Out-of-hospital cardiac arrest (OHCA) is a life-threatening emergency and one of the leading causes of death worldwide. Survival depends critically on how quickly help arrives and whether cardiopulmonary resuscitation (CPR) and defibrillation are started early. In recent years, many regions in Europe and Italy, including Emilia-Romagna, have introduced systems that alert nearby volunteers, called First Responders (FRs), through smartphone applications (such as DAE RespondER). These individuals can reach the patient before emergency medical services (EMS) and begin life-saving actions.

The FIRSST-RER study aims to evaluate whether the intervention of First Responders improves survival in patients with out-of-hospital cardiac arrest in Emilia-Romagna.

This is a multicentre observational study involving approximately 5,000 adult patients who experienced cardiac arrest between 2018 and 2025 and were treated by the regional emergency system.

The study compares two groups of patients:

those who received help from at least one First Responder activated via the app those who did not receive First Responder intervention

The main objective is to determine whether First Responders increase survival at 30 days after cardiac arrest.

Additional objectives include evaluating:

survival at 6 months and 1 year neurological outcomes (brain function recovery)

Data for this study are collected from existing healthcare and emergency system databases, including EMS dispatch records, national health data systems, and the DAE RespondER platform.

For patients who survive, follow-up information may be collected through medical records or telephone contact.

Importantly, this study does not involve any experimental treatments or changes in patient care. It is based entirely on data already collected during routine emergency care, and therefore does not pose additional risks to patients.

All data are handled securely and in compliance with privacy regulations. Personal identifiers are replaced with coded information (pseudonymisation), and only authorised researchers can access the data. Patients who are alive may be contacted to provide consent and additional follow-up information; participation is voluntary, and consent can be withdrawn at any time.

The results of this study will provide important evidence on the effectiveness of citizen responder systems and may help guide future improvements in emergency response organisation, public health strategies, and training programmes. Ultimately, the goal is to increase survival and improve outcomes for people experiencing cardiac arrest in the community.

Inclusion Criteria:

• Age ≥18 years
• Confirmed out-of-hospital cardiac arrest (OHCA)
• Emergency medical services activation with Advanced Life Support dispatch (Code Red Advanced Blue)
• OHCA confirmed by return code 2-3-4 and/or NSIS codes (C0208 or C0205)
• Event occurring between 1 January 2018 and 31 December 2025
• Activation of the regional emergency medical system (Emilia-Romagna 118 system)

Exclusion Criteria:

• Age <18 years
• Cardiac arrest not confirmed after EMS assessment
• Cases with return code <2 (misclassified as cardiac arrest)

Adult patients (≥18 years) with confirmed out-of-hospital cardiac arrest managed by the regional emergency medical services (118 system) in Emilia-Romagna between 1 January 2018 and 31 December 2025. Patients are identified through EMS dispatch databases and national health information systems and include cases in which a First Responder was activated via the DAE RespondER system. The study population reflects a real-world, unselected cohort of OHCA patients treated within an integrated regional emergency care system.

Individual participant data (IPD) that underlie the results reported in this study, after de-identification, will be made available to researchers upon reasonable request.

Data will be available beginning 6 months and ending 5 years following publication of the study results.

Researchers who provide a methodologically sound proposal will be eligible to access the data for purposes of achieving the aims of the approved proposal.

Proposals should be directed to the corresponding investigator. Data will be shared following approval by the study steering committee and in compliance with applicable data protection regulations (GDPR).

Data will be provided in a de-identified format, and a data sharing agreement will be required.

Data will be available beginning 6 months after publication of the primary results and ending 5 years after publication.

Access to de-identified individual participant data and supporting documents will be granted to researchers who provide a methodologically sound research proposal.

Requests will be reviewed by the study steering committee. Data will be shared following approval and in compliance with applicable data protection regulations (GDPR).

A data sharing agreement will be required. Data will be provided in a secure format, and access may be restricted to ensure confidentiality and appropriate use.