Statistical analysis
The data are expressed as median (interquartile range). Survival probabilities were evaluated using the Kaplan-Meier method. The Mann–Whitney U-test was used to compare continuous variables between the PVO and non-PVO groups; an effect size greater than 0.50 was considered a large difference. The χ2 test was used to compare the categorical data between the two groups. The overall accuracy of the potential variables in predicting postoperative PVO was summarized using the area under the receiver operating characteristic (AUROC) curves. The optimal cut-off values of the postoperative V-A angle and ratio to predict postoperative PVO were determined by the Youden index. Differences were considered statistically significant if the P-value was <0.05. All data were analyzed using IBM SPSS Statistics, Version 24.0 (IBM Corp, Armonk, NY).