The study aims to increase proportions of bystander defibrillation during out-of-hospital cardiac arrest (hereof referred to as cardiac arrest) in residential areas with a high density of cardiac arrests. The intervention consists of Automated External Defibrillators (AEDs) and residents' involvement in resuscitation through training and enrollment as citizen responders.

Survival decreases by 10% for every minute that passes after a cardiac arrest until defibrillation. Despite an increasing number of available AEDs, survival and defibrillation rates in residential areas remain poor. Efforts to increase bystander defibrillation has the potential to improve survival. Through the strategic deployment of AEDs, training, and recruitment of residents as citizen responders, we aim to improve proportions of bystander defibrillation and 30-day survival in densely populated residential areas with a high density of cardiac arrests.

Inclusion Criteria:

• All witnessed cardiac arrests recognized registered in the Danish Cardiac Arrest Registry occurring within the study sites. This excludes non witnessed cardiac arrests.
• Witnessed cardiac arrest
• Non-traumatic etiology, this excludes intoxication, drowning or suicide.

Exclusion Criteria:

• Cardiac arrest occurring in a nursing home
• Cardiac arrest not treated by the EMS due to ethical reasons or obvious signs of death
• Not true cardiac arrest (suspected, but not verified)
• Cardiac arrests witnessed by the emergency medical personnel

Due to the nature of the study, treatment allocation is not masked to rescuers or patients. The steering committee will, during the study, receive information on total numbers of arrests and overall success of providing defibrillation. The steering committee will not receive information on whether defibrillation took place in areas allocated to intervention or no intervention. Only the safety committee will have access to all information.