Developing slow wave activity saturation as a marker of depth of anaesthesia

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General anaesthesia is delivered to individuals during an operation to stop them being aware of what is going around them, prevent any pain and immobilise the body so surgery can be carried out safely. Unfortunately the anaesthetists, who are the specialist doctors responsible for delivering the anaesthesia, do not currently have a reliable way of measuring the exact point when an individual's brain becomes unconscious during the surgery. They tend to judge the amount of anaesthetic they give depending on when the average person would lose consciousness. They then increase or decrease the dose for that person depending on how their heart or the lungs react during the operation. We believe that we have discovered an interesting change in the brain's electrical activity that indicates the point when an individual person having surgery loses perception of what is happening in the outside world. This potentially means that anaesthetists could give just the right amount of drug for that particular person. This is important because, whilst anaesthesia is very safe, some patients who are older or particularly sick may suffer from long-term side effects if they are given too much anaesthesia. It will also prevent the very rare event that someone is aware during the operation. Our interesting observation is that when the anaesthetic dose is increased, slow waves in the brain reach a maximum level, and then do not increase any further even though much more anaesthetic drug is given. These slow waves are low frequency oscillations in the brain (at around 1 Hz or 1 cycle per second) and are also an important feature of deep sleep. We have called this observation slow wave activity saturation (or SWAS) and it can be measured by applying electrical sensors to the scalp - a technique called electroencephalography (or EEG for short). When we discovered SWAS, we also performed simultaneous brain imaging with a technique called functional magnetic resonance imaging (FMRI), and found that brain's response to pain and words altered dramatically when the individual's electrical activity reached this SWAS level. The brain network activated in response to these stimuli at SWAS was very different to the one that was activated when they were awake, or even the brain network that was activated at lower anaesthetic concentrations. It was this change in how stimuli are processed in the brain that makes us believe that the person is no longer aware of the outside world.We have recently developed a mathematical model that, when applied to an EEG system, can dynamically track the changes in slow wave activity in real-time. The model will allow us to predict when an individual has entered this SWAS state. We hope that, by delivering the anaesthesia to achieve this state, we can make sure everyone who has surgery is unaware of what is going but also doesn't receive too much medication so that it takes them longer to recover. To test this, we plan to use our system in 200 patients having surgery and deliver just enough anaesthesia so that their brain's activity reaches the saturation state. We will then check whether SWAS is a good measure to assess how deeply someone is anaesthetised in two ways. Firstly, we will use a technique called the isolated forearm test before the surgery to confirm that the patient is not aware of what is going on around them. Secondly, we will check how they recover from the operation by measuring how sick they feel afterwards and how much pain they are in. We hope that we can show that patients who receive anaesthesia delivered to the SWAS state have an improved recovery after surgery than patients who have anaesthesia delivered in the usual way. If we can show that this study is a success, we hope that in the long-term we can create a depth of anaesthesia monitor that will enable patients all over the world to be given just the right amount of anaesthetic for their operations.