BackgroundVentricular arrhythmias (VA) after atrial fibrillation (AF) electrical cardioversion (ECV) have been reported. ObjectiveAssess incidence, timing, and clinical characteristics of patients with post-AF ECV-related VAs. MethodsMulticenter observational retrospective study including 13 centers, incorporating patients with VA or sudden cardiac death (SCD) within 10 days of ECV. Total number of ECVs performed during the collecting period was provided. Patients with pre-ECV VA were excluded. ResultsTwenty-three patients with VAs were identified out of 11897 AF ECVs performed in 13 centers during a median 2-year period, suggesting post-ECV VA incidence of 0.2%. Patient’s age was 71+11 years, 13 (56.5%) females. AF duration prior to ECV was 71+54 days. Congestive heart failure (CHF) and hypertension were both found in 17 (74%) patients. QT prolonging drugs were used by 17 (74%). Index VA occurred 28.5 [5.5,72] hours post-ECV, including Torsades de Pointes, non-sustained polymorphic ventricular tachycardia, and sudden cardiac death in 17 (74%), 5 (22%), and 1 (4%) patient, respectively. Post-ECV heart rate was slower and QT duration longer compared to pre-ECV (57+11bpm vs. 113+270bpm; p<0.001; QT 482+61ms vs. 390+60ms; p<0.001). VA reoccurred in 9 (39%) patients, 11 [3,13.5] hours post-index VA. Two patients had an arrhythmic death within 72 hours post-ECV. ConclusionVA post-AF ECV are rare, occur within 3-72 hours post-ECV, and are potentially fatal. Our study gives a ‘signal of caution’ favoring prolonged monitoring in small subset of patients as CHF patients treated with class III AAD, with post-ECV bradycardia, especially (but not exclusively) when QT prolongation noted.