Purpose
Improvements in survival from out-of-hospital cardiac arrest (OHCA) are often attributed to early defibrillation by bystanders or first responders. However, it is unclear if outcomes are also improving over time for patients shocked by bystanders and first responders. 

Methods
Between 2000 and 2016, we included consecutive adult OHCA patients from the Victorian Ambulance Cardiac Arrest Registry who presented to emergency medical services (EMS) with an initial shockable rhythm and received an attempted resuscitation. Cases witnessed by EMS were excluded. Adjusted logistic regression analyses were used to assess trends in survival to hospital discharge across patients first shocked by paramedics, first responders and bystanders.

Results
Of the 9,872 initial shockable arrests, 1,194 (12.1%) were first shocked by bystander or first responder (increasing from 6.0% in 2000 to 19.9% in 2016, p<0.001). Unadjusted survival to hospital discharge varied according to who shocked first: 21.0% for paramedics; 31.3% for first responders, and; 44.1% for bystanders (p<0.001). Between 2000 and 2016, survival increased for all groups: from 15.7% to 29.8% for patients first shocked by paramedics; from 4.8% to 37.0% for patients first shocked by first responders, and; from 16.7% to 52.1% for patients first shocked by bystanders (p<0.001 for all trends). After adjustment, the odds of survival increased independently of other arrest factors, by 8.5% per year (OR 1.09, 95% CI: 1.07, 1.10) for patients first shocked by paramedics, 8.4% per year (OR 1.08, 95% CI: 1.04, 1.13) for patients first shocked by first responders, and 11.5% per year (OR 1.11, 95% CI: 1.05, 1.18) for patients shocked by bystanders.

Conclusion
Although early defibrillation by bystanders and first responders is helping to improve overall survival from OHCA, trends in survival are also increasing independent of these initiatives. Patients shocked by bystanders are observing the highest growth in survival outcomes.

8 - Long-term trends in survival from out-of-hospital cardiac arrest according to who provided the first shock: a 17-year observational study.