Background Defibrillation of a shockable rhythm improves patients’ chance of survival following cardiac arrest. Evidence is limited regarding optimal shock energy level and delivery strategy. Most studies are based on old resuscitation regimes and use a variety of endpoints, making meaningful comparison difficult. The POSED study seeks to determine whether it is feasible to conduct a randomised, pragmatic clinical effectiveness trial in UK ambulance services to identify the optimal energy for defibrillation.

Methods This is a pragmatic, cluster-randomised, open label, controlled trial. Defibrillators within a single UK Ambulance service will be randomised to one of three groups delivering shock strategies in current UK use. Adult (≥18 yrs) patients will be eligible if they sustain a cardiac arrest in the out-of-hospital environment that requires defibrillation. Defibrillators will be randomised to deliver one of the following strategies to 90 participants (30 in each group): 120-150-200J, 150-200-200J, 200-200-200J. Relatives of non-survivors of trial participation will be actively informed. Focus groups will explore the experiences of paramedics recruiting patients. The primary feasibility outcome is the proportion of eligible patients who received the randomised study intervention.

Results Baseline variables will be reported. The secondary feasibility outcomes to be reported are the recruitment rate, rate of adherence to allocated treatment, data completeness of clinical outcomes and issues identified by ambulance staff with suggestions for study optimisation. Clinical outcomes are Return of an Organised Rhythm (ROOR) at 2 minutes post-shock, refibrillation rate, Return of Spontaneous Circulation (ROSC) at hospital handover, survival and neurological outcome at 30 days.

Conclusions The POSED study will assess the feasibility and required sample size of a large-scale trial and will explore opportunities to optimise the trial protocol.