The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Claire A. Hawkes, Inès Kander, Terry P. Brown, Scott Booth, Abraham Contreras and Chen Ji, and Gavin D. Perkins are employed by the University of Warwick, which receives grants from the British Heart Foundation (BHF) and the Resuscitation Council (UK) for the conduct of the Out-of-Hospital Cardiac Arrest outcomes project. Terry P. Brown is supported by the National Institute for Health Research (NIHR) Applied Research Collaboration (ARC) West Midlands. A. Niroshan Siriwardena currently receives research funding from the NIHR, the Medical Research Council, Health and Care Research Wales, and Health Education England. Rachael T. Forthergill is employed by the London Ambulance Service NHS Trust. Estelle Stephenson is employed by the British Hearth Foundation. Gavin D. Perkins is an Editorial Board Member for Resuscitation Plus and is supported by the National Institute for Health Research (NIHR) Applied Research Collaboration (ARC) West Midlands and Director of Research for the Intensive Care Foundation. The remaining authors have no disclosures to report. The views expressed in this paper are those of the authors and not necessarily those of the NIHR, Department of Health and Social Care, the BHF, or the Resuscitation Council UK.