This analysis plan has been written while the data collection from the DIFFICAIR trial was still on-going and trial data not accessible. The data analysis of the main publication will follow this plan. The statistical analysis was approved by the DIFFICAIR steering committee on 29 December 2013. The last day of data collection was 31 December 2013. The involved departments were given one additional month to ensure registration of all patients in the Danish Anaesthesia Database. On 31 January 2014 the database was locked and data extracted. The statistical analysis plan was published on www.clinicaltrials.gov before the last data entry and before data was extracted and data management commenced.