Data analysis
The main endpoint of the study was any episode of ventricular tachycardia (VT) or ventricular fibrillation (VF) during the study period (see Supplementary Material).
Programming of detection zones, counters, and discrimination algorithms were set in any patient according to indication, routine practice and patient’s history. Device detected ventricular episodes labelled as VT or VF were adjudicated by two expert cardiac electrophysiologists; in case of discordance a third electrophysiologist was asked to classify the event and the episode was labelled according to majority vote. Episode data were categorized into binary variables indicating the presence or absence of a VT, VF, treatment with ATP or with shock during the study period. Data analyses were conducted separately for VA episodes self-terminating and/or treated with ATP or with at least one shock.
Descriptive statistics for continuous variables were reported as mean (standard deviation) and binary variables as percentages. The event rate was calculated as the number of patients having an event in the study period divided by the total number of patients. Event rate in the study period was compared to that of the reference period using chi-squared test. The mean number of events in the study period was calculated as total number of events divided by the total sample size; comparison between periods was carried out by t t-test. Univariate and multivariate logistic regressions were conducted in order to examine the associations between variables of interest and the occurrence of an arrhythmic event. For all tests, a significance level of 0.05 was used. All analyses were performed using the Stata software version 13.0 (StataCorp, Texas, US).