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BackgroundDespite defibrillation being proven to increase patients’ chances of survival following out-of-hospital cardiac arrest (OHCA) with shockable rhythm, the best shock energies are unknown. Literature comprises studies with outdated resuscitation protocols and a variety of endpoints. We explored the feasibility of a trial of optimal shock energy for OCHA. Our primary objective was to establish the number of eligible and recruited patients; secondary outcomes were adherence and data completeness.MethodsWe conducted a three-arm parallel group cluster randomised controlled trial in South Central ambulance service. Adult patients in OHCA treated for a shockable rhythm were included. Zoll X series defibrillators (clusters) were randomised to deliver 120–150–200 J, 150–200–200 J, or 200–200–200 J shock strategies.ResultsBetween March 2022 and February 2023, we randomised 38 eligible patients (120–150–200 J (n = 12), 150–200–200 J (n = 10), 200–200– 200 J (n = 16)). The recruitment rate (cluster/month) was 0.07 . Treatment adherence was 93% and completeness of secondary outcomes was 86%. At 30 days, 3/36 (8.3%) patients survived. Due to the limited data storage on defibrillators, the window of opportunity for data collection was short. Data collection was challenging when defibrillators became separated from their allocated vehicles.ConclusionWe demonstrated the feasibility of a cluster randomised controlled trial of optimal shock energy for defibrillation in a UK ambulance service. We identified strategies that would aid data collection. The location of study defibrillators could be monitored using Bluetooth tracking or radio frequency identification (RFID) scanning. Remote upload to a data repository or download to a USB device at the end of each shift would help to ensure that defibrillator data were obtained before being overwritten.