Tutorial of the week update in anaesthesia




















One way to try and reduce the incidence of awareness is to use some form of depth of anaesthesia monitoring. For most anaesthetists, this takes the form of relatively continuous monitoring of physiological variables — requiring the continuous presence of an anaesthetist. This can be increased by the use of monitoring of end-tidal concentrations of inhaled anaesthetic agents.

Despite the fact that MAC refers to a motor response, it is widely accepted that the risk of awareness is dramatically reduced if a patients are exhaling 0. Anaesthetic agents suppress the normal EEG pattern is a dose related manner.

A conventional EEG is complex and requires specialist interpretation. One of the most well known is the BIS Bi-spectral Index, Aspect Medical monitor, which calculates and presents a single number on a scale of 0 to no activity to an awake patient. This number corresponds to the hypnotic state of the patient but not to motor responses to stimuli which are partially mediated at a spinal level.

Whilst processed EEG devices monitor cerebral function — which may be more closely related to awareness than MAC — there are limitations. At present, the devices are not widely available in many institutions in the UK. As with all electrical signals, interference may occur, particularly in the electrically noisy theatre environment.

The EEG is not suppressed by nitrous oxide whilst ketamine increases electrical activity. In addition, BIS has been marketed as a tool which enables lower usage of anaesthetic agents, which in turn may lead to faster wake up times with shorter and better quality recovery stays and subsequent cost reductions 11, As yet, this has not been shown to translate into other outcomes such as a faster discharge from hospital.

Some have expressed concern that the use of such monitoring may encourage lighter anaesthesia and actually increase the risk of awareness.

So does this sort of monitoring actually reduce awareness? There are many studies in the literature with varying conclusions. Some fail to detect any awareness at all whilst others demonstrate a reduction in awareness with depth of anaesthesia monitoring An interesting recent publication randomised high risk patients to receive either BIS guided anaesthesia targeted range between 40 and 60 or end-tidal guided anaesthesia maintained above 0.

The study showed an identical incidence of awareness in both groups leading the authors to suggest that their findings did not support the use of BIS monitoring in standard practice. Evoked electrical potentials have their uses in medicine. It is possible to detect nervous system activity as a result of central stimulation by measuring auditory or somatosensory potentials but these are little affected by anaesthetics and they are more commonly used to detect nervous system tissue integrity during spinal or neurosurgery.

The isolated forearm technique, measurement of oesophageal motility and forehead galvanometry are experimental techniques that are relatively unreliable and are now confined to history. Anaesthetic awareness is a rare but potentially devastating complication of general anaesthesia. It may not be possible to prevent every case from happening, but it is possible to identify high risk cases and modify anaesthetic technique accordingly.

For all other cases, prevention of awareness requires meticulous attention to detail to eliminate errors. Whilst depth of anaesthesia monitors that process EEG signals may help to reduce the incidence of awareness, they are not widely used in this country and the evidence is still not conclusive.

This does not mean that they are not useful but it may be that a strict protocol driven anaesthetic delivery may be as good. Get notified when a new tutorial is published! Tutorial of the Week: Awareness under Anaesthesia Dr. PDF Download. Offline Reading. Available Offline. Tutorial Outline. Published November 30, Learn More. Loading Tutorials.

PDF Download. Take Quiz. Offline Reading. Available Offline. No tutorials match your criteria. We are pleased to say that the volume and interest in submitting articles to ATOTW is steadily growing and consequently the need for further editors is increasing. We invite anaesthesiologist around the globe to submit their interest and intent in supporting this project.

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