Oxford Meeting - November 2002

Measuring depth of anaesthesia: Bispectral index

Dr Anthony Absalom, Consultant Anaesthetist, Norfolk & Norwich University Hospital

Why measure anaesthetic depth?

The practise of total intravenous anaesthesia is becoming more popular, but there is no intravenous equivalent of the end-tidal inhalational agent monitor. Accurate prediction of dose requirements is made difficult by large inter-individual variations in pharmacokinetics and pharmacodynamics; and changing stimulus levels during the course of a surgical procedure. Awareness has devastating consequences for patients, hospitals and anaesthetists.

What is anaesthesia?

Logically, we should have had a full understanding of anaesthesia before trying to measure it. Our understanding of anaesthesia remains limited - there are many unanswered questions... Generally we are only able to define anaesthesia in terms of what it isn't. Measuring anaesthetic depth in different ways may help us understand the "anaesthetic" state.

Bispectral Index - background information?

The BIS can be measured using a monitor manufactured by Aspect Medical Systems (Newton, USA). It produces a number between 0 and 100 (100 represents the fully awake state, and zero no cortical activity), based on the weighted sum of three sub-paran1eters. These are derived from time domain-, frequency- and bispectral analysis. Pattern recognition algorithms are used to dynamically alter the weightings.

What properties does BIS have?

The BIS correlates with depth of sedation and anaesthesia, and can predict the likelihood of response to commands and recall. BIS values correlate with end-tidal volatile agent concentrations, and with blood and effect-site propofol concentrations. It is not very good at predicting movement in response to painful stimuli.

What can BIS monitoring do for patients, doctors and hospitals?

BIS monitoring can be used to guide the titration of sedative, analgesic and anaesthetic agents and has been shown to reduce anaesthetic agent doses and costs, reduce recovery and discharge times, and also to improve quality of recovery. It has also proved of value in the study of drug interactions, and may assist in the diagnosis of dementia and brainstem death. BIS has been used as the control variable in automated anaesthetic delivery systems. Three large trials are investigating the crucial issue of whether or not BIS monitoring can reduce the incidence of awareness. The "B-aware" trial may produce the best quality evidence because it is a randomised controlled trial. BIS monitoring can be used to guide the titration of sedative, analgesic and anaesthetic agents and has been shown to reduce anaesthetic agent doses and costs, reduce recovery and discharge times, and also to improve quality of recovery.

Conclusion