Awareness During General Anaesthesia

Awareness during general anaesthesia is still a remarkable problem in modern anaesthetic practice. Fortunately, the incidence of awareness with explicit recall of severe pain is low, with a frequency of fewer than 1 event in 3000 anaesthetic episodes. For modern routine anaesthesia the incidence of explicit recall of intraoperative perception is only slightly higher, at about 1 event in 1000 procedures. General anaesthesia for caesarean section, cardiac operations, acute trauma management or bronchoscopy seems to carry a higher risk of intraoperative awareness. The incidence of subconscious awareness with implicit recall of auditory perception, and of amnesic but conscious awareness, is methodologically very hard to assess but can be as high as 100%, depending on the anaesthetic regimen used. The highest incidence of intraoperative awareness is associated with the use of specifically acting, mostly receptor-mediated drugs, such as opioids, benzodiazepines or the weak anaesthetic nitrous oxide, given alone or in combination. In contrast, volatile anaesthetics such as halothane, enflurane, isoflurane, sevoflurane and desflurane, as well as potent intravenous anaesthetics such as methohexital, thiopental, etomidate and propofol in appropriate concentrations, successfully block any intraoperative perceptions. Volatile anaesthetics offer the advantage that the concentration of the drug is easily controlled by monitoring end-expiratory gas concentrations. On the other hand, sophisticated dosage strategies for the use of intravenous agents, for example propofol and alfentanil, offer the chance to achieve an incidence of awareness during total intravenous anaesthesia as low as that seen for volatile anaesthetics. Constant attention to the possibility of awareness, along with an appropriate monitoring of the depth of the anaesthesia, helps to avoid awareness. Although observing clinical signs is the most common method, it is a very poor indicator for intraoperative awareness. Electrophysiological measurements can help to guide the anaesthesiologist’s decisions. Derived parameters of the raw electroencephalogram, such as the spectral edge frequency, may help to assess relevant individual pharmacokinetic or pharmacodynamic properties for the anaesthetics used. Midlatency auditory evoked potentials seem to be a promising measure for assessing depth of anaesthesia, although further studies are needed to confirm their clinical usefulness. Any instance of intraoperative awareness should be treated sympathetically and seriously by the responsible anaesthesiologist. Aid should be provided to the patients to minimise long lasting psychological sequelae. In the worst case a post-traumatic stress disorder syndrome may develop, requiring intensive psychological or even psychiatric treatment for the patient.