Statistical Analysis
Statistical analyses were performed using STATA Version 15 (Statistical Software, College Station, TX) and R version 3.4.3 (https://www.R-project.org/). Preoperative and operative characteristics and postoperative outcomes were summarized. For continuous variables, comparisons between groups were conducted based on normality with either a two-sample independent t-test or Wilcoxon rank sum test. Categorical comparisons were by Chi square or Fisher’s exact tests. Overall survival analyses were conducted by Kaplan-Meier methods, with log-rank comparison between surgical treatment groups. Cox proportional hazard modeling was conducted to identify pre and intraoperative factors associated with overall mortality. All variables with a univariable p value <0.2 were considered in creating a multivariable Cox proportional hazard model of overall mortality. The independent variable of interest was type of aortic arch repair (hemiarch versus total arch). Model diagnostics included use of Schoenfeld residuals to examine the proportional hazard assumption, martingale residuals to assess linearity, and Cox-Snell residuals to examine overall model fit. Influential observations were identified through use of delta beta and likelihood displacement. The proportional hazard assumption was found to be violated by the “other” category of surgical indication, therefore the final multivariable model is stratified by this covariate. A sensitivity analysis excluding key influential observations, and a separate analysis excluding the “other” category of surgical indication, confirmed the primary findings (see supplemental material). Results are presented as hazard ratio (HR) and 95% confidence intervals. Non-parametric estimates of the cumulative incidence of aortic reintervention were calculated9. Statistical analyses of need for aortic reintervention used competing risks and considered death as a competing outcome and are presented as sub-hazard ratios (SHR) and 95% confidence intervals10. Statistics with a two-sided p value less than 0.05 were considered statistically significant.