PROJECT SUMMARY / ABSTRACT The public health burden of out-of-hospital cardiac arrest (OHCA) is enormous, affecting ~350,000 individuals each year in the U.S. and is the third leading cause of death. To improve our understanding of OHCA, the Cardiac Arrest Registry to Enhance Survival (CARES) was launched by Emory University and the Centers for Disease Control and Prevention. Through >70 peer-reviewed publications over the past 2 decades, CARES has transformed our understanding of the epidemiology and outcomes of patients with OHCA. Yet, little is known as to how some emergency medical service (EMS) agencies achieve higher survival rates for their patients with OHCA than other agencies. While variation in OHCA survival is largely believed to be due to adoption of key care processes by EMS agencies in the prehospital setting, how these processes are employed in routine practice, the underlying factors contributing to their successful implementation, and effective interactions between EMS agencies with the community, first responders (e.g., police, fire), 911 dispatchers, and hospitals have not been systematically defined. Moreover, despite the fact that Black and Hispanic patients have lower survival rates for OHCA than White patients, there is scant research on how to reduce disparities in OHCA survival and how top-performing EMS agencies that work in majority Black or Hispanic communities overcome additional barriers to prehospital OHCA response and care. Accordingly, we propose the RED-CASO (Reducing Ethnic-racial Disparities in Cardiac Arrest Survival Outcomes) study to address these critical gaps in knowledge and to lay the foundation for quality improvement efforts to reduce disparities in OHCA survival. First, we will identify top-performing and bottom-performing EMS agencies in OHCA survival, with a focus on those that work in majority Black or Hispanic communities. Second, we will identify ‘best practices’ at top-performing EMS agencies in OHCA survival through detailed interviews with key stakeholders during site visits. We will intentionally sample EMS agencies which work in majority Black or Hispanic communities, and these agencies will comprise at least half of the sites visited. And third, we will validate these best practices associated with EMS agencies with the highest survival rates for OHCA. We will use a sequential mixed-methods approach with a focus on identifying best practices at top- performing EMS agencies which work in communities where most residents are Black or Hispanic. We will build on work by our investigative team to systematically understand factors associated with survival for other emergency settings (in-hospital cardiac arrest) and leverage our team’s multidisciplinary expertise in cardiology, emergency medicine, epidemiology, and mixed methods research. Findings from this study will be used to construct a ‘best practices guide’ for OHCA that can be implemented by EMS agencies both within CARES and with nationa...