Statistical Analysis
Statistical analyses were performed using SAS version 9.4 (SAS Institute, Inc., Cary, NC., USA). For the two surgical approaches, baseline and postoperative outcomes were analyzed using frequency tables and histograms. Continuous variables are reported as mean and standard deviation if normally distributed, or as median and interquartile range if not normally distributed. Continuous variables were compared between the two groups using either two sample independent t-tests or the Wilcoxon rank sum test, depending on normality. Categorical variables were compared using the Chi square or Fischer’s exact test, depending on expected values.
Unadjusted survival between groups was compared by Kaplan-Meier methods and comparisons made between groups by the Log-Rank test. Death was accounted for in models of cumulative incidence of mitral reoperation and progression of MR as described in the Fine-Grey Model [15,16].
Multivariable logistic regression was performed to determine which preoperative variables were associated with a minimally invasive surgical approach. A minimally invasive surgical approach was modeled as the dependent variable and the following independent variables were included in the model based on the univariate analysis, the medical literature, and biologic plausibility: age, race (white vs. non-white), previous cardiac surgery (yes vs. no), need for concurrent cardiac procedure (yes vs. no), location of MV leaflet involvement on preoperative echo (anterior vs. posterior vs. bileaflet), MV lesion etiology (degenerative vs. non-degenerative), and diabetes (yes vs. no). The C-statistic and Hosmer and Lemeshow goodness-of-fit test were reported for the model.
To estimate the impact of surgical approach on the outcomes of interest, the multivariable logistic regression model was used to calculate a subject’s propensity for having a minimally invasive surgical approach. Propensity scores were then entered in to a separate Cox proportional hazard regression model to calculate adjusted hazard ratios for each outcome. Statistics with a p value less than 0.05 were considered statistically significant.