Depth of anaesthesia
The discovery of anaesthesia transformed the human condition, and unplanned awareness returns a patient to the nightmare that was surgery before anaesthesia and analgesia. Significant advances in the pharmacology and technology of anaesthesia have still not brought us much closer to a reliable means of monitoring its depth, although because awareness is such a serious complication considerable research effort has been dedicated to the search for methods of detection. Many remain research tools or are not yet in widespread use, but you should have some idea about which may in due course find their way into clinical practice.
You may be asked about the types of awareness under general anaesthesia, the commonest causes, patients particularly at risk, and possible sequelae.
Definitions: awareness can be ‘explicit’ or ‘implicit’. Explicit awareness is defined by spontaneous or prompted recall of intraoperative events, which may or may not include pain. Its accurate incidence is hard to determine, but commonly quoted figures are up to 0.2% in non-obstetric and non-cardiac anaesthesia, up to 0.4% in emergency caesarean section under general anaesthesia, and up to 1.5% in cardiac surgery. (The Royal College of Anaesthetists patient information leaflet quotes an incidence of 0.1–0.2%.)
Causes: its causes lie in equipment and its (mis)use, in pharmacology and its application, and, very rarely, in the physiology of patients.
Equipment and apparatus: awareness may result from a failure of the apparatus to deliver adequate concentrations of anaesthetic agent. The anaesthetic machine must deliver an accurate fresh gas flow via an appropriate breathing system using a vaporizer.