The "Intravenous vs. Intraosseous Vascular Access During Out-of-Hospital Cardiac Arrest (IVIO)"-trial is an investigator-initiated, randomized, parallel group, patient and outcome assessor-blinded, superiority trial of intravenous vs. intraosseous vascular access during adult out-of-hospital cardiac arrest. The intraosseous group will be further randomized to humeral vs. tibial access. The trial will be conducted in the Central Denmark Region. The primary outcome will be sustained return of spontaneous circulation, and 762 patients will be included. Key secondary outcomes include survival at 30 days and survival at 30 days with a favorable neurological outcome.

Inclusion Criteria:

1. Out-of-hospital cardiac arrest
2. Age ≥ 18 years
3. Indication for intravenous or intraosseous vascular access during cardiac arrest

Exclusion Criteria:

1. Blunt or penetrating traumatic cardiac arrest
2. Prior enrollment in the trial
3. Intravenous or intraosseous vascular access already in place and working when the first trial-participating unit arrives on site

Six months after the publication of the last results, all de-identified individual patient data will be made available for data sharing.