Volunteer Responders Boosted CPR Rates but Not Survival, Danish Trial Finds

Community first responder systems, in which smartphone apps alert nearby volunteers to perform CPR and defibrillation before an ambulance arrives, are becoming more common worldwide. Until now they had been backed only by observational studies. The randomized HeartRunner trial, presented in a Hot Line session at ESC Congress 2026, tested the approach directly in the Capital Region of Denmark. “We conducted a randomized study first to evaluate the impact of community responders on patient outcomes, and second to answer the question of whether using untrained volunteers is safe,” said principal investigator Fredrik Folke of Copenhagen University Hospital.

The trial randomly assigned 2,060 suspected out-of-hospital cardiac arrests to either community first responder activation plus standard emergency response, or standard response alone. Cases were split into two groups by how far away the nearest volunteer was estimated to be: under three minutes' travel, or three to nine minutes. The trial's main result was neutral. Thirty-day survival was 15% with volunteer activation versus 17% without it when responders were under three minutes away, and 13% versus 14% when they were three to nine minutes away — neither difference was statistically significant.

Volunteer activation did significantly increase bystander action. When responder arrival time was three to nine minutes, bystander CPR rose from 75% to 86%, and bystander defibrillation more than doubled, from 6% to 14%. Rates of the heart restarting and of survival with a favorable neurological outcome were similar between the two groups. Folke suggested the neutral result partly reflects how fast ambulances already were: “The trial was conducted in an area where ambulance response times were very short, around seven minutes.”

Folke said the trial shows that community first responder systems can meaningfully improve bystander CPR and defibrillation, but that their effect on survival depends heavily on how much time responders actually save.