Abstract: Europace (2020) 22, 1855–1863 doi:10.1093/europace/euaa216 CLINICAL RESEARCH Arrhythmias and COVID-19 Arrhythmic safety of hydroxychloroquine in COVID-19 patients from different clinical settings 1 Heart Rhythm Center, Centro Cardiologico Monzino, IRCCS, Milan, Italy; 2University Heart Center, University Hospital Zürich, Zürich, Switzerland; 3Unit of Cardiology, Department of Experimental, Diagnostic and Specialty Medicine-DIMES, University of Bologna, Bologna, Italy; 4Department of Cardiology, ASST-Fatebenefratelli Sacco, Luigi Sacco Hospital, University of Milan, Milan, Italy; 5Department of Cardiovascular Disease, Policlinico Umberto I Hospital, Rome, Italy; 6III Division of Infectious Diseases, ASSTFatebenefratelli Sacco, Department of Biomedical and Clinical Sciences, University of Milan, Italy; 7Division of Cardiology, Spedali Civili Hospital, Brescia, Italy; 8Department of Anesthesiology, Emergency and Intensive Care Medicine, University of Göttingen, Göttingen, Germany; 9Department of Systems Medicine, University of Rome ‘Tor Vergata’, Rome, Italy; 10Department of Internal Medicine, Luigi Sacco Hospital, University of Milan, Milan, Italy; 11 Unit of Infectious Diseases, Department of Medical and Surgical Sciences, S. Orsola Hospital, University of Bologna, Bologna, Italy; and 12 Department of Clinical Sciences and Community Health, University of Milan, Milan, Italy Received 6 May 2020; editorial decision 2 July 2020; accepted after revision 3 July 2020; online publish-ahead-of-print 24 September 2020 Aims The aim of the study was to describe ECG modifications and arrhythmic events in COVID-19 patients undergoing hydroxychloroquine (HCQ) therapy in different clinical settings. ................................................................................................................................................................................................... Methods COVID-19 patients at seven institutions receiving HCQ therapy from whom a baseline and at least one ECG at and results 48þ h were available were enrolled in the study. QT/QTc prolongation, QT-associated and QT-independent arrhythmic events, arrhythmic mortality, and overall mortality during HCQ therapy were assessed. A total of 649 COVID-19 patients (61.9 ± 18.7 years, 46.1% males) were enrolled. HCQ therapy was administrated as a home therapy regimen in 126 (19.4%) patients, and as an in-hospital-treatment to 495 (76.3%) hospitalized and 28 (4.3%) intensive care unit (ICU) patients. At 36–72 and at 96þ h after the first HCQ dose, 358 and 404 ECGs were obtained, respectively. A significant QT/QTc interval prolongation was observed (P < 0.001), but the magnitude of the increase was modest [þ13 (9–16) ms]. Baseline QT/QTc length and presence of fever (P = 0.001) at admission represented the most important determinants of QT/QTc prolongation. No arrhythmic-related deaths were reported. The overall major ventricular arrhythmia rate was low (1.1%), with all events found not to be related to QT or HCQ therapy at a centralized event evaluation. No differences in QT/QTc prolongation and QT-related arrhythmias were observed across different clinical settings, with non-QT-related arrhythmias being more common in the intensive care setting. ................................................................................................................................................................................................... Conclusion HCQ..