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Background: Cardiac autonomic dysfunction after myocardial infarction identifies patients at high risk despite only moderately reduced left ventricular ejection fraction. We aimed to show that telemedical monitoring with implantable cardiac monitors in these patients can improve early detection of subclinical but prognostically relevant arrhythmic events., Methods: We did a prospective investigator-initiated, randomised, multicentre, open-label, diagnostic trial at 33 centres in Germany and Austria. Survivors of acute myocardial infarction with left ventricular ejection fraction of 36-50% had biosignal analysis for assessment of cardiac autonomic function. Patients with abnormal periodic repolarisation dynamics (≥5·75 deg 2 ) or abnormal deceleration capacity (≤2·5 ms) were randomly assigned (1:1) to telemedical monitoring with implantable cardiac monitors or conventional follow-up. Primary endpoint was time to detection of serious arrhythmic events defined by atrial fibrillation 6 min or longer, atrioventricular block class IIb or higher and fast non-sustained (>187 beats per min; ≥40 beats) or sustained ventricular tachycardia or fibrillation. This study is registered with ClinicalTrials.gov, NCT02594488., Findings: Between May 12, 2016, and July 20, 2020, 1305 individuals were screened and 400 patients at high risk were randomly assigned (median age 64 years [IQR 57-73]); left ventricular ejection fraction 45% [40-48]) to telemedical monitoring with implantable cardiac monitors (implantable cardiac monitor group; n=201) or conventional follow-up (control group; n=199). During median follow-up of 21 months, serious arrhythmic events were detected in 60 (30%) patients of the implantable cardiac monitor group and 12 (6%) patients of the control group (hazard ratio 6·33 [IQR 3·40-11·78]; p<0·001). An improved detection rate by implantable cardiac monitors was observed for all types of serious arrhythmic events: atrial fibrillation 6 min or longer (47 [23%] patients vs 11 [6%] patients; p<0·001), atrioventricular block class IIb or higher (14 [7%] vs 0; p<0·001) and ventricular tachycardia or ventricular fibrillation (nine [4%] patients vs two [1%] patients; p=0·054)., Interpretation: In patients at high risk after myocardial infarction and cardiac autonomic dysfunction but only moderately reduced left ventricular ejection fraction, telemedical monitoring with implantable cardiac monitors was highly effective in early detection of subclinical, prognostically relevant serious arrhythmic events., Funding: German Centre for Cardiovascular Research (DZHK) and Medtronic Bakken Research Center., Competing Interests: Declaration of interests AB received funding from Medtronic Bakken Research Center as co-funding for the SMART-MI trial (providing implantable cardiac monitors and staff cost for implantable cardiac monitors core lab); and speaker honoraria from Bayer, Boerhinger Ingelheim, Edwards, Medtronic, and Novartis. MSt received consulting fees, speaker honoraria, and travel expenses from Medtronic. RW received grants from German Centre for Cardiovascular Research, Bristol Myers Squibb–Pfizer, and Grant Boston Scientific; speaker honoraria from Biotronik, Boston Scientific, Medtronic, Abiomed, Bristol Myers Squibb–Pfizer, Boehringer Ingelheim, Daiichi Sankyo, Bayer, and Novartis; and travel expenses from Boston Scientific, Bristol Myers Squibb–Pfizer, Boehringer Ingelheim, Daiichi Sankyo, and Bayer. RW participated on advisory boards for Biotronik, Philips, Boehringer Ingelheim, and Daiichi Sankyo. ALu received grants and consulting fees from Boston Scientific and Biosense Webster; speaker honoraria from Boston Scientific, Biosense Webster, and Medtronic; travel expenses from Boston Scientific; and participated on data safety monitoring boards and societies for Boston Scientific. NG received grants from Boston Scientific and Medtronic and travel expenses from Bayer Vital. PC received research grants from Philips. LSM received grants from the German Research Foundation and the EU; speaker honoraria from Bayer, Astra Zeneca, Pfizer, Bristol Myers Squibb, Daiichi Sankyo, and Boehringer Ingelheim; travel expenses from Servier, Boehringer Ingelheim, and Vifor; and participated on data safety monitoring boards for Else Kröner-Fresenius-Stiftung. LSM is stock holder of Bayer and Fresenius Medical Care. MCB received consulting fees from Medtronic and Boston Scientific; speaker honoraria from Medtronic, Boston Scientific, and St Jude Medical; travel expenses from Medtronic, Jonhson & Johnson, Boston Scientific, and St Jude Medical; and participated on advisory boards for Medtronic. CL is member of the Expert Panel for medical devices for the European Commission. CU received consulting fees and speaker honoraria from Medtronic. JRE received consulting fees and speaker honoraria from Medtronic, Abbott, and Boston Scientific. UM received grants from German Centre for Cardiovascular Research (DZHK). All other authors declare no competing interests., (Copyright © 2022 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license. Published by Elsevier Ltd.. All rights reserved.)