IntroductionErrors during treatment may affect patient outcomes and can include errors in treatment algorithms, teamwork, and system errors. In-hospital cardiac arrests (IHCA) require immediate and effective treatment, and delays are known to reduce survival. In-situ simulation is a tool that can be used to study emergency responses, including IHCA. We investigated system errors discovered during unannounced in-situ simulated IHCA.MethodThis multicenter cohort study included unannounced, full-scale IHCA in-situ simulations followed by a debriefing based on PEARLS with plus-delta used in the analysis phase. Simulations and debriefings were video-recorded for subsequent analysis. System errors observed were categorized by thematic analysis and analyzed for clinical implications. Errors related to treatment algorithm and clinical performance were excluded.ResultsWe conducted 36 in-situ simulations across 4 hospitals with a total discovery of 30 system errors. On average, we discovered 0.8 system errors per simulation within the categories: human, organizational, hardware, or software errors. Of these, 25 errors (83%) had direct treatment consequences. System errors caused treatment delays in 15 cases, a need for alternative actions in 6 cases, omission of actions in 4 cases, and other consequences in 5 cases.ConclusionUsing unannounced in-situ simulations, we identified almost one system error per simulation, and most of these errors were deemed to impact treatment negatively. The errors affected treatment by either causing delays, need for alternative treatment options, or omitting treatment actions. We suggest that hospitals focus on the need for regular testing of the emergency response by conducting full-scale unannounced in-situ simulations. This should be a priority to improve patient safety and care.