Statistical analysis
The original statistical analysis plan for the primary trial is reported elsewhere; while this analysis was not pre-planned, it was conducted in accordance with the original analysis plan. Categorical variables are reported as counts and percentages and compared with Fisher exact tests; continuous variables are reported as median (interquartile range [IQR]) and compared with Wilcoxon rank-sum tests. Patients were analysed according to the group in which they were randomised in the original trial, and the analysis dataset included all patients who were randomised and received general anaesthesia for eligible surgery. The quantity of missing data for the primary outcome was small; therefore, only a complete case analysis was conducted, and no assumption for missing data was made.The effect of the intervention on the primary outcome is reported as numbers and percentages with risk differences and 95% confidence intervals (CIs), calculated using a generalised linear model with binomial distribution and an identity link function. All other binary outcomes were analysed identically. The effect of the intervention on the length of ICU stay and hospital stay was estimated with generalised linear models considering a Gaussian distribution.For all outcomes, multivariable analyses were performed using the models described above and incorporating adjustments for independent covariates of age, sex, baseline oxygen saturation (measured by pulse oximetry), body mass index and the Assess Respiratory Risk in Surgical Patients in Catalonia (ARISCAT) score [All continuous variables were standardised to improve convergence—the results represent the change in the outcome according to the increase in one standard deviation of the continuous predictor. All analyses were not adjusted for multiplicity, and the confidence intervals (CIs) should not be used to infer definite differences between the groups. The rate of missing data is shown in eTable 