Primary Care Clinical Sciences, School of Health and Population Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. d.mccahon@bham.ac.uk. Primary Care Clinical Sciences, School of Health and Population Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. Critical Care and Perioperative Medical Research Group, Queen Mary University of London, Mile End Road, London, E1 4NS, UK. School of Medical and Dental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.