NAP5 and depth of anaesthesia monitoring

Abstract

We read with interest the editorial by Avidan and Mashour 1 accompanying the National Audit Project (NAP)5 baseline report by Pandit et al. 2. We are somewhat surprised at the criticism of the methodology of this study and the inference that this may lead to an under-reporting of the true incidence of awareness. Considering that surgical procedures are commonly performed under regional anaesthesia or monitored anesthesia care in fully awake or sedated patients, we question the significance of awareness during surgery that is not reported. If it isn't reported then it could be contested that it is not something that has distressed the patient, especially when this is a topic that is widely reported in the media and when UK patients do not appear reluctant to complain or even litigate. It is also important, from the same perspective, to differentiate between awareness with and without pain, the latter, as with regional anaesthesia, being unlikely to trouble patients. It is also difficult to understand how the results from the NAP5 baseline report support Avidan and Mashour's elaboration on total intravenous anaesthesia (TIVA), bispectral index (BIS) and end-tidal anaesthetic concentration. Firstly, the assertion that TIVA is associated with a higher incidence of awareness with recall is based on a study from Spain (in which the interview details were not published) 3 and one from China 4. In the former, the incidence of awareness with recall in TIVA was even less than the average US incidence reported in the AIM trial 5, and the latter reports exceptionally high incidences of awareness with recall no matter what technique of anaesthesia was used. Neither of these studies could provide details on the TIVA technique and the dose of anaesthetic. Such figures suggest serious problems with the way anaesthesia is being delivered generally and make it impossible to know whether TIVA was being used appropriately. Other studies suggest no difference in the incidence of awareness with TIVA compared with inhalation anaesthesia 6-8. Secondly, TIVA has meanwhile gained in popularity. In many earlier audits, it was an uncommon technique mainly performed by ‘enthusiasts’, hence the risk of awareness was difficult to quantify in comparison with inhalation anaesthesia techniques. Although TIVA requires careful monitoring of the intravenous infusion site there is, however, no other reason why TIVA should increase awareness risk. In fact, the ability to titrate to clinical effect at induction using target controlled infusions is a distinct advantage over inhalational anaesthesia, induction of which is usually with an intravenous drug and subsequently delivered according to minimum alveolar concentration (MAC), the statistically ‘mean’ dose. Although it is possible to measure end-tidal concentrations of inhalational agents, anaesthetic management based on concentration has not been shown to reduce awareness in itself. In the B-unaware study 9, awareness with recall occurred even when BIS values and end-tidal anaesthetic concentrations were within the target ranges. Indeed, the incidence of explicit recall was no different whether end-tidal anaesthetic concentration was used or not in a large prospective Scandinavian study 7. The inherent variability of MAC 10 makes it, therefore, problematic to suggest protocols to reduce awareness with recall based on end-tidal anaesthetic concentration only. Thirdly, it is erroneous to recommend that a processed EEG-based monitor (pEEG) be used in order to decrease the likelihood of awareness with recall if TIVA is chosen. These monitors may be useful irrespective of the anaesthesic technique used but also have significant limitations. The only reason that they could be more useful with TIVA is for the same reason that TCI can be titrated. As for pEEG, for intravenous and volatile agents alike, at least with TCI this can be titrated during induction. Not only does pEEG show remarkable intra-individual variability at loss of consciousness, it also varies according to the anaesthetic drug(s) used 10. With TIVA the same drugs are used during induction and maintenance. Consequently, if used, pEEG can be titrated to clinical signs of loss of consciousness, potentially increasing reliability and effectiveness. When intravenous induction is followed by volatile maintenance, this advantage is lost and, again, effectiveness is based on statistical probability. Bispectral index (BIS) monitoring is probably the most commonly used pEEG monitor. Only two relevant prospective, randomised, controlled trials, concluded that BIS monitoring was of benefit to high-risk patients, the B-aware study (supported in part by the manufacturer) and the BIS cardiopulmonary bypass study, which was underpowered to detect an effect 11, 12. Although brain function monitors in general may add a certain value in titrating anaesthetic drugs, the low utilisation of these devices found in the NAP5 baseline report still appears to be associated with a lower incidence of awareness with recall than expected. Therefore, it seems that more evidence needs to be produced to promote their use solely to prevent awareness with recall.