Selective smartphone alerts helped improve bystander response time for cardiac arrest

Research Highlights:

• Selecting which volunteer community first responders were alerted to the scene of a cardiac arrest was overall more successful when alerting responders who had shorter estimated arrival times than only sending out alerts to responders who were geographically closer to the scene and also reduced dispatching volunteers likely to get to the scene after EMS, according to preliminary research conducted by researchers in Freiburg, Germany.
• Because "smart" alerting considered obstacles related to how fast the responder could get to the scene, this method nearly doubled the chance of early volunteer arrival compared with alerts sent only to responders within the nearby geographical area.
• The researchers said further studies are needed to determine whether smart alerting improves cardiac arrest survival.
• Note: This trial is simultaneously published today as a full manuscript in the peer-reviewed scientific journal, Journal of the American Heart Association.

Embargoed until 3:15 p.m. CT/4:15 p.m. ET, Saturday, Nov. 7, 2026

CHICAGO, Nov. 7, 2026 - Being "smart" about which volunteer, community first responders were alerted of a cardiac arrest in progress increased the likelihood of volunteers reaching the patient before emergency medical services (EMS) arrived, while reducing dispatches of volunteers likely to get to the scene after EMS, according to late-breaking science presented at the American Heart Association's Resuscitation Science Symposium 2026. The meeting will be held Nov. 7-8 in Chicago and will feature the most recent advances related to treating cardiopulmonary arrest and life-threatening traumatic injury.

When someone's heart suddenly stops while they are outside a hospital, every minute without CPR lowers their chance of survival. In many areas worldwide, emergency dispatch centers taking calls, such as 911 in the U.S. and 112 in Europe, also alert trained volunteer community first responders through a smartphone app aiming to achieve the fastest response with CPR and automated external defibrillator (AED) support before an ambulance arrives.

Researchers in Freiburg, Germany, tested three different strategies for alerting trained community volunteers to determine which one most often got a trained volunteer to the patient before the ambulance. Alerts were sent to either:

• A small geographic circle within about 600 meters (about 0.4 miles) of the scene of the cardiac arrest;
• A larger circle within about 2,500 meters (about 1.6 miles) of the scene; or
• A location determined with a "smart alerting" calculation that took into account the expected travel time for a trained volunteer to reach the patient compared to the expected arrival of the ambulance.

For seven months (Jan. 1-July 31, 2024), the alert system randomly switched among the three strategies every day. Neither the dispatchers nor the volunteers knew which strategy was active on any day. Arrival times were measured automatically using the volunteers' smartphone GPS data.

What were the study's key findings?

• When alerts were sent to a small geographic circle close to the scene of the cardiac arrest, a trained volunteer arrived before the ambulance in fewer than 1 in 3 (31%) emergencies.
• When alerts were sent to a larger geographic circle, a trained volunteer arrived before the ambulance about 57% of the time.
• When a "smart alerting" strategy was used, a trained volunteer arrived before the ambulance about 58% of the time.

Not only was the arrival time similar in the large geographic circle and the smart alert strategies, both strategies independently also more often resulted in more than one trained volunteer arriving to the patient. This is important because CPR is physically demanding and having a second rescuer available allows for taking turns with chest compressions.

"Our study results indicate that how these trained volunteers are chosen and assigned to cardiac emergencies really matters. 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., the senior study author and chair of the department of anesthesiology, intensive care and emergency medicine at St. Josef's Hospital in Freiburg, Germany. "Small, fixed geographic radii based on the patient's location are still widely used internationally. 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."

Smart alerting was also more selective: 31% of the trained volunteers who accepted an alert were deliberately not sent because calculations showed they would most likely arrive after the ambulance. Only 69% of trained volunteers received a task in the smart alerting process, compared with 90% for the small geographic circle group and 94% for the larger geographic circle group, even though smart alerting achieved nearly identical high arrival rate as the large circle approach. The smart app calculations made the emergency response more efficient: fewer volunteers were sent on trips that would most likely have resulted in arrival after EMS.

"We were struck by how poorly straight-line distance predicts actual arrival. A relevant number of volunteers who were relatively far away by road still reached the patient quickly, for example by car, whereas some volunteers who appeared to be closer when measured by straight-line distance would not have made it in time. A circle on a map simply cannot capture this. Smart alerting can, and it separated trained volunteers who could realistically make a difference from those who could not: almost one in three trained volunteers who accepted an emergency assignment was not sent because the ambulance would most likely have been faster," said Mueller, who also chairs the Region of Lifesavers (Region der Lebensretter e.V.), a German non-profit association that operates an app-based, smartphone-triggered first responder network.

