Selecting volunteer community first responders for cardiac arrest emergencies based on estimated travel time rather than geographic proximity can nearly double the chance of early arrival compared with small, fixed-radius alerts, according to late-breaking research presented at the American Heart Association's Resuscitation Science Symposium 2026 in Chicago. The findings, simultaneously published in the Journal of the American Heart Association, suggest that smartphone alerting systems could be optimized to get trained volunteers to the scene faster, potentially improving survival rates.
Researchers in Freiburg, Germany, conducted the ALARM randomized clinical trial to compare three strategies for alerting trained volunteers via smartphone app: a small geographic circle within about 600 meters of the scene, a larger circle within about 2,500 meters, and a "smart alerting" calculation that considered the expected travel time for each volunteer relative to the expected ambulance arrival. Over seven months in 2024, the system randomly switched among the three strategies daily, with neither dispatchers nor volunteers aware of which was active.
The results showed that when alerts were sent to a small geographic circle, a trained volunteer arrived before the ambulance in only 31% of emergencies. In contrast, the larger circle and smart alerting strategies achieved arrival rates of 57% and 58%, respectively. Smart alerting was also more efficient: it deliberately did not send 31% of volunteers who accepted an alert because calculations showed they would likely arrive after EMS, reducing unnecessary dispatches. Overall, 69% of volunteers received a task under smart alerting, compared with 90% for the small circle and 94% for the large circle, yet smart alerting maintained a high arrival rate similar to the large circle approach.
"Our study results indicate that how these trained volunteers are chosen and assigned to cardiac emergencies really matters," said Michael Mueller, M.D., senior study author and chair of the department of anesthesiology, intensive care and emergency medicine at St. Josef's Hospital in Freiburg. "When the app evaluated who can actually get there fastest, instead of just who is closest on a map, more than half of patients had a trained volunteer providing care before the ambulance arrived, and often two volunteers instead of one." Mueller also chairs the Region of Lifesavers, a German non-profit that operates the app-based first responder network.
The study has limitations. It measured system performance, such as arrival before EMS, not patient survival. It was conducted in one region of Germany with a particular medical system and high numbers of automated external defibrillators (AEDs), so results may differ elsewhere. The researchers noted that further studies are needed to determine whether smart alerting improves cardiac arrest survival.
Cardiac arrest is a time-critical emergency; every minute without CPR lowers survival chances. The American Heart Association's 2025 Emergency Cardiovascular Care Guidelines emphasize that prompt CPR and defibrillation significantly improve survival. Research suggests immediate CPR can double or triple survival rates. For more information on cardiac arrest treatment, see the AHA's Treatment of Cardiac Arrest page. The public can also Join the Nation of Lifesavers by learning CPR today.
Smart alerting could have broader implications for emergency response systems worldwide. Many dispatch centers still use fixed geographic radii to alert volunteers, potentially missing faster responders who are farther away but can reach the scene quicker by car or other means. "Small, fixed geographic radii based on the patient's location are still widely used internationally," Mueller said. "Our data suggest that such systems may be leaving many potential lifesavers untapped and that there is room to improve response times, which would hopefully also improve survival rates."
The research was conducted by the Region of Lifesavers nonprofit in collaboration with medical schools, a hospital, emergency services and volunteer first responders. The study area covered about 1,531 square kilometers and roughly 493,000 people, including urban, suburban and rural areas, making it a realistic laboratory for testing. The smartphone system was activated more than 820 times for suspected cardiac arrest, and more than 1,480 trained volunteers were dispatched. At least one volunteer accepted the alert in 56% of emergencies for the small circle, 83% for the large circle and 87% for smart alerting. A trained volunteer reached the scene in 49%, 78% and 79% of cases, respectively.
For more news from the Resuscitation Science Symposium 2026, visit the AHA's heart.org or learn about stroke at stroke.org.
