Selecting volunteer community first responders for cardiac arrest based on estimated travel time rather than geographic proximity improved the likelihood of a trained volunteer reaching the patient before emergency medical services (EMS) arrived, according to late-breaking research presented at the American Heart Association’s Resuscitation Science Symposium 2026. The findings, from the ALARM randomized clinical trial conducted in Freiburg, Germany, suggest that widely used fixed-radius alerting systems may be missing opportunities to save lives.
When a heart stops outside a hospital, every minute without CPR lowers survival chances. Many dispatch centers alert trained volunteers via smartphone apps, but most use simple geographic circles. The Freiburg study compared three strategies: a small circle of about 600 meters, a larger circle of about 2,500 meters, and a “smart alerting” calculation that considered expected travel time to the patient versus ambulance arrival. For seven months, the system randomly switched strategies daily, with neither dispatchers nor volunteers aware of the active method.
Results showed that a volunteer arrived before EMS in only 31% of emergencies with the small circle, versus 57% with the large circle and 58% with smart alerting. Smart alerting also reduced unnecessary dispatches: 31% of volunteers who accepted an alert were not sent because they would likely arrive after EMS, compared with 90% and 94% dispatched in the small and large circle groups, respectively. “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,” said Michael Mueller, M.D., senior study author and chair of anesthesiology at St. Josef’s Hospital in Freiburg.
The implications are significant for emergency response systems worldwide. Small, fixed radii are still widely used internationally, and Mueller noted they may be “leaving many potential lifesavers untapped.” Smart alerting also increased the number of volunteers reaching the scene, which is important because CPR is physically demanding and having a second rescuer allows for taking turns with chest compressions. The American Heart Association’s 2025 Emergency Cardiovascular Care Guidelines emphasize that prompt CPR and defibrillation greatly improve survival, and research suggests immediate CPR can triple survival rates.
However, the study only measured response times, not patient survival. “Whether smart alerting improves survival odds needs to be examined in future research,” Mueller said. The trial, possibly the first to directly compare alerting algorithms for smartphone-based responder systems, was conducted in one German region with a high density of AEDs, so results may differ elsewhere. Still, the findings highlight a practical, low-cost refinement to existing systems. For more information on cardiac arrest treatment, see the Treatment of Cardiac Arrest page, and to learn CPR, consider joining the Nation of Lifesavers. The American Heart Association’s financial information is available here.
