Oxford Meeting - November 2002

Monitoring Anaesthetic Depth - Auditory Evoked Potentials

Dr Harris Mantzaridis, Glasgow

A simple definition for general anaesthesia is the drug induced state that includes hypnosis, analgesia, muscle relaxation and attenuation of autonomic responses. Depth of anaesthesia is much more difficult to define. Loss of consciousness is quantal. It is an all-or-none phenomenon, since it is a threshold event. However, there is a graded response thereafter. This response varies with relative balance of hypnosis and analgesia on one side and surgical stimulation on the other.

When muscle relaxants are used, an indication of the depth of anaesthesia is given by a combination of autonomic responses. The deepest planes are defined by circulatory depression. However, there is no clear upper limit, since the patient cannot move. If for any reason anaesthesia becomes lighter, the patient may regain consciousness without necessarily the anaesthetist noticing it.

Awareness under general anaesthesia can cause significant morbidity to the patient and is considered indefensible. An anaesthetic depth monitor is, therefore, highly desirable.

Any anaesthetic depth monitor should provide similar values for different anaesthetic agents at equipotent doses. The values at induction of anaesthesia should be similar to those recorded at recovery. It should alter its signal appropriately during surgical stimulation. It should be unaffected by small temperature changes, cardioactive and vasoactive drugs, hypotension or tachycardia. Finally it should provide reliable, reproducible, unambiguous results with an obvious transition from consciousness to unconsciousness and vice versa with no awareness and recall of events during anaesthesia.

Auditory Evoked Potentials (AEPs) are the electrical responses of the brain to auditory stimuli. They correlate well with depth of anaesthesia but are difficult to acquire and analyse automatically. An index, the Auditory Evoked Potential Index (AEPex) has been proposed. This index is derived from the AEP and fulfills most (if not all) of the criteria of an ideal anaesthetic depth monitor.

The AEPex has been used in several clinical studies as the input signal to a Closed-Loop Anaesthesia system and to derive the Effect Site Kinetics of propofol. It also provided an unbiased measurement in many research projects including comparison of the pharmacokinetic and pharmacodynamic effects of propofol, the propofol-sparing effect of nitrous oxide during surgery and the propofol-sparing action of local blocks. Future projects will use the system to compare volatile with intravenous anaesthesia and to assess different anaesthetic techniques such as the cardio-respiratory effects of different intraoperative analgesics.