Monitoring depth of anaesthesia by EEG

Depth of anaesthesia is a dynamic balance between effect-site concentration of hypnotic and analgesic drugs, and intensity of surgical stimulation. Awareness during surgery is one of the most feared complications of anaesthesia but excessive depth of anaesthesia is equally undesirable, given the narrow therapeutic index of anaesthetic drugs and the reduced physiological reserve of many patients. The quoted incidence of awareness with explicit recall in noncardiac and non-obstetric surgery is 0.07–0.2%, increasing to 0.4% for emergency Caesarean section performed under general anaesthesia and 1.1–1.5% for cardiac surgery. Awareness refers to the capacity to perceive and process incoming sensory information and thus be cognisant of one’s environment. In the context of anaesthesia, it is often defined as the ability to respond purposefully to intra-operative stimuli, and is usually assessed by the ability to obey commands. This implies processing and understanding of sensory input, as distinct from reflex movements which are mediated at a spinal or brainstem level. Such a definition is useful when attempting to quantify the occurrence of apparent intra-operative awareness but suffers the obvious drawback that it cannot be used in paralysed patients. Anaesthetists have traditionally relied upon signs of autonomic stress (tachycardia, hypertension, lachrymation, sweating) to indicate inadequate depth of anaesthesia in paralysed patients but these are neither sensitive nor specific markers, either in isolation or when combined in a scoring system. Patients may have spontaneous or prompted memory for intra-operative events. Such memory is said to be explicit. ‘Recall’ is synonymous with explicit memory. Implicit memory describes a process whereby patients may have no conscious recollection for intraoperative events but may nevertheless demonstrate evidence of intra-operative memory processing at a subconscious level. It is suggested that this can have a deleterious effect on behaviour, emotions and thought processes, although the patient does not connect any such disturbances to a memory of intra-operative awareness. The distinction made between intra-operative awareness and postoperative ability to remember being aware raises the contentious issue of which is more important. In studies using the isolated forearm test, patients who have clearly been able to respond to commands intra-operatively may subsequently demonstrate neither explicit nor implicit memory for intra-operative awareness. As the brain is the target organ of drugs that produce general anaesthesia, it is logical that changes in neurophysiological parameters might provide indicators of anaesthetic depth. The characteristics of the ideal depth of anaesthesia monitor are listed in Table 1. This article describes the EEG techniques available for monitoring depth of anaesthesia, highlighting their strengths and shortcomings.