How to Measure First Responders' Arrival Time in a Smartphone Alerting System? An observational study.

BACKGROUND: Out-of-hospital cardiac arrest remains a time-critical emergency in which early cardiopulmonary resuscitation and defibrillation are key to survival. Smartphone Alerting Systems (SAS) mobilize community first responders (CFR) rapidly, but accurate measurement of their arrival times remains challenging for system evaluation and optimization. METHODS: In this observational study, all activations of the SAS "Region of Lifesavers" in Freiburg, Germany, were analysed over a 12-month period (1/12/2023 - 30/11/2024). Response time was defined as the interval from system activation by the dispatch center to one of three arrival indicators: GPS entry into a 100-meter radius (GPS100m), GPS entry into a 50-meter radius (GPS50m), or manual confirmation via the app. Descriptive statistics, the Friedman test, Wilcoxon signed-rank test, and Bland-Altman plots were used to compare the response time measurement methods. RESULTS: Of 1,498 alerts, 2,480 CFR accepted, the median arrival times were GPS100m = 04:21 [02:58, 06:00] min, GPS50m = 04:40 [03:19, 06:25] min, and manual confirmation 05:50 [04:03, 08:04] min (1,577 complete data cases). Significant differences were observed between all methods (p<0.001 for all pairwise comparisons). Manual confirmation was documented in only 69.2% of cases. Bland-Altman analysis showed a smaller bias for GPS50m than for GPS100m, with comparable limits of agreement. CONCLUSION: GPS-based tracking provides a reliable, objective measure of CFR arrival times. The 50m geofence offers the highest practical applicability and consistency. Manual confirmation was often delayed or missing, limiting its reliability. GPS50m could be considered a standardized metric for arrival measurement in SAS for further evaluation and optimization. (DRKS00032957, 30/10/2023).