b'2021 CARES Annual Report 35 35 Arrest Characteristics and Outcomes Survival outcomes differed markedly across etiology, presenting rhythm, and witness status categories.Patients with an arrest of presumed cardiac etiology had an unadjusted survival rate to hospital discharge of 8.2%. Survival among patients with an arrest caused by a respiratory mechanism or drowning was slightly higher (11.3 and 12.2%, respectively), whereas patients with an overdose-related arrest had a survival rate of 18.4%. Survival was lowest among patients with an arrest due to exsanguination or hemorrhage (3.2%) (Figure 17). Figure 17. Unadjusted survival outcomes by arrest etiology. Patients that present with an initial shockable rhythm of ventricular fibrillation (VF) or ventricular tachycardia (VT) have a much higher chance of survival than patients who present with a non-shockable rhythm such as asystole or pulseless electrical activity (PEA) (Figure 18). Patients who presented in a shockable rhythm had a survival to hospital admission rate of 44.5%, compared with 32.1% for those in PEA and 14.4% for those in asystole. Similarly, patients presenting in a shockable rhythm had a greater chance of being discharged alive (26.0%), compared with 10% of patients presenting in PEA and 2.3% of patientsFigure 18. Unadjusted survival outcomes by presenting arrest rhythm. in asystole. Arrest witness status also has a significant impact on patient outcomes, as witnessed arrests have more opportunity for bystander intervention and early delivery of care. OHCA patients with a 911 Responder witnessed arrest had the highest chance of survival to hospital discharge (15.9%), followed closely by those with a bystander witnessed arrest (13.5%). In contrast, unwitnessed events had a survival rate of 4.3% (Figure 19).Figure 19. Unadjusted survival outcomes by arrest witness status.'