b'Witness Status 11.8%The witness status of an arrest 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 (51.5%), while 36.7% were witnessed by a bystander and 11.8%51.5%were witnessed by a 911 Responder (Figure 8). Patients with a bystander witnessed arrest were over three times36.7%more likely to survive their event compared to unwitnessed arrests (15.6% vs 4.6%, respectively; p.0001), while patients with a 911 Responder witnessed arrest were approximately four times more likely to survive comparedUnwitnessedto unwitnessed arrests (18.2% vs 4.6%, respectively;Bystander Witnessedp.0001). Witnessed by 911 ResponderFigure 8. Arrest witness status.Initial RhythmUpon initial cardiac rhythm monitoring following OHCA, patients may present with either 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 significantly better survival rates among patients presenting with a shockable rhythm compared to those with a non-shockable rhythm (28.8% vs 6.3%, p.0001).17.5% of patients presented with an initial shockable rhythm of ventricular fibrillation (VF) or ventricular tachycardia (VT), while 82.5% of patients presented in an unshockable rhythm, with asystole being the most common (51.7%). The presenting rhythm differed markedly by arrest witness status, with bystander witnessed patients much more likely to present in a shockable rhythm compared to unwitnessed patients (28.4% vs 9.8%, respectively; p.0001) (Figure 9).80%67%60%52% 49%40% 38%28% 25% 27%17% 22% 18%20% 14%9% 9% 10% 9% 6%0%All Bystander Witnessed Unwitnessed Witnessed by 911 ResponderArrest Witness StatusVF/VT/Unknown Shockable Asystole Idioventricular/PEA Unknown UnshockableFigure 9. Presenting arrest rhythm by arrest witness status.32 33'