The study is aimed to assess (a) the incidence of hyperoxia at the point of return of spontaneous circulation (ROSC) and (b) the role of arterial blood oxygen partial pressure to brain oxygenation during out-of-hospital cardiac arrest. 80 adult patients will be recruited in a physician staffed helicopter emergency medical services. Brain regional oxygen saturation and invasive blood pressure are monitored until hospital admission and arterial blood gases are analyzed immediately when the unit arrives to the patient and again at the time of ROSC.

Inclusion Criteria:

• out-of-hospital cardiac arrest of presumed cardiac origin
• resuscitation will be continued at least 5 minutes (if return of spontaneous circulation not achieved earlier) after arrival of physician team

Exclusion Criteria:

• do-not-attempt-resuscitation (DNAR) order or withdrawal of treatment by prehospital critical care physician
• return of spontaneous circulation achieved before physician-led critical team arrives
• external cause for cardiac arrest (e.g. trauma, suffocation)
• workload too high or environment too dangerous to perform study procedures

All adult patients met in cardiac arrest by the physician-led prehospital critical care team are to be screened and recruited based the eligibility criteria. The patients include patients in all primary rhythms.