Annemarie Docherty1, John Emelifeonwu2, Peter J D Andrews3. 1. Department of Anaesthesia, Critical Care and Pain Medicine, University of Edinburgh, Chancellor's Building, Little France, Edinburgh, United Kingdom. 2. Department of Neurosurgery & Centre for Clinical Brain Sciences, University of Edinburgh & NHS Lothian, Western General Hospital, Edinburgh EH4 2XU, United Kingdom. 3. Centre for Clinical Brain Sciences, University of Edinburgh & NHS Lothian, Western General Hospital, Edinburgh EH4 2XU, United Kingdom.