The application KATRETTER demonstrates for the first time the feasibility and advantages of automatic app-based first responder activation, crowdsourcing Basic Life Support treatment for OHCA patients in an entire German federal state. In line with the guidance of national and international medical bodies, it constitutes another indispensable element of crucial public health endeavors to enhance survival rates following out-of-hospital cardiac arrest (OHCA), in concert with telephone-guided CPR, regular CPR workshops in educational institutions and places of work, and the widespread use of publicly accessible AEDs [11, 13].

Smartphone-based first responder apps are in this context not to be viewed in a conventional sense as an alerting tool for professional help, but rather represent a form of digital call for help in the vicinity of the emergency location to bridge the time to EMS arrival with bystander CPR. Contrary, this very “call for help” was previously limited to sight or calling distance from the emergency scene [17]. The potential assistance from first responders mobilized through such applications is not restricted to executing basic life support measures. Instead, it could extend to tasks like directing emergency services to the crisis site, tending to family members, or providing the control centre with more comprehensive data on the situation, occurrences, and patient conditions. In this context, the question whether early bystander CPR or rather early defibrillation before arrival of EMS resources on scene will have the most impact on patient-centered outcomes remains to be answered. The results of a randomized-controlled trial done in Stockholm showed improved bystander CPR rates through activation of CFR. However, a survival benefit could not be demonstrated [18]. The same applies to a current study by Gregers et al. who, despite clearly showing increased rates of bystander CPR and defibrillation through an intensified CFR activation strategy, could not note improvements in return of spontaneous circulation or survival to hospital discharge. Study designs like these may often be under-powered to detect a change in secondary patient-centered outcomes like chance of survival [19]. A concept to especially boost early defibrillation rates by handing out ultraportable AED devices to CFR with the goal of earlier AED use in a higher number of OHCA activations is currently being studied in a trial in Australia (The First Responder Shock Trial; ACTRN12622000448741) with the primary outcome set to see a change in 30-day survival [20].

As for limitations, during the time period shortly after the first launch, cases were registered in which there was a premature termination during a mission due to a software error. Additionally, KATRETTER typically dispatches first responders to an OHCA only if the OHCA is also identified as such a (suspected) case during the emergency call. There may be instances where an OHCA is not identified despite the use of a standardized emergency call query during the emergency call taking process. Additionally, the cardiac arrest may have taken place after the emergency call had ended. Factors like the notification function of KATRETTER alarms being silenced or the geolocation function being turned off can impact compliance between the system and user. It is possible that there was an under-reporting of first responders arriving at the scene due to the absence of arrival timestamps sent by the first responder.

This analysis demonstrates that a significant quantity of first responders can be recruited within 24 months, without extensive advertising or publicity, and despite the pressures of a global pandemic. Nonetheless, as with other CFR systems such as Pulsepoint and myResponder, it is clear that the proportion of registered users accepting activations and responding is still relatively low [21, 22]. Significant contrasts can be observed between the registration process and requirements of the KATRETTER system in Berlin and other comparable applications across the globe [22]. In contrast, Pulsepoint in the USA generally restricts registration to professional helpers such as EMS personnel and healthcare professionals, while GoodSam in the UK mandates proof of a current BLS course. No special evidence is necessary to register with the HEARTRUNNER in Denmark or myResponder app in Singapore, which is equivalent to Berlin's KATRETTER system, allowing for registration through SMS validation on a low-threshold basis [21,22,23,24,25].

Other applications already implemented regionally in Germany, such as the “MOBILE RETTER”, “REGION LEBENSRETTER” or “CORHELPER” app, as well as previous implementation processes of the KATRETTER system in areas outside the state of Berlin, also require proof of qualification in a healthcare profession, and in some parts of Germany even additional specific induction and BLS courses [26,27,28,29].

Even in systems where individuals can register without proof of medical qualification, our data confirms that the vast majority of those who register possess some form of CPR qualification. Many of these individuals are medical professionals with regular experience in resuscitation. The benefits of a significantly reduced effort required by system operators in qualification management, as well as a lower threshold for qualified personnel to register, seem to outweigh other concerns.

Further functions distinguish the various nationally and internationally available apps, other than the registration process. For example, in addition to registering first responders, several apps can integrate other individuals or publicly accessible locations of an Automated External Defibrillator (AED), resulting in the simultaneous activation of a person to bring the nearest available AED to the scene of an out-of-hospital cardiac arrest (OHCA). Additional functions comprise the capability of initiating a video emergency call to the control centre or selecting in the app's first responder profile one's current mode of transport, including on foot, by bicycle or by car for calculating the optimal route and navigation [22, 23].

Recommendations for app-based cardiac first responder (CFR) activation systems in out-of-hospital cardiac arrest (OHCA) have been developed during a European consensus conference. This has exposed a considerable inconsistency in the use and design of these systems in Europe. It is generally agreed among the medical societies in Europe that country-wide systems should be implemented and consistently utilised. It is emphasised that bi-directional communication between first responders and control centres during and after activation for an OHCA is of particular importance [30]. This may be in line with observations from other CFR apps, some of which have identified a special psychosocial aftercare need for first responders after a mission, but further research is needed to confirm the extent of psychosocial aftercare demands [31, 32]. One of the foreseeable greatest challenges in Germany regarding the necessary nationwide implementation of CFR applications is, in addition to adequate funding, the possibility of connecting first responders across different control centers and different CFR apps throughout the country. Here, KATRETTER offers the prospect of better integration, connectivity, and cooperation through standardized, transparent interfaces, the ability to integrate first responders from other control center areas, and the fact that it has already been fully implemented in three German states.

Further, it is important to note that the invitation of citizens into first aid alerting systems presumably helps to strengthen individual medical emergency competency, but also more broadly speaking general health-related action ability and thus resilience of the population [33].