BackgroundSimulation training in cardiopulmonary resuscitation (CPR) is effective but active practice time is limited given the large number of students and the learning effect size remains small. To improve learning during observation, the use of an observer tool (OT) has been advocated. The aim was to assess the value of OT to improve medical students' learning outcomes during CPR simulation training.MethodsThis prospective, randomized study took place during CPR training of medical students. The workshop targeted recognition of unconsciousness, absence of breathing, call for help, cardiac massage and defibrillation. Students practicing in dyads were randomized to use an OT (i.e., a checklist summarizing CPR skills and step-by-step actions) (OT +) or not (OT-) when observing others. At the end of the training, the global performance of the dyad was assessed by an evaluator using the OT checklist (primary outcome). The non-technical skills (NTS), chest compression quality, perceived improvement in knowledge and skills and knowledge score (MCQ) were also recorded.ResultsThe student dyads were included (OT + : n = 40 and OT-: n = 41). Immediately after training, the global performance was similar between the two groups: OT + : 24 [23—25] and OT-: 23 [21—24] (out of 25), p = 0.052. However, better learning of breathing assessment and cardiac massage performance, as well as a better knowledge score, were found in the OT + group. No significant difference was observed for NTS or perceived improvement in knowledge and skills. Satisfaction was higher in the OT- group.ConclusionsThe use of an OT during CPR simulation did not show any pedagogical benefit on the global performance of medical students. However, a potential benefit was found for several important secondary outcomes. Further studies are needed to confirm these positive results.