A similar BIS value does not mean a similar depth of anaesthesia
Hull, UK
E-mail: i.f.russell{at}hull.ac.uk
Editor—The recent editorial1 and accompanying two studies2 3 provide more intriguing information as regards the complex nature of general anaesthesia. However, there are two aspects of the terminology used within the papers which I believe have the potential to cause confusion.
First, there is the suggestion that equi-anaesthetic concentrations of a drug can be defined by the same bispectral index (BIS) value. I have no concerns if an author's methodology describes titrating anaesthetic drugs to a particular BIS value, but authors should not believe that because patients have the same BIS values this indicates a similar depth of anaesthesia. There is abundant evidence to the contrary. The onset of anaesthesia is generally defined as loss of consciousness (LOC) and this is an easily observed endpoint. The available data do not support BIS or any depth of anaesthesia (DoA) monitor as being able to define LOC with a particular BIS number. In the two papers2 3 discussed in the editorial,1 the BIS value at LOC for sevoflurane and propofol were 70 vs 612 and 80 vs 63.3 These are significantly different BIS values indicating inter-individual variability. To add further confusion, data currently under analysis, which I obtained during a study of BIS-guided anaesthesia backed up with the isolated forearm technique not only confirm this inter-individual variability (patients responding to command during surgery with BIS values anywhere between the low 40 s up into the 80 s), but the same patient may respond at quite different BIS values at different times during the procedure.
Secondly, Mourisse and colleagues3 state that ‘unconsciousness was assessed with the validated method of BIS reflecting the depression of the cerebral cortex’. To back up this statement, they refer the reader to a paper discussing the validation of ‘DoA’ monitors.4 Although this is a well-written article, it is essentially descriptive and in the concluding paragraph the authors write: ‘When validated, DoA monitors can be integrated into future anaesthetic advisory and feedback systems’. To me, this suggests that DoA monitors are yet to be validated!
Nijmegen, The Netherlands
* E-mail: j.mourisse{at}anes.umcn.nl
Editor—We thank Dr Russell for his response to the editorial and his interest in our articles.1–3 He expresses concerns about the terminology we have used. We wish to respond in three ways.
First, we did not use the term ‘DoA’ in the papers.2 3 ‘DoA’ suggests that anaesthesia is a variable that is somehow measurable. In fact, we first have to define anaesthesia, then as a next step, we can try to measure anaesthesia. If we can measure anaesthesia, we can control it.
We therefore were seeking evidence that general anaesthesia has cortical (amnesia and unconsciousness) and subcortical components (antinociception, immobility, and autonomic stability).2 3 LOC and inhibition of movement in response to a noxious stimulus are two separate endpoints of anaesthesia, although mutual influences between components are possible. This concept5 is not new and was recently discussed in this Journal.4 We have used BIS as a measure of cortical activity. The blink reflex and the withdrawal reflex were used as measures of subcortical activity.2 3 BIS does not test ‘DoA’, but rather the hypnotic component of the state of general anaesthesia.6
Secondly, Dr Russell is right that LOC does not correspond to a particular BIS value. We do not agree that LOC is an easily observed endpoint. An exact determination of the time of LOC depends on the frequency of asking, and asking itself can influence the time of LOC. Furthermore, determining LOC is subjective and, around the LOC, the dose–response curve for BIS is at its steepest. All these factors contribute to a large variability. That does not alter the fact that we found a discrepancy between the BIS values at LOC for sevoflurane and propofol. However, we believe we can make a valid comparison between dose–response curves for BIS against sevoflurane or propofol concentrations, as we excluded periods of excitation and burst suppression. We are uncertain about the place of the isolated forearm technique in our concept. Apart from being subjective to the interpretation by the anaesthetist, it is not always clear whether a response is initiated by cortical or subcortical structures. Therefore, it is not a surprise to see responses at low BIS values.
Thirdly, as the BIS monitor can reduce the incidence of awareness,7 8 it is validated for this application. That is what we are referring to in our introduction.3 BIS could not be validated in a straightforward way as a measure of the state of arousal of a patient because there is a lack of an objective gold standard. Therefore, one has to rely on indirect indices, such as anaesthetic drug concentrations or clinical scales.4 BIS—the most widely evaluated index—is not a perfect measure, if only because of the time that is required for the computation from the raw EEG.9
In conclusion, we have abandoned the term ‘DoA’. The increasing evidence for the crucial role of subcortical functions, particularly at the level of the spinal cord, suggests that the time has come to stop using the term ‘DoA monitor’ to indicate a device for solely monitoring cortical functions.
References
1 Sneyd JR, Rigby-Jones AE. Effect site: who needs it? Br J Anaesth (2007) 98:701–4.
2 Mourisse J, Lerou J, Struys M, Zwarts M, Booij L. Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: BIS and blink reflex. Br J Anaesth (2007) 98:737–45.
3 Mourisse J, Lerou J, Struys M, Zwarts M, Booij L. Multi-level approach to anaesthetic effects produced by sevoflurane or propofol in humans: BIS and tetanic stimulus-induced withdrawal reflex. Br J Anaesth (2007) 98:746–55.
4 Bruhn J, Myles PS, Sneyd R, Struys MMRF. Depth of anaesthesia monitoring: what's available, what's validated and what's next? Br J Anaesth (2006) 97:85–94.
5 Glass PS. Anesthetic drug interactions: an insight into general anesthesia—its mechanism and dosing strategy. Anesthesiology (1998) 88:5–6.[Web of Science][Medline]
6 Degoute CS, Macabeo C, Dubreuil C, Duclaux R, Banssilon V. EEG bispectral index and hypnotic component of anaesthesia induced by sevoflurane: comparison between children and adults. Br J Anaesth (2001) 86:209–12.
7 Ekman A, Lindholm ML, Lennmarken C, Sandin R. Reduction in the incidence of awareness using BIS monitoring. Acta Anaesthesiol Scand (2004) 48:20–6.[CrossRef][Web of Science][Medline]
8 Myles PS, Leslie K, McNeil J, Forbes A, Chan MT. Bispectral index monitoring to prevent awareness during anaesthesia: the B-Aware randomised controlled trial. Lancet (2004) 363:1757–63.[CrossRef][Web of Science][Medline]
9 Lerou JG, Mourisse J. Applying a physiological model to quantify the delay between changes in end-expired concentrations of sevoflurane and bispectral index. Br J Anaesth (2007) 99:226–36.
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