We read with much interest the recent report “When Is a Bispectral Index of 60 Too Low?” by Manyam et al.  1They wrote that 1% sevoflurane is usually sufficient to produce clinically adequate anesthesia with an effect site concentration (Ce) of 5 ng/ml remifentanil. They used the Observer's Assessment of Alertness/Sedation scale to assess the sedation level simultaneously measuring the Bispectral Index (BIS) and auditory evoked potential index (AAI), and they concluded that targeting a BIS less than 60 or an AAI less than 30 may result in an excessively deep anesthetic state during sevoflurane–remifentanil anesthesia.

Previous reports2,3have shown that opioids reduced the Ce of anesthetics at loss of consciousness. We think this phenomenon does not always indicate that opioids decrease the adequate Ce of anesthetics for maintenance of anesthesia. We cannot assess the level of hypnosis at the surgical level of anesthesia with the Observer's Assessment of Alertness/Sedation scale, because the scale is always 0 when BIS decreases below 50, 40, or less. Currently, there is no direct evidence whether opioids can reduce the Ce of anesthetics to maintain an adequate level of hypnosis for surgery when a medium to high dose of opioids is used. At the surgical level, we cannot tell whether electroencephalogram-derived parameters, such as the BIS, are adequate to indicate the level of hypnosis, because we cannot assess the adequacy of level of hypnosis other than by using the monitors based on electroencephalographic or evoked potentials.

The major goal of general anesthesia is to maintain unconsciousness and amnesia during surgery. And we should pay much attention to memory, especially “implicit memory.” In most cases, the concentration of anesthetic that prevents explicit memory is lower than the minimum alveolar concentration required to follow commands (MACawake). However, implicit memory formation does occur even when the concentration is above MACawake. Iselin-Chaves et al.  4reported that there was evidence of memory for words presented during adequate anesthesia (BIS 41–60) but not during deep anesthesia (BIS 21–40). At this point, there is no consensus about whether we should prevent implicit memory; our view is that we should prevent it because it might have influence on patients' postoperative emotion or behavior. To prevent memory formation, it is important to block sensory inputs from reaching the brain, especially noxious inputs and auditory inputs. Patients who experience awareness during anesthesia often report pain sensations or hearing sounds or voices during surgery. Remifentanil, 5 ng/ml, is sufficient to block the noxious stimuli in most cases. As for auditory inputs, we can assess them by auditory evoked potentials. AAI is calculated from the track length of auditory evoked potential waveform in a specific period. There is the possibility that auditory inputs are processed in the cortex and implicit memory might be retained when AAI values are high. Therefore, it would be better to keep the AAI values less than 30 if we wish to prevent implicit memory. Considering these facts, it is too early to draw the conclusion that targeting a BIS less than 60 with the Ce of remifentanil at 5 ng/ml results in an excessively deep anesthetic state.

By the way, BIS values become 40–50 when keep the end-tidal concentration of sevoflurane at 1.2–1.4% with 0.2 μg · kg−1· min−1remifentanil. At this level, AAI becomes less than 30.5Katoh et al.  6reported that, during plain sevoflurane anesthesia, the average value of BIS was 60 at 1.0% and 40 at 1.5%. When we keep the end-tidal sevoflurane at 1.2–1.4%, most patients can quickly recover from anesthesia after surgery. Compared with the data by Katoh et al. ,6BIS values shown in figure 5 or figure 7 seemed to be higher, which suggested that the Ce of sevoflurane would not reach equilibrium in the current study. So the relation between BIS values and end-tidal concentration of sevoflurane would not be correct considering the steady state.

Finally, we think it would be wise to keep the end-tidal concentration of sevoflurane at 1.2–1.4% (a little higher than 1.0%) until enough data are accumulated.

*Osaka University Graduate School of Medicine, Osaka, Japan. hagihira@anes.med.osaka-u.ac.jp

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