This study looked at cardiac arrest that happens outside a hospital. Cardiac arrest means the heart suddenly stops pumping blood. When this happens, a person collapses and needs help right away.

The study looked at care given by the emergency medical service of the Canton of Zug in Switzerland. The main goal was to learn what makes it more likely that the heart starts to pump blood again on its own. Doctors call this return of spontaneous circulation (ROSC). The researchers also wanted to describe how well the service performed over four years.

The researchers did not test any new treatment. They did not contact any people. Instead, they reviewed records that the service already had, from the years 2022 to 2025. All records were coded so that no person could be identified.

The study included 240 people who were treated for cardiac arrest. In about 4 out of 10 people (43 out of 100, or 43%), the heart started to pump blood again during care.

The researchers checked whether some factors were linked to this result. These factors were:

the heart rhythm found at the start of care how quickly the ambulance reached the person whether someone saw the person collapse whether a bystander or an emergency call handler started cardiopulmonary resuscitation (CPR), which is chest compressions and rescue breaths whether a paramedic or a doctor led the care

The heart rhythm at the start was the factor most strongly linked to the heart starting again. The result was similar whether a specially trained paramedic or a doctor led the care.

The researchers also wanted to know how well these factors can predict whether the heart will start again. They built two simple tools to estimate the chance of this result. One tool uses information known during the emergency call. The other tool uses information known when the ambulance team arrives. They tested how well the tools worked, and they compared the second tool with a tool that already exists. These tools are early versions. They need to be tested on new patients before they can be used in care.

The researchers also looked at whether the result differed by place within the service area and whether it changed over the four years.

This study cannot show that one factor causes another. It looked back at past records from one service, so the results may not apply to other places. The study did not follow people after they reached the hospital, so it does not report longer term survival. The findings can help the service review its own care and plan future work.