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Background: Shock synchronized to the QRS-complex is essential to avoid cardioversion induced ventricular fibrillation. Several defibrillators are used in clinical practice, but little is known about their ability to synchronize shocks in real life use. In a randomized study, comparing two defibrillators, a major adverse event occurred and was related to synchronization failure of one device. Aim: To investigate if shocks are delivered asynchronous during synchronized cardioversion. Methods: A post-hoc analysis of a randomized study enrolling patients admitted for elective cardioversion was performed. Patients were randomized to receive cardioversion by LIFEPAK 20 (Physio-Control Inc., Redmond, WA, USA) or Schiller Defigard 5000 (Schiller AG, Baar, Switzerland). All shocks were delivered with the defibrillators in synchronization modus. Post-hoc, we investigated the ability to synchronize, i.e. the delay between the R-wave and shock delivery, by reviewing electrocardiographic shock recordings. According to the Association for the Advancement of Medical Instrumentation and the American National Standards Institute, a synchronized shock should be delivered within 60 milliseconds after the R-wave. Results: A total of 134 patients were randomized; 65 patients received cardioversion by LIFEPAK 20 (mean age: 67 years, 14% atrial flutter), and 70 patients Schiller Defigard 5000 (mean age: 66 years, 14% atrial flutter). Overall, 138 shocks were delivered by LIFEPAK 20, all of them delayed more than 60 milliseconds (range: 80-200 milliseconds). All LIFEPAK 20 shocks were however delivered between the R-wave and the T-wave upstroke. In total, 193 shocks were delivered by Schiller Defigard 5000. Of these, 169 (88%) shocks were delivered within 60 milliseconds (range: 0-450 milliseconds), 180 (93%) between the R-wave and the T-wave upstroke, and 13 (7%) in the vulnerable part of the T-wave. Two of these shocks resulted in malignant ventricular tachycardia. Conclusion: Synchronized shocks in cardioversions are not uncommonly delivered asynchronous, including in the vulnerable phase of the T-wave. Of these shocks, two resulted in cardioversion induced malignant ventricular tachycardia.