Abstract Sun1406: Early Onset of Harlequin Syndrome Following Prehospital Extracorporeal Cardiopulmonary Resucitation for Refractory Out-of-Hospital Cardiac Arrest

Case Presentation: A 37-year-old man with no known medical history presented to a pharmacy with agitation and severe chest pain radiating to the back. At 16:00, he experienced a witnessed cardiac arrest. Despite advanced life support, he had recurrent ventricular fibrillation without return of spontaneous circulation. At 17:22, prehospital ECMO was initiated. Coronary angiography showed a lesion in the left anterior descending artery, treated with coronary angioplasty and stent placement. At 20:00, he was admitted to the intensive care unit, ventilated with FiO2 50% and PEEP 5 mbar, with the ECMO membrane oxygenator FiO2 set at 60%. Initial inhospital blood gas analysis revealed severe hypoxemia: PaO2 35.2 mmHg, SaO2 57%. Despite lung-protective ventilation, FiO2 100%, PEEP up to 20 mbar while maintaining a plateau pressure of 30 mmHg, and ECMO FiO2 100%, hypoxemia persisted. ECMO flow was maintained at 2.5 L/min, with a high dose of norepinephrine infusion. Differential Diagnosis: -Severe lesionnal ARDS with preserved cardiac function causing Harlequin syndrome -Arterial cannula malposition -Right heart failure with intracardiac shunt -ECMO circuit malfunction Transthoracic echocardiography confirmed preserved left ventricular function, preserved cardiac output, normal right heart, and correct venous cannula placement. No oxygenator dysfunction was found. CT showed bilateral infiltrates consistent with a post cardiac arrest lesionnal ARDS. The most likely diagnosis was early-onset Harlequin syndrome. Treatment and Management: As the safest and fastest option, conversion to veno-arterio-venous ECMO via percutaneous jugular access was initiated, allowing rapid correction of the differential hypoxemia. Outcome and Follow-Up: The patient remained on ECMO for 4 days and required mechanical ventilation for 6. He was transferred to the cardiology ward on day 10, then to a rehabilitation center. At intensive care unit discharge, he had good neurological outcomes (CPC 1). Teaching Points: -Harlequin syndrome can occur early during ECMO after refractory out-of-hospital cardiac arrest. -Early conversion to VAV-ECMO is a fast and safe option when surgical strategies are not feasible. -Timely diagnosis, repeated echocardiography, and blood gas monitoring are critical to prevent neurological damage.