Out-of-hospital cardiac arrest (OHCA) remains a leading global emergency condition with low survival to hospital discharge despite advances in cardiopulmonary resuscitation. Return of spontaneous circulation (ROSC) rates have improved; however, 30-50% of patients experience rearrest after ROSC, which is associated with significantly reduced survival. Preventable physiologic factors related to prehospital care - including hypoxia, hypotension, and hyperventilation - are frequently identified prior to rearrest. Evidence-based post-ROSC clinical bundles exist mainly for in-hospital settings, while structured prehospital post-resuscitation care protocols are limited, particularly in resource-constrained environments.

The RE-ARREST project aims to develop, implement, and evaluate an evidence-based prehospital post-resuscitation care protocol designed for paramedic-led Emergency Medical Services. The intervention includes structured monitoring, tailored oxygenation and ventilation targets, vasopressor use criteria (norepinephrine), fluid management decision support, teamwork communication, and operational training workshops using simulation.

This is a quasi-experimental pre-post interventional study conducted at the Siriraj Emergency Medical Service (SiEMS), Thailand. The study compares outcomes from retrospective pre-implementation cases with prospective post-implementation cases, including both patient-centered outcomes and provider compliance. Adult OHCA patients with ROSC achieved prehospital and transported to Siriraj Hospital are eligible. The estimated sample size is 318 participants (pre-intervention 212; post-intervention 106) over two years.

The primary outcome is the incidence of rearrest within 1 hour after ROSC during prehospital care and initial emergency department management. Secondary outcomes include protocol compliance, survival-to-admission, and survival-to-hospital-discharge. The protocol emphasizes feasibility, safety, and replicability to inform scalable EMS clinical practice guidelines.

This research is expected to provide novel evidence on targeted prehospital post-ROSC care and has the potential to reduce rearrest, improve neurologically favorable survival, and strengthen EMS system quality improvement efforts in Thailand and other low-to-middle-resource settings.

Inclusion Criteria:

• Adults aged 18 years or older.
• Patients with out-of-hospital cardiac arrest (OHCA) who received resuscitative care from the Siriraj Emergency Medical Services Center (SiEMS).

Patients who achieved return of spontaneous circulation (ROSC) prior to hospital arrival.

- Patients subsequently transported to the Emergency Department of Siriraj Hospital for further treatment.

Exclusion Criteria:

• Suspected traumatic cardiac arrest.
• ROSC patients whose legally authorized representative declined vasopressor administration, despite clinical indication according to the prehospital post-resuscitation care protocol.
• ROSC patients who had not undergone endotracheal intubation, and whose legally authorized representative declined endotracheal intubation, despite protocol-based indication.
• ROSC patients without rearrest whose legally authorized representative declined further blood investigations or additional procedures after arrival at the Emergency Department.
• Patients whose legally authorized representative declined participation in the study.
• Patients without an available legally authorized representative to provide consent for study participation.

only IPD used in the results publication.