Monitoring (un)consciousness: the implications of a new definition of ‘anaesthesia’

What should we properly monitor, when we monitor the brain for ‘anaesthesia’? Any answer is likely to depend on what we mean by ‘anaesthesia’. The article by Escallier et al. in this issue of Anaesthesia is an important review of the status of processed EEG (pEEG) monitoring in anaesthesia [1]. Central to the role of pEEG (or any other type) of ‘depth of anaesthesia’ monitors is their putative ability to detect when a paralysed patient is suitably anaesthetised or not; apparently a simple binary decision-making process. Yet, this article contains a profound sentence whose implications, if widely accepted, are likely to change our entire view of ‘anaesthesia’, for reasons I will explain in this editorial. The apparently innocuous sentence is: “There is a growing consensus that intra-operative awareness is a spectrum of brain states” [my emphasis]. The following questions immediately come to mind: what is the basis for this new consensus? What does this consensus imply for mechanisms of anaesthesia? And what does it imply for monitoring of the anaesthetic state? The emerging consensus that intra-operative awareness is a spectrum of brain states Traditionally, anaesthesia has been regarded as an all-or-nothing, binary phenomenon. This view was most clearly proposed by Prys-Roberts when he wrote: “There cannot be degrees of anaesthesia nor for that matter can there be variable depths of anaesthesia” [2], a statement that was unsupported by other references but a sentiment that became nevertheless widely repeated in standard texts [3]. Superficially, this makes sense: either you are anaesthetised or you are not. Once you are anaesthetised, it is difficult to conceive then how you can be ‘more’ anaesthetised. It is not (to borrow Sleigh’s phrase [4]) as if ‘the patient is a submarine’! Yet, things are never really so simple and this traditional view is now challenged in several ways. Assuming that anaesthetic drugs act at protein channel receptor targets, we know that dose-response pharmacology is not binary or allor-nothing. Rather, the drug-doseresponse relationship is characteristically continuous, described by relatively simple models in which the drug effect is non-linearly proportional to drug concentration, up to some maximum receptor effect. At some concentration of drug lower than this maximum, the active drug-receptor combination reaches a threshold that triggers the intended response (in this case, ‘anaesthesia’). If there were no variability in individual organism sensitivity or receptor state, then all animals of a species would become anaesthetised at exactly the same anaesthetic concentration. We know that this is not true: at a given clinically relevant concentration, there will always be some proportion of animals not anaesthetised (this proportion dependent upon the steepness of the population ‘dose-response’ relationship for the drug) [5]. In this way, Dilger has elegantly summarised how continuous dose-response relationships at molecular level can translate into near (but not quite) binary relationships at population level [6] (Fig. 1). Figure 1 also raises another question. Even if anaesthesia is