Abstract Body (Do not enter title and authors here): Background: Atrial fibrillation (AF) outcomes are strongly associated with continuous measures of AF burden.

Objectives: To assess the association between changes in maximum daily AF duration (MDAFD) and the combined endpoint of ischemic stroke and mortality among patients with cardiac implantable electronic devices (CIEDs) capable of continuous AF detection.

Methods: The Optum^® de-identified electronic health record (EHR) (2007-2021) was linked with the Medtronic CareLink^® heart rhythm database for this study. CIED patients with healthcare activity recorded in the EHR >12 months pre- and >6 months post-CIED implant were included, excluding those with oral anticoagulation (OAC) prescription. MDAFD was assessed during the first 30 days post-implant (baseline period) and the 30 days prior to censoring or an event. Associations between change in MDAFD and the combined endpoint were evaluated, adjusting for the individual components of CHA₂DS₂-VASc, baseline MDAFD category and chronic kidney disease (CKD).

Results: Of 26,400 patients (68±13 years; 60% male; follow-up 2.6±1.6 years) analyzed, 2,544 (9.6%) had AF during baseline. In all patients, increased (vs. stable or decreased) MDAFD in follow-up was associated with a higher adjusted rate of stroke and mortality (HR 1.80 [1.61-2.01]). There was no association between decreased MDAFD in follow-up and the combined endpoint. Subgroup analysis by baseline MDAFD category demonstrated that increased MDAFD in follow up was associated with a greater risk of stroke or mortality among patients with no AF at baseline, and decreased MDAFD in follow-up was associated with a lower adjusted risk of stroke or mortality among patients with baseline MDAFD 1hour-<5.5hours and 5.5hours-<23.5hours (Figure).

Conclusions: Results from the present study suggest that progression from low burden AF is associated with negative outcomes, and that there may be threshold effects of baseline AF duration that benefit from enhanced rhythm control. These data encourage continued monitoring of AF duration over time to enhance risk stratification and further inform treatment efforts.