Bystander Defibrillation and Survival According to Emergency Medical Service Response Time After Out-of-Hospital Cardiac Arrest: A Nationwide Registry-Based Cohort Study

The impact of emergency medical services (EMS) response times when integrating bystanders’ automated external defibrillator (AED) use into established response systems remains unclear. This study aims to investigate 30-day survival probabilities for different EMS response times for bystander and non-bystander defibrillated patients and identify for which EMS response times bystander defibrillation improves 30-day survival probability.

Data on patients with bystander witnessed out-of-hospital-cardiac arrest (OHCAs) with initial shockable rhythm who received bystander cardiopulmonary resuscitation were retrieved from Danish Cardiac Arrest Registry for years 2016–2022. Proportions of 30-day survival were calculated for five intervals of EMS response time for patients who received bystander defibrillation and those who did not. The causal inference framework utilizing targeted maximum likelihood estimation was used to estimate 30-day survival probability for each interval of EMS response time and when comparing cases where bystander defibrillation was performed with those where it was not. This analysis was adjusted for relevant confounding factors and conducted separately for residential and public OHCAs.

The study included 3,924 patients with OHCA. Bystander defibrillation was more frequent in public than in residential OHCAs (64.1% vs. 35.9%). Short EMS response times had higher 30-day survival probability. Bystander defibrillation resulted in higher probability of 30-day survival for EMS response times of 7–9 min (survival ratio 1.24 [95% CI: 1.03; 1.49]) in public OHCAs in the adjusted model, when compared to non-bystander defibrillated patients.

With EMS response times of 7–9 min, we detected a clear 30-day survival benefit for bystander defibrillated patients in public locations. No 30-day survival benefits were seen for other EMS response time intervals or in residential locations.