Some of the patients affected by Out-of-hospital cardiac arrest (OHCA) with ventricular fibrillation (VF)/ventricular tachycardia (VT) do not respond to initial defibrillation. The survival decreases with number of defibrillations required to terminate VF/VT. In 2022, one prospective cluster randomized trial showed increased survival among (OHCA) patients in refractory VF using Double Sequential Defibrillation (DSD).

To evaluate feasibility and safety this randomized pilot trial will compare the effect of double defibrillation strategy initiated as soon as possible after the first defibrillation with continued resuscitation using standard defibrillation, in patients with Out of Hospital Cardiac arrest (OHCA). The results from this pilot trial will form the basis for design of a larger multicenter survival study.

The Dubbel-D study is an academic, investigator initiated, open-label pilot study with a randomized controlled trial (RCT) design conducted in the prehospital emergency medical services, i.e. ambulance organizations. The trial will be conducted by participating ambulance units attending OHCA´s. These units will perform screening for inclusion, randomization, intervention or control treatment and initial follow-up.

In all cases of OHCA a defibrillator should always be attached with the standard pad placement, anterio-lateral (A-L) position first. This is in accordance with standard of care. If there is VT/VF or an automated external defibrillator (AED) suggests defibrillation, defibrillation should be performed, and immediate chest compressions resumed. Thereafter, the patient can be screened for inclusion. If two study specific defibrillators (Corpulse 3) on site and no exclusion criteria (age below 18 years, obvious pregnancy, known preexisting Do Not Attempt Resuscitation order) the patient can be included and randomized.

Randomization will be performed by drawing a scratch-card with concealed allocation that will be stored with the EMS defibrillators. All scratch-cards will be pre-randomized in a 3:1 ratio in blocks consisting of 4-8-12 and stratified by region and ambulance provider.

If the patient is randomized to the intervention group, the ambulance crew team will apply the second defibrillator with electrodes placed in the anterio-posterior (A-P) position as soon as possible. Defibrillation is performed by one person defibrillating both defibrillators in a sequential manner "Double Sequential Defibrillation" (DSD). All consecutive defibrillations will thereafter be performed with the DSD strategy until ROSC, termination of resuscitation or decision to move the patient to hospital.

If randomized to the control group, the ambulance crew team will continue Advanced Life Support (ACLS) in accordance with standard of care. Defibrillation is performed with standard electrode placement (A-L position) using a single defibrillator. If an AED is the first defibrillator attached to the patient, the ambulance crew should shift from an AED to their own manual defibrillator, but the mode of defibrillation should remain in A-L position and only one defibrillator should be used for each defibrillation and continue until ROSC, termination of resuscitation or decision to move the patient to hospital.

Inclusion Criteria:

• Out-of-Hospital patients with Ventricular Fibrillation/ Ventricular Tachycardia at rhythm analysis after ambulance arrival and at least one defibrillation performed in standard (Antero-Lateral) position

Exclusion Criteria:

• Age below 18 years
• Obvious pregnancy
• Known preexisting Do Not Attempt Resuscitation order