b'Witness Status Arrest witness status has significant implications for patient outcomes, as witnessed arrests have more opportunity for bystander intervention and early delivery of care.Approximately half of arrests were unwitnessed (51.3%), while 37.1% were bystander witnessed and 11.6% were witnessed by a 911 Responder (Figure 8). Patients with a bystander witnessed arrest were more than 3 times as likely to survive their event compared with unwitnessed arrests (14.2% vs 4.1%, respectively; p.0001), while patients with a 911 Responder witnessed arrest were approximately 4 times as likely to survive compared with unwitnessed arrests (17.1% vs 4.1%, respectively; p.0001). Figure 8. Arrest witness status. Initial Rhythm When the cardiac rhythm is first monitored after OHCA, a patient may present in a shockable rhythm (ventricular fibrillation or ventricular tachycardia) or non-shockable rhythm (asystole or idioventricular/pulseless electrical activity [PEA]). Treatment and prognosis depend on presenting rhythm, with better survival after OHCA among patients with a shockable rhythm (26.8% vs 5.8%, p.0001). 17.0% of patients presented with an initial shockable rhythm of ventricular fibrillation (VF) or ventricular tachycardia (VT), while 83.0% of patients presented in an unshockable rhythm, with asystole being the most common (52.7%). Presenting rhythm differed markedly by arrest witness status, with bystander witnessed patients being much more likely to present in a shockable rhythm than unwitnessed patients (27.5% vs 9.4%, respectively; p.0001) (Figure 9). Figure 9. Presenting arrest rhythm by arrest witness status. 29'