DoubleD-Trial is an academic, investigator-initiated, open-label randomised trial. The aim is to evaluate whether early double sequential defibrillation (DSD) improves survival compared to standard defibrillation in out-of-hospital cardiac arrest (OHCA). Patients with an initial shockable rhythm, who remain in cardiac arrest after at least one standard shock are screened for inclusion. The trial is coordinated by the Centre for Resuscitation Science, Karolinska Institutet (sponsor). For more information, see the pilot study and the main trial.