Taking on TIVA . Why we need guidelines on total intravenous anaesthesia

Le résumé fourni par la source

Whatever you think about total intravenous anaesthesia (TIVA), it is essential for us to be able to use this important modality with confidence and safety. Quality of recovery is one of the most obvious, well-known and tangible benefits of propofol-based TIVA, but others are becoming apparent. Patients appear to have less pain than after inhalational anaesthesia 1 and there is less impact on neurocognitive function 2. There may even be survival advantages in cancer surgery 3, with plausible mechanistic support 4. These apparent advantages are, however, still apparently insufficient to sway anaesthetists to adopt TIVA for their patients more readily 5. It is interesting to speculate as to why this may be the case and to determine factors which are preventing adoption 6, but there is no doubt that familiarity and safety concerns are paramount. However, more than 20 years after the introduction of propofol-based TIVA, there continues to be an apparent deficiency in the quality of care when using this technique, as identified by the NAP5 report 5, with TIVA accounting for a disproportionate share of cases of accidental awareness under general anaesthesia. If familiarity breeds contempt, it is unlikely to be the case with propofol-based TIVA as it accounts for only 8% of general anaesthetics 5. Inadequate education and training is still one of the main contributing factors to TIVA-related accidental awareness under general anaesthesia 5. Why is this still the case? The genesis of a TIVA training gap perhaps resides more in the historical than the technical or intellectual realm. When TIVA was first introduced into the clinical arena, delivery by manual weight-based algorithms was cumbersome and potentially inaccurate in regard to dosing. The introduction of target-controlled infusions (TCI) with pharmacokinetic modelling made this much easier, although commercially available TCI pumps were initially restricted to expensive prefilled glass syringes. At the same time, there was a limited choice of affordable and potent short-acting opioids and together this significantly increased the costs of anaesthesia. Anaesthetists of that generation were trained to use inhalational agents and were certainly more familiar with those techniques, while the choice of volatile anaesthetic agents was also improving. Total intravenous anaesthesia was a mysterious world of eccentrics that liked discussing pharmacokinetics and non-linear mixed-effect modelling. Technology for measuring end-tidal anaesthetic agent concentrations had become routine and there were concerns of awareness with TIVA, probably compounded by collective inexperience with the then new technique. This may have prevented the development of critical masses of local experts that are necessary to provide widely available and sustainable training opportunities for TIVA. The adverse conditions described above are also conducive to the perpetuation of misconceptions that further dissuade its use, thus feeding into a negative cycle of collective inexperience leading to lack of training opportunities, circling back to inexperience. Perhaps an even more powerful factor in the development of misunderstandings regarding TIVA is the anchoring effect that learning an inhalational technique first has. There is always the temptation to crowbar an equivalent concept of a minimum alveolar concentration (MAC) when dosing in TIVA but this concept is not directly transferable to TIVA. Minimum alveolar concentration is defined by a response to pain and most inhalational agents have both antinociceptive and anaesthetic properties. However, with TIVA, we control anaesthesia/consciousness with one drug (propofol) and analgesia with another (a titratable opioid such as remifentanil). Propofol is a powerful hypnotic drug but not a conventional analgesic, although it does have activity at hyperpolarisation-activated cyclic nucleotide-gated channel-1 (HCN1) receptors and anti-inflammatory effects which reduce postoperative pain 1. This is an important distinction but one which may also have advantages now that there is increasing concern over the adverse effects of excessively deep anaesthesia 7. It is, therefore, not appropriate and perhaps even dangerous to use propofol on its own to attenuate the stress of surgery. Even titrating the effective concentration of propofol using response to painful stimuli as the goal could result in a far higher dose than necessary, also leading to delayed emergence. Antinociception should be achieved with either opioids, for example, remifentanil, or the use of regional block. Propofol should be given in sufficient quantity to produce loss of consciousness and, having achieved that, generally target concentrations need little adjustment during the course of surgery, whereas the opioid concentration may vary a lot depending on the surgical stimulation 8. TIVA is also a method where induction and maintenance are provided with the same drug, thereby it is one of the only anaesthesia techniques that can be truly personalised, unlike the use of MAC with inhalational agents. In the last 20 years, delivery systems have matured in terms of safety and choice, drug costs have plunged, better opioids have been developed and depth of anaesthesia monitoring is widely available. Intravenous (i.v.) anaesthesia with TCI is relatively easy. The practitioner does not need a detailed knowledge of pharmacokinetics any more than they need to know oil:gas solubility coefficients to administer inhalational agents. These technological advances and changes in market conditions should make it ripe for a ‘renaissance’ in TIVA. Along with the apparent deficiency in training and education, the guidelines by Nimmo et al. from the Society for Intravenous Anaesthesia (SIVA) and the Association of Anaesthetists are timely and apposite as they clarify a number of important safety issues and should help clinicians feel more confident in their practice 9. The guidelines have been produced by expert members of a Working Party established by the Association of Anaesthetists and SIVA, some of whom were also involved in the NAP5 audit where a number of preventable problems with TIVA delivery were apparent. Although there are limitations to consensus opinion 10, there is nothing particularly controversial in these guidelines which are, essentially, good, common sense advice that should be followed. The advantages and use of TCI over manual techniques are emphasised without getting bogged down in the arguments over pharmacokinetic models. Preparation, familiarity with equipment and safe delivery techniques are clearly presented as they are a common cause of awareness. Irrational pharmacological practice such as mixing opioids and propofol together in the same syringe for infusion are explained. There is some discussion of more advanced techniques such as rapid sequence induction. In our experience, this is actually relatively easy as most TCI pumps can calculate patient blood and effect-site concentrations if you use the pump to deliver a manual bolus, should you wish to hasten the onset of effect. It needs to be borne in mind that propofol does have a relatively slow onset compared with thiopentone and we suggest that TCI is actually a logical and safe technique to use with propofol induction. In most cases, pre-oxygenation can be achieved safely under light/moderate propofol sedation when cricoid pressure can be applied and the dose increased to loss of consciousness. A common theme in these guidelines is encouraging the use of processed EEG monitoring. We agree that these monitors are useful, as they are with any form of general anaesthesia, although not essential for TIVA, and should be considered in difficult clinical situations, for example, obesity, where there is difficulty in monitoring the infusion site or when neuromuscular blocking agents are being used. These monitors have limitations 11, not least the dimensionless and non-s

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.