Background: This study describes and evaluates advanced paramedic practitioner (APP) use of focused cardiac ultrasound (FoCUS) in out-of-hospital cardiac arrest (OHCA), and relates ultrasound findings with decisions to terminate resuscitation. The authors report characteristics of patients who do/do not undergo a FoCUS examination by APPs, ultrasound probe positions used and whether FoCUS findings were associated with decisions to terminate resuscitation or to convey patients to an emergency department (ED) with ongoing resuscitation. Method: A retrospective, observational cohort study of all adult medical OHCA patients attended by APPs in Greater London during 2018 was carried out using data from emergency medical services (EMS) and APP databases. Results: Twenty-eight APPs attended 1444 OHCA patients in 2018, of whom 744 underwent FoCUS. The subcostal probe position was used most frequently (74%), followed by the parasternal long axis (19%), with significantly smaller use of the parasternal short axis and apical windows. Absence of spontaneous cardiac motion (SCM) was associated with resuscitation being terminated (333 out of 391; 85%) and the presence of SCM was associated with conveyance to the emergency department (213 out of 264 patients; 80%). All decisions to terminate resuscitation were within the APP scope of practice. Conclusion: The authors believe this is the largest prehospital study involving FoCUS in OHCA. An association between FoCUS findings and decisions made to either convey patients to hospital or terminate resuscitation was found. The SC window was most used and ROLE decisions were deemed to be in accordance with local guidance and practice.