M L Fabritius1, O Mathiesen2, J Wetterslev3, J B Dahl4. 1. Department of Anaesthesiology, Centre of Head and Orthopaedics, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark. 2. Department of Anaesthesiology, Zealand University Hospital, Koge, Denmark. 3. Copenhagen Trial Unit, Centre for Clinical Intervention Research, Copenhagen University Hospital, Rigshospitalet, Copenhagen, Denmark. 4. Department of Anaesthesiology and Intensive Care Copenhagen University Hospital, Bispebjerg and Frederiksberg Hospitals, Copenhagen, Denmark.