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This research could lead to the first personalised method for administering appropriate doses of anaesthetic during operations and potentially reduce the risks associated with being under a general anaesthetic. Professor Hugh Perry, Chair of the Neurosciences and Mental Health Board at the Medical Research Council, said: ‘With the growing use of anaesthetics in the elderly and other at-risk groups, understanding the minimal dose required to induce the necessary level of anaesthesia is hugely important.’
The work was carried out at the centre for Functional Magnetic Resonance Imaging of the Brain (FMRIB) and was funded by the Medical Research Council, the Wellcome Trust, International Anesthesia Research Society and the National Institute of Academic Anaesthesia. The results are published online today in the journal Science Translational Medicine.
The research team administered a standard anaesthetic over an extended period of time to 16 subjects. They exposed the subjects to different kinds of stimuli, while recording the brain’s electrical activity using electroencephalography (EEG). They found that there were common patterns of behaviour among the group as they lost consciousness – but that these behaviours happened at different times unique to each individual. Beyond the stage where the subjects stopped responding to standard external stimuli, they all reached a state where the slow-wave activity in the brain stayed the same, even as higher doses of anaesthetic were given.
The researchers also identified a relationship between slow-wave saturation amplitude and grey matter volume in the frontal parts of the subjects’ brains. Using a combination of EEG and functional magnetic resonance imaging (fMRI), the team measured slow-wave brain activity in the group. The fMRI data helped reveal that at the point of ‘saturation’ of the slow-wave activity, the brain became ‘isolated’ from the external world. In other words, the brain regions that would usually be expected to respond to standard stimuli were no longer activated. This indicates that there is an optimum depth of anaesthesia producing perception loss beyond which further anaesthetic is not needed.
Professor Irene Tracey, Director of FMRIB and senior author of the paper, said: ‘Despite the hundreds of thousands of anaesthetics administered daily to patients, remarkably there is no robust, individualised indicator of perceptual awareness available. While we can indirectly gauge whether a patient physically responds to their environment, this imaging method offers a much more nuanced approach. The next stage in the development of this method will be to see if it is similarly illuminating when we monitor anaesthetised patients undergoing surgery.’