But what of slightly slower increases? Slow increases in the Narcotrend stage suggesting anaesthesia is lightening were not uncommon but many such increases did not go on to reach stage ‘C,’ were unaccompanied by any isolated forearm response and, after a short period without any intervention, reverted to a lower stage. In the absence of the isolated forearm technique, it is possible that these non-significant increases in the Narcotrend stage would be treated—but most of this treatment would be an unnecessary increase in the delivered anaesthetic agent.
In conclusion, the current study is too small to make any predictions as to the sensitivity or specificity of any particular Narcotrend stage, but the fact that 12/12 patients responded to command at levels defined as ‘surgical anaesthesia’ would suggest that the Narcotrend cannot be relied upon to determine the presence or absence of consciousness during the whole course of an anaesthetic when neuromuscular blocking drugs are being used.
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Acknowledgments |
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I am grateful to Schiller AG for the loan of the Narcotrend anaesthesia brain monitor.
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Footnotes |
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This article is accompanied by the Editorial. 
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References |
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- Utility does not require complete accuracy
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