Loss of Consciousness Monitoring
A JOINT STATEMENT BY THE ROYAL COLLEGE OF ANAESTHETISTS AND THE ASSOCIATION OF ANAESTHETISTS OF GREAT BRTAIN AND IRELAND
Establishing depth of anaesthesia has been taught ever since the inception of anaesthesia using physiological assessments which mainly indicate the level of sympathetic stimulation, for example sweating, tear formation, direct observation of the pupil size and changes in cardiovascular measurements. Awareness is very rare in patients who are not paralysed during anaesthesia. The principal concern is in patients in whom neuromuscular blocking drugs are used and therefore the ability to move is lost. The isolated forearm technique evolved to address this problem but, although enthusiastically advocated by some, it is still viewed apprehensively by others because of prolonged arterial occlusion.
Anaesthetists now use a variety of different indicators to monitor depth of anaesthesia, many of which rely upon monitoring more accurately the changes in normal physiological variables such as heart rate and blood pressure. Although variations in these parameters can be associated with variations in the level of anaesthesia, many studies have demonstrated that they are not completely reliable. Accurate monitoring of the level of consciousness and the potential awareness of an anaesthetised patient require different techniques. Volatile agent monitoring, linked to the concept of Minimum Alveolar Concentration (MAC), is employed currently as a reliable measurement for routine use in this area, and yet the incidence of intraoperative awareness has not changed in recent years from about 1-2 per 1000 cases. Indeed, a major study demonstrated the frequency of intraoperative awareness was the same whether or not end-tidal agent monitoring was used during the general anaesthetic. Clearly, agent monitoring is of no value when total intravenous anaesthesia is used.
Understandably, many anaesthetists and patients ask if the more recent techniques which have been proposed for monitoring loss of consciousness should be used routinely.
Specific devices designed to monitor loss of consciousness using adaptations of either surface EEG monitoring or auditory evoked potentials have become available. However, their routine use has yet to be fully considered as part of our recommended minimum monitoring standards. The American Society of Anesthesiologists recently published a report from a task force set up to assess the use of brain function monitoring to prevent intraoperative awareness. This report summarised the state of the literature and reported the opinions derived from task force members, expert consultants, open forums and public commentary. It concluded that “brain function monitoring is not routinely indicated for patients undergoing general anesthesia, either to reduce the frequency of intraoperative awareness or to monitor depth of anesthesia.” It was the consensus of the task force that the decision to use a brain function monitor should be made on a case-by-case basis by the individual practitioner for selected patients. The task force reported that patients may experience intraoperative awareness in spite of monitored values which would imply an adequate depth of anaesthesia.
The AAGBI and RCoA emphasise that close vigilance of all aspects of the patient and anaesthetic equipment together with observations of changes to normal physiological variables, should remain the principal factors in monitoring of awareness. Brain function monitors should be considered in high risk cases. Examples of these would include patients with a previous experience of awareness, general anaesthesia for caesarean section or cardiac surgery and trauma patients undergoing surgery. Technological improvement, together with critical evaluation of the place, value and reliability of new methods of brain function monitoring should be reviewed constantly to ensure the highest quality and safety of anaesthetic care for our patients.
04 Jan 2006