2.3. Statistical Analysis

Data are presented as absolute numbers (percentages), mean values ± standard deviation (SD), or medians and interquartile ranges (IQRs) [IQR25–IQR75], where appropriate. Statistical analysis was performed using the Student’s t-test for continuous data with a normal distribution or the Mann–Whitney test for non-parametric comparisons between groups. Spearman’s correlation test was used to evaluate univariate correlates of a given variable. Univariate and multivariate Cox proportional hazards regression analyses were performed to identify independent variables associated with the endpoint. In the multivariate Cox analysis, RV–arterial coupling values were multiplied by ten to enhance the clinical significance of 0.1 variation. Optimal cut-off values for RV coupling associated with the outcome were calculated by time-dependent receiver–operator characteristic (ROC) curves by calculating the area under the curves. Kaplan–Meier analysis was used to compare the differences in survival and was implemented with the log-rank test. The cumulative survival time was calculated as the time from the date of the first observation (first diagnosis) to the date of death or censorship on the date of the last follow-up visit. The null hypothesis was rejected at p ≤ 0.05. Statistical analysis was performed with SPSS version 21 (SPSS Inc., Chicago, IL, USA).