This study aimed to determine the impact of bystander CPR on clinical outcomes in patients with increasing response time from collapse to EMS response.

A population-based observational study was conducted in patients with witnessed out-of-hospital cardiac arrest (OHCA) of presumed cardiac etiology from 2012 to 2014. The time interval from collapse to CPR by EMS providers was categorized into quartile groups: fastest group (<4min), fast group (4 to <8min), late group (8 to <15min), and latest group (15 to <30min). The primary outcome was hospital discharge and the secondary outcome was survival with good neurological outcome. Multivariable logistic regression analysis was performed to evaluate the interaction between bystander CPR and the time interval from collapse to CPR by EMS providers.

A total of 15,354 OHCAs were analyzed. Bystander CPR was performed in 8591 (56.0%). Survival to hospital discharge occurred in 1632 (10.6%) and favorable neurological outcome in 996 (6.5%). In an interaction model of bystander CPR, compared to the fastest group, adjusted odds ratios (AORs) (95% CIs) for survival to discharge were 0.89 (0.66–1.20) in the fast group, 0.76 (0.57–1.02) in the late group, and 0.52 (0.37–0.73) in the latest group. For favorable neurological outcome, AORs were 1.12 (0.77–1.62) in the fast group, 0.90 (0.62–1.30) in the late group, 0.59 (0.38–0.91) in the latest group.

The survival from OHCA decreases as the ambulance response time increases. The increase in mortality and worsening neurologic outcomes appear to be mitigated in those patients who receive bystander CPR.