Statistical Analysis
Data analyses were performed using SAS 9.4 software (SAS institute,
Cary, NC). Figures of odds and hazard ratios were prepared using Prism 7
(GraphPad Software, Inc., La Jolla, CA, USA). Data are presented for the
overall cohort, and the two groups stratified by timing of onset of deep
sternal wound infection after index cardiac procedure. Categorical data
were summarized as the number and percentage of patients. For continuous
data, the mean value and standard deviations were reported for normally
distributed data while the median value and interquartile ranges were
reported for non-normally distributed data. Continuous variables were
compared between the two groups using either Student’s t-test or the
Wilcoxon rank sum test, depending on data normality. Categorical
variables were compared using the chi square or Fisher’s exact test,
depending on expected values. Unadjusted survival was assessed by
Kaplan-Meier methods for the overall cohort and various strata. Between
strata comparisons were made by log-rank. Multivariable Cox proportional
hazard modeling was used to model predictors of death in the overall
cohort.
Multivariable logistic regression was performed to determine which
preoperative variables were predictive of onset of DSWI within 30 days
of index cardiac procedure. Early onset DSWI was modeled as the
dependent variable and the following independent variables were included
in the model based on univariate analysis, the medical literature, and
biologic plausibility: sex, smoking (yes vs. no), previous cardiac
surgery to the index operation for the DSWI (yes vs. no), positive wound
culture at time of initial DSWI operation (yes vs. no), and urgency of
initial DSWI operation (yes vs. no). The C-statistic and Hosmer-Lemeshow
goodness-of-fit test were reported for the model.
To estimate the impact of timing of onset of deep sternal wound
infection after cardiac surgery on subsequent mortality, the
multivariable regression model was used to calculate a subject’s
propensity for having an early onset DSWI. Propensity scores were then
entered into a separate Cox proportional hazard regression model to
calculate adjusted hazard ratios for estimates of mortality.