The American Heart Association's 2025 Emergency Cardiovascular Care Guidelines detail that people experiencing cardiac arrest who receive prompt medical attention including CPR to increase the blood flow to the heart and brain, and/or an electrical shock from a defibrillator, are much more likely to survive than those who do not receive swift medical intervention. Research suggests cardiac arrest victims who received CPR immediately have 2 to 3 times higher survival rates compared to those who do not.

"For our study, we only looked at the response time to getting bystander CPR to people quickly, we did not look at patient survival rates," Mueller said. "Whether smart alerting improves survival odds needs to be examined in future research."

What are the study's details, background, design and limitations?

• The research comes from the ALerting Strategies for SmArtphone-based First Responder SysteMs or ALARM randomized clinical trial . It may be the first trial to directly compare different alerting algorithms/processes of a smartphone-based first responder system.
• The Region of Lifesavers (Region der Lebensretter e.V.) nonprofit, which conducted the study, operates the smartphone alerting system in the Freiburg region along with medical schools, a hospital, emergency services and volunteer first responders.
• The study period ran from January through July 2024 in the area covered by the Freiburg emergency dispatch center in southwestern Germany. The region covers about 1,531 square kilometers (about 590 square miles) and roughly 493,000 people: the city of Freiburg (about 230,000 residents) and the surrounding district of Breisgau-Hochschwarzwald (about 263,000 residents). It includes urban, suburban and rural areas, which, researchers said, made it a strong real-world laboratory because travel conditions, and whether volunteers go by car, bike or on foot varies significantly.
• For the seven-month study, one of the three alerting strategies was chosen randomly and was active at the emergency dispatch center for 24 hours, starting at midnight. Up to four volunteers could be assigned to each emergency.
• Dispatchers did not know which strategy was active and triggered the system based on identical criteria each day. Trained volunteers received alerts without knowing which strategy selected them.
• 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 emergency alert assignment in 56% of emergencies for the small geographic circle vs. 83% for the large geographic circle vs. 87% for smart alerting system.
• At least one trained volunteer reached the scene in 49% of emergencies in the small geographic circle, 78% in the large geographic circle and 79% via the smart alerting protocol.
• A trained volunteer brought an AED to the emergency before the ambulance in 11% of cases for the small geographic circle, 28% for the large geographic circle and 27% in the smart alerting group.
• The study includes several limitations. It measured system performance, such as who arrived before the ambulance, not patient outcomes after cardiac arrest. Also, the study was conducted in one region in Germany with a particular medical system and high numbers of AEDs, meaning results may differ in other regions and countries with different medical and emergency service systems.

Co-authors, disclosures and funding sources are listed in the manuscript.

Statements and conclusions of studies that are presented at the American Heart Association's scientific meetings are solely those of the study authors and do not necessarily reflect the Association's policy or position. The Association makes no representation or guarantee as to their accuracy or reliability. Abstracts presented at the Association's scientific meetings are not peer-reviewed, rather, they are curated by independent review panels and are considered based on the potential to add to the diversity of scientific issues and views discussed at the meeting. The findings are considered preliminary until published as a full manuscript in a peer-reviewed scientific journal.

The Association receives more than 85% of its revenue from sources other than corporations. These sources include contributions from individuals, foundations and estates, as well as investment earnings and revenue from the sale of our educational materials. Corporations (including pharmaceutical, device manufacturers and other companies) also make donations to the Association. The Association has strict policies to prevent any donations from influencing its science content and policy positions. Overall financial information is available here.

Additional Resources:

• Multimedia is available on the right column of the release link
• Link to abstract and the American Heart Association's Resuscitation Science Symposium 2026 Online Program Planner
• American Heart Association news release: Updated CPR guidelines tackle choking response, opioid-related emergencies and a revised Chain of Survival (Oct. 2025)
• American Heart Association health information: Treatment of Cardiac Arrest
• American Heart Association health initiative: Join the Nation of Lifesavers by learning CPR today
• About Resuscitation Science Symposium 2026
• For more news at the American Heart Association's Resuscitation Science Symposium 2026, follow us on X @HeartNews, #ReSS26

###

About the American Heart Association

The American Heart Association is a relentless force for a world of longer, healthier lives. Dedicated to ensuring equitable health in all communities, the organization has been a leading source of health information for more than one hundred years. Supported by more than 35 million volunteers globally, we fund groundbreaking research, advocate for the public's health, and provide critical resources to save and improve lives affected by cardiovascular disease and stroke. By driving breakthroughs and implementing proven solutions in science, policy, and care, we work tirelessly to advance health and transform lives every day. Connect with us on heart.org, Facebook, X or by calling 1-800-AHA-USA1.

For Media Inquiries and American Heart Association Expert Perspective:

American Heart Association Communications & Media Relations in Dallas: 214-706-1173; [email protected]

Sarah Johnson: [email protected]

For Public Inquiries: 1-800-AHA-USA1 (242-8721)

heart.org and stroke.org