Memory, Awareness and Intravenous Anesthetics

Abstract

It is clear that there are increased risks of accidental awareness under general anaesthesia when a total intravenous anaesthetic or even target-controlled infusion technique is being used (Pandit et al., Br J Anaesth 113:540–548, 2014; Pandit et al., Br J Anaesth 113:549–59, 2014) especially in the presence of neuromuscular blockade. Typically, the problems include compromise of infusion patency due to tube kinking, infiltration at the cannula site, underdosing or even frank human error such as failure to turn on the pump, connect the pump or drug administration lines or incorrect pump programming. In contrast to volatile inhaled anaesthetic techniques, where end-tidal agent concentration monitoring is available, for TIVA, there are no indicators of adequate dose or alarms which alert the anaesthetist to a problem. It is therefore imperative that the anaesthetist regularly checks the infusion line and ensures that it is visible. The cannula site should also be checked frequently to ensure patency of infusion. The pump itself also needs to be checked throughout that it is actually delivering the infusion and the correct programme parameters have been selected. In addition, it is a National Institute for Health and Care Excellence (NICE UK) recommendation that some additional technique for monitoring depth of anaesthesia is employed, especially when neuromuscular blockade is included (NICE, depth of anaesthesia monitors—bispectral index (BIS), E-Entropy and Narcotrend-Compact M, 2012). In most cases, this will be a processed EEG monitor such as the bispectral index (Medtronic, Minneapolis, USA). The Royal College of Anaesthetists’ National Audit Project 5 (Accidental Awareness during General Anaesthesia) expert panel also recommends the use of the isolated forearm technique, despite the general reluctance on the part of the profession to use this (Pandit et al., Br J Anaesth 113:549–59, 2014). However, the IFT remains the only direct and reliable indication of mental state and consciousness during a general anaesthetic when a muscle relaxant is administered. It is relatively simple to implement, making use of equipment that can be found in all operating theatres and anaesthetic rooms, and is inexpensive. Nevertheless, there are many myths about the IFT which give rise to a lack of enthusiasm for the technique (Russell, Anaesthesia 68:677–688, 2013). These include the fallacies that it cannot be used for more than 20 min, it does not correlate with post-operative patient interview, and it cannot be used when both arms are needed for surgical or anaesthetic purposes. Another important obstacle for many is concern about losing face in front of surgical colleagues: shouldn’t the competent anaesthetist know that their patient is unconscious without having to interrogate the patient on the table? A simple way to overcome this concern is to use a recurrent pre-recorded message supplied through headphones using a digital recorder, thus making verbal commands inconspicuous. Nevertheless, it is always important to observe the tourniqued limb throughout to identify movement. NAP5 has also emphasised the importance of the use of a nerve stimulator to monitor paralysis.