Continuous brain monitoring called Bispectral Index monitoring or BIS for short, is a relatively new medical technology that can reduce common side effects of anaesthesia. I have asked Dr Stephen Rous, specialist anaesthetist in Cape Town to enlighten us about the BIS technology and how it works.
Medical technology has been a little slow to get on the bandwagon. Consider the stethoscope. It is used to listen to breath sounds and the heart sounds. It was developed in 1816 in France by René Laennec. The invention was born out of necessity – he was uncomfortable placing his ear on the chest of a women when listening to her breathing and beating heart. (Excuse my stereotyping, but isn’t that surprising coming from a Frenchman? Nonetheless that is the story of the stethoscope).
FROM OLD TO NEW
Another traditional practice, introduced in 1937, is still used to determine level or depth of anaesthesia. It is called the Guedels classification. Anaesthetists have used physiological variables such as heart rate and blood pressure, along with the measurement of the expired level of anaesthetic gas to estimate the level of anaesthesia.
Advances in monitoring brain activity and clever algorithms have helped medical science develop a new monitor – the Bispectral Index (BIS) . This is the first monitor to directly use brain activity as a measure of anaesthetic depth.
The technology is based on monitoring brain activity using an electroencephalogram. This tongue-twister is thankfully abbreviated to EEG. By playing a few sticky electrodes onto the forehead a reading of the brain’s electrical activity can be taken and displayed as a tracing. An EEG is regularly used to study the electrical brain activity in patients suffering from epilepsy and in sleep studies.
THE DEVELOPMENT OF THE BIS
The Lebanese-born brain child of the BIS monitor, Nassib Chamoun became fascinated by EEGs in 1985, and while at Harvard he collected EEG readings from subjects during various levels of anaesthesia. Using bispectral analysis, which finds underlying patterns in waveforms, he decoded the masses of data gained from his EEG readings. After 25 years of work and millions of dollars of venture capital, he developed a proprietary algorithm to determine the depth of anaesthesia. The BIS monitor was cleared for use in 1996 for assessing the hypnotic effects of general anaesthetics and sedatives by the US Food and Drug Administration (FDA).
The BIS monitor gives a range of readings from 0 to 100 (which represents coma to fully awake). A reading between 40 and 60 indicates an appropriate level for general anaesthesia. And in this range a patient will not experience awareness – which is the unpleasant state of being awake but paralysed during surgery.
BIS IS NOT UNIVERSALLY USEFUL
In an interview in Time magazine, Chamoun called the BIS monitor “anaesthesia’s Holy Grail”. But it is not without controversy. And is not universally useful.
Firstly it does not seem to work well in children. Because the EEG data and BIS algorithm was based on readings in healthy adults, and so it cannot automatically be extrapolated to young children.
Some people have divergent readings that do not correspond at all with their level of anaesthesia. It also does not work well while using certain anaesthetic agents such a Ketamine which seem to interfere with readings.
The real benefit in using BIS monitoring say Dr Rous “is that it reduces the amount of anaesthesia needed”. This is especially useful in lengthy microvascular reconstruction cases where up to 30% decrease in dose of intravenous anaesthesia is given.
The obvious benefits of his “less-is-more” philosophy is that patients have fewer negative effects from anaesthesia. Nausea and vomiting is one of the most unpleasant side effects of anaesthesia which is positively influenced by using the BIS monitor. And titrating anaesthesia to level of anaesthesia allows patients to wake up smoothly and comfortably just as the surgeon has placed the final stitch.
Get in touch with Dr Stephen Rous if you have any questions