Statistical analysis
The data are presented as a number (percentage) and mean ± standard
deviation. Clinical characteristics between the study groups were
compared by a t-test for continuous variables and chi-squared test for
categorical variables. The first occurrence of RR-NSVT during the
follow-up period was considered as a time-dependent covariate. For
cluster RR-NSVT, the time interval until the occurrence of the third
RR-NSVT in the first cluster episode was taken into account. The first
appropriate ICD therapy was credited to the (cluster or frequent)
RR-NSVT only if it occurred after the first episode to determine whether
each form of RR-NSVT was associated with the following VTAs. Cox
regression analyses were performed to identify associations with the
occurrence of VTAs and were expressed as hazard ratios (HRs) and 95%
confidence intervals (CIs). In the multivariate models, adjustment was
performed for significant outcome confounders identified by stepwise
selection from the baseline variables with the model entry significance
limit set at 0.100. The cumulative incidence of VTA occurrence and
admission for HF and cardiovascular mortality was estimated using the
Kaplan-Meier method and compared using a log-rank test between groups.
All statistical analyses were performed using statistical software (SPSS
for Windows, version 22). A two-sided p-value ≤0.05 was considered
significant